Damp timber related articles

Damp & Timber Related Articles

The following articles have been written by experts within the damp timber industry.

Damp Timber Article 1.

RISING DAMP – Tim Hutton

This article is reproduced from The Building Conservation Directory 1998 and revised February 2012

Rising damp is widely misdiagnosed in existing buildings, based on the incorrect interpretation of visual evidence and the readings of moisture meters. Because of a highly successful sales campaign for over 30 years by specialist remedial contractors installing injected ‘chemical damp proof courses’, this misdiagnosis of rising damp has also become synonymous with a diagnosis of a lack of an ‘injected chemical damp-proof course’. Although this has been very good for business, it has often resulted in a waste of the clients’ money and resources; original plasters and finishes have been destroyed in the process of installation, and unnecessary damage has been caused to original structures by the drilling of irrigation holes. In addition, money that might have been spent on more cost-effective maintenance or repair works has been wasted.

Whilst injected chemical damp-proof courses may provide some protection for certain types of structure if properly specified, their general application is rarely the most cost-effective way of controlling damp problems in buildings, and may be wrongly specified and ineffective. In particular the more generally available water based products may only form an effective ‘hydrophobic band’ if applied to a dry wall after it has dried out. This can prevent their effective installation in damp walls.

CAUSE AND EFFECT

Rising damp actually describes the movement of moisture upward through permeable building materials by capillary action. It becomes a problem if the moisture penetrates vulnerable materials or finishes, particularly in the occupied parts of a building. This moisture will dissolve soluble salts from the building materials such as calcium sulphate, and may also carry soluble salts from its source. If the moisture evaporates through a permeable surface, these salts will be left behind and form deposits on or within the evaporative surface. Where there is a large evaporative surface, salt crystals are deposited as a harmless flour-like dusting on the surface. If evaporation is restricted to localised areas such as defects in an impermeable paint finish, then salt deposition is concentrated, forming thick crystalline deposits with the appearance of small flowers; hence the term ‘efflorescence’. When evaporation occurs within the material, salts can be deposited within the pores. The expanding salt crystals in these locations may result in fractures forming in the material and spalling of the surface. This type of decay may be seen in porous brickwork or masonry.

When there has been a long-term problem with moisture penetration, evaporation at the edge of the damp area leads to a distinctive ‘tide mark’ as a result of salt deposition. Where this occurs at the base of a wall, the tide mark is often taken as a typical diagnostic feature of ‘rising damp’. However, these salt accumulations may remain even when the water penetration that originally caused them has long gone. Similarly, water penetration may have occurred from causes other than ‘rising damp’.

The most common source of moisture in the base of the walls of buildings is from defective ground and surface drainage. This is present to some degree in almost every building in the country, due to a combination of such factors as rising ground levels, the failure of ground drainage systems, and the increased use of concrete or finishes around buildings without consideration of drainage slopes.

The accumulation of ‘moisture reservoirs’ in the foundations may also arise as the result of chronic plumbing leaks or floods from catastrophic plumbing or drainage defects.

Damp conditions at the foot of walls may be greatly increased by condensation. This occurs when warm moisture-laden air cools to dew point (the temperature at which moisture condenses) against a cold surface. Such cold surfaces commonly occur when the insulation value of the external wall is reduced by water penetration, as described above. Intermittent occupancy with intermittent heating provides the conditions for condensation of further water on these cold damp surfaces, particularly in ground floor bedrooms. These phenomena are the main causes of damp in the base of walls rather than ‘rising damp’ alone.

Concentrations of hygroscopic salts, which are often found in masonry, can also absorb moisture from the air, especially at relative humilities above 75 per cent. In a room that is sometimes unoccupied, with fluctuating relative humidity levels, this can result in the regular appearance of salt blooms on the surface (‘cyclical efflorescence and deliquescence’), resulting in damage to vulnerable materials, and giving the appearance of rising damp.

Damp masonry at the base of walls may lead to a number of problems:

  • The moisture content of the structure may rise to a level at which decay organisms may grow, or the materials themselves may be adversely affected. For example, timber skirting boards or built-in bonding timbers along the base of walls may become infected and decayed by dry rot, wet rot, weevils or woodworm.
  • In very damp conditions, the inorganic materials themselves may lose their structural strength. This occurs most spectacularly with walls made of cob (earth) soaked with water.
  •  Damp conditions on the surface of walls, particularly in conjunction with condensation, allow the growth of moulds both on the surface and within porous or fibrous materials, such as wallpapers or carpets fitted against the base of the wall. Not only is this aesthetically unacceptable and damaging to finishes, but it can be a significant health hazard to occupants.
  • Where evaporation takes place, the deposition of soluble salts on the surface or within the pores of materials can cause aesthetic and structural damage.

Examples of moisture sources within a property    Example of moisture reservoirs within a property

TREATMENT OPTIONS

As described above, ‘rising damp’ is only one of many mechanisms resulting in high moisture levels in the base of walls, and even when it is a significant factor, it is rarely the primary source of moisture. The management of problems due to high moisture levels requires the proper identification of the moisture source and the defect responsible, before the most cost-effective solution to the problem can be determined.

Damp and its effects may then be controlled by adopting one or more of the following measures:

  • The provision of suitable moisture sinks to dissipate the moisture at its source without causing problems to the structure or occupants, and the repair of any contributing defects acting as moisture sources, such as broken pipes.
  • The introduction of either physical barriers using damp-proof membranes or materials to form a ‘damp-proof course’ or hydrophobic (water-repellent) materials as in ‘chemical damp proof courses’.
  • The isolation of vulnerable materials such as timber and interior finishes from damp fabric.

Moisture Barriers

The control of moisture movement using either damp-proof or hydrophobic materials to create a relatively less permeable ‘moisture barrier’ is not necessarily a cost-effective option in controlling damp problems and may even be counter-productive. This is because use of relatively impermeable materials will restrict moisture movement and hence drying. As a result, moisture may be ‘locked’ into damp materials for many years causing chronic problems. Moisture may also be prevented from dissipating from permeable materials, resulting in the build-up of moisture or even damper conditions in localised areas. This may result in moisture moving into previously dry structures or evaporating from previously unaffected surfaces, causing further salt efflorescence. One reason why those injecting ‘chemical damp-proof courses’ generally insist on re-plastering treated masonry with a salt-proof and waterproof mixture, is to cover up these potential problems.

A relatively common example of the effect of inserting a damp-proof material into a structure is the appearance of fresh ‘rising damp’ in walls following the laying of a new concrete floor with a damp-proof membrane. This is most often done when a suspended floor structure is replaced by a solid floor, or when a breathable stone slab floor is lifted and re-laid. Before the alteration of the original floor, moisture would have been able to evaporate off a large surface, without affecting internal finishes. However, a new impermeable membrane allows the water to accumulate beneath, forcing it to the sides of the room and into the base of the walls. This causes damp timber decay problems unless appropriate ventilation has been provided at the floor/ wall junction. These damp problems are then often used as justification for the injection of a moisture-barrier and the removal and replacement of plaster with remedial mixes. In fact, the more cost effective solution would have been to allow the floor structure to continue to breathe. This can be done with a suspended floor or by re-detailing the floor/wall junction in such a way as to allow moisture to dissipate, for example, with a vented skirting detail.

If it is decided that a moisture-barrier at the base of the wall is essential, the most reliable method is to introduce a physical barrier rather than a chemical one. This involves cutting in a layer of damp-proof material to form a barrier which is continuous with the damp-proof membrane under the floor. As the wall above this barrier will remain damp for some time, it is then necessary to isolate all vulnerable materials above as well as below the barrier, such as skirting boards, from the base of the wall with a damp-proof membrane or ventilated air gap.

However, a damp-proof barrier is always vulnerable to local failure and will tend to concentrate moisture and damp problems at these points. This is a general characteristic of all impermeable materials, including those used in tanking systems, which are generally found to fail at some point or at some time. This results in more ‘concentrated’ moisture at the points of failure, and hence more severe damp problems locally when they fail. Because of this, the more robust, fail-safe, and traditional building techniques rely on the use of permeable materials and ventilation details in order to dissipate moisture and prevent it coming into contact with vulnerable materials or interiors.

‘Chemical damp-proofing’ may provide a useful barrier to damp in the short to medium term, or at least a ‘nominal damp proof course’, where the walls are of uniform construction such as sound brickwork laid with strong cement mortar; especially if they are combined with a ventilated dry lining system or other building detail which allows moisture to dissipate. However, any gaps which are left, or which appear over time as the material deteriorates, may lead to an accelerated rate of decay.

This method is most unreliable where walls are of natural stone, because the injected hydrophobic material will follow the lines of least resistance and may not accumulate in sufficient quantities where it is needed. This is particularly true when the wall is made up of materials of different permeability, as is common in the thicker walls of older buildings where the bricks and mortar may be of variable consistency and the structure may include cavities, particularly when the wall consists of brickwork or masonry skins containing a rubble infill.

Surface Water Drainage

The most cost-effective way of preventing damp problems in buildings, including those resulting in damp masonry at the foot of walls, is to minimise moisture sources and provide adequate passive moisture sinks to dissipate any penetrative moisture so as to make the system fail-safe. This should start with the provision of adequate ground drainage around the building to minimise water penetration to the foundations, and the re-detailing of surface drainage so as to ensure surface water is drained clear of the foot of the walls.

It has become fashionable to specify ‘French drains’ to help with this process. However, these are often poorly specified and soon become ‘French ponds’ in UK conditions. This may be because the base of the drain has been inadequately levelled or drained to keep water out of the foundations and the gravel infill has become contaminated with soil and debris, preventing proper moisture drainage and evaporation from the foot of the wall.

In the UK, the more traditional and more effective detail is to use a ventilated and drained ‘dry area’ around the foot of the wall. These are commonly covered with York stone slabs in order to prevent debris accumulating in the drained dry area and to minimise maintenance.

Alternatively, a perforated plastic land drain can be laid to falls in a trench lined with geo-textile and back filled with ‘beach cobles’ or large diameter hard core. Proprietary external ‘drained cavity systems are also available.

Ventilation to base of church

A traditional ventilated and drained ‘dry area’ around the foot of the wall of a church, partially covered with stone slabs.

Wall Construction

The use of impermeable finishes, such as sand/ cement renders, around the base of external walls is a common cause of damp timber decay problems. These prevent moisture evaporating from the foot of the wall, forcing it into the interiors. As with all impermeable materials, they eventually fail, generally due to cracking. This allows water to penetrate into the foot of the wall, but prevents drying. The use of more traditional breathable lime mortar renders, and the correct detailing of renders to shed water clear of the base of the wall and to prevent ‘bridging’ of any existing damp-proof course, would be the preferred solutions.

Cavity wall construction may provide a way of dissipating moisture and preventing it penetrating into the building, provided the cavity is through ventilated. This may be compromised by debris or the ill-advised injection of proprietary insulation foams. These defects may also bridge existing damp-proof courses, allowing water to penetrate to interior finishes. In some cases, the most cost effective solution is to reinstate a through-ventilated cavity.

Generally, failures in existing damp-proof courses are the result of bridging by inappropriate repairs and alterations, by raised ground levels or by localised damage due to structural movement or poor building work. If a damp-proof course is an original design detail to control moisture movement in the structure, it may be necessary to carry out local repairs. This is best done by ‘cutting in’ a new layer of damp-proof material locally rather than by the general injection of hydrophobic solutions into the masonry to create a ‘moisture movement restricting barrier’.

Ventilation

Traditional buildings built in damp or potentially damp sites commonly included through-ventilated sub-floor cavities, cellars or basements. These act as sumps to allow the evaporation and dissipation of moisture from the structure before it reaches occupied areas or vulnerable finishes. Indeed, in some parts of the country it is not uncommon to find streams running through the cellars or basements in old farmhouses. These were presumably retained as a source of water for domestic use. However, if the ventilation of a basement, cellar or sub-floor cavity has been restricted, moisture can build up and penetrate vulnerable structures. This can occur, for example, by earth and plants clogging air bricks or by the ill-advised application of relatively impermeable materials. The solution to these problems if they develop, is to re-establish ventilation, not to start applying further damp-proof materials.

Example of moisture sinks within a property

As described earlier, the reinstatement of a through-ventilated suspended floor is generally preferable to its replacement with a concrete slab. The requirement for the continued dissipation of moisture does not preclude the use of basements and cellars as occupied areas, but means that walls should be kept ventilated and not sealed. This can be achieved by using through-ventilated dry lining systems rather than impermeable finishes or tanking materials, which would only force moisture into adjacent structures above or to the side. Traditionally, dry lining has been produced by the use of timber panelling spaced from the masonry with battens or the use of lath and plaster. In all cases, the cavity behind should be ventilated at the top and at the bottom to allow through-ventilation to dissipate moisture, as otherwise moisture will accumulate to cause damp timber decay problems. This commonly happens when insulation material or debris is allowed to block the cavity behind lath and plaster or when impermeable paint layers accumulate over timber panelling. These defects are easily solved and the traditional ‘farmhouse’ technique of timber panelling to dado level can be an attractive and cost-effective solution to problems of damp penetration or condensation affecting the foot of masonry walls. Modern materials and techniques may be used to achieve the same end, and many products are available on the market to allow the cost-effective provision of through-ventilated dry lining systems, including specialist plasterboard systems and studded plastic membranes which can be used to form vertical damp proof course details behind the dry lining.

 

 

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Damp Timber Article 2.

Woodworm (Anobium Punctatum)  –  Tim Hutton

This article is reproduced from The Building Conservation Directory 2008. It provides a practical guide to woodworm infestation and its eradication

Anobium Punctatum, generally known as the common furniture beetle or ‘woodworm’, has been perceived to be the main cause of damage to timber in the UK over the last 100 years. During the last 50 years, insecticidal treatments have been widely marketed and used to ‘treat and preserve’ timbers in buildings thought to be at risk from this organism. The perceived risk of woodworm infection and decay has become so integral to the culture of property management and building repair in the UK that most buildings which are more than 50 years old have been treated at least once, and many have been treated repeatedly on each change of ownership. This became almost automatic as mortgage lenders became convinced of the requirement for ‘guarantees’ that woodworm was not active in a building before issuing loans.

Woodworm galleries in floor timbers
Anobium Punctatum, generally known as the common furniture beetle or ‘woodworm’, has been perceived to be the main cause of damage to timber in the UK over the last 100 years. During the last 50 years, insecticidal treatments have been widely marketed and used to ‘treat and preserve’ timbers in buildings thought to be at risk from this organism. The perceived risk of woodworm infection and decay has become so integral to the culture of property management and building repair in the UK that most buildings which are more than 50 years old have been treated at least once, and many have been treated repeatedly on each change of ownership. This became almost automatic as mortgage lenders became convinced of the requirement for ‘guarantees’ that woodworm was not active in a building before issuing loans.

IDENTIFICATION AND LIFE CYCLE

Anobium punctatum is one of a large number of beetles that have evolved to exploit the cellulose in timber in temperate climates. It occurs naturally in the wild in the temperate woodlands of northern Europe and may have colonised other similar temperate environments, particularly in New Zealand and the east coast of North America. The adults are small oval brown beetles approximately 4-6mm long. When viewed from above, the head and eyes are invisible beneath the thorax and the wing cases have relatively straight parallel sides rather than an oval or round appearance. When viewed under the microscope, the surface of the wing covers are seen to be covered with fine yellowish hairs and longitudinal rows of pits are visible. The antennae should be visible extending from beneath: these have eleven segments with the last three segments enlarged so that these three together are longer than the combined remaining segments.

Picture of woodworm beetle Anobium punctatum

Anobium punctatum adult, typically 4-6mm long (Image: BRE, from Recognising Wood Rot and Insect Damage in Buildings, BR453; all other images: Hutton & Rostron Environmental Investigations Ltd)

The adult beetles emerge from infected timber in the spring, generally between May and August in the northern hemisphere, leaving a small round hole of approximately 1-2mm in diameter on the surface of the wood. The adult beetles mate soon after emergence: first, the female beetle appears to seek out suitable timber to lay her eggs and for the larvae to feed on, and the male then seeks out the female by tracking the pheromones she releases, giving preference to visual cues for standing timber. The adult beetles then die without causing further damage to timber.

The small pearl-like eggs may be seen with the naked eye in clusters of up to 50. These are only laid on dead timber where the bark has been removed and where there are suitable egg laying sites, such as cracks, crevices, exposed end grain or previous emergence holes. Anobium punctatum specialises in infecting the sapwood of temperate softwoods and hardwoods that have been dead for at least five years, but may also infect the heartwood of timbers such as beech, birch, cherry, alder and spruce, or timbers that have been modified by fungal attack. The eggs of Anobium punctatum generally hatch within six to ten days under suitable environmental conditions.

As with many other insects, the majority of the lifecycle of Anobium punctatum is spent as larvae. These are greyish white in colour with a narrow dark band over the mouth parts and grow to about 6mm long. The front part of the body appears relatively thick or hunched and has three pairs of visible legs. The rear section of the body is thinner, with a rounded tail-end. There are transverse bands with two rows of spinules on the first six segments and a single row of spinules on the seventh segment. In the wild, the larvae generally spend a year excavating tunnels usually approximately 1-2mm in diameter and generally parallel to the grain of the timber. These tunnels are backfilled with the residues of the timber consumed, forming a cream-coloured powdery material consisting of lemon-shaped pellets when viewed with a microscope, which may feel gritty to the fingers if relatively fresh. It is in the larval stage that Anobium punctatum causes most of the damage to timber.

Consuming cellulose from timber in this way poses many potential problems for the larvae, not least the fact that growing trees deposit chemicals within their timber to prevent or discourage attack by insects and other organisms. Cellulose is generally indigestible to insects or other animals. Anobium punctatum, like other cellulose-consuming animals, therefore relies on commensal micro-organisms within its gut to help digest the cellulose and produce the proteins and sugars that it requires to grow. The presence and absence of relative proportions of these and other chemicals within the timber appears to be crucial to whether they are able to flourish within particular timbers. This accounts for the apparent preference of Anobium punctatum for sapwood over heartwood, as the former is likely to contain more residual sugars and proteins of use to the growing larvae, while the latter is likely to contain more potentially toxic chemicals deposited by the tree during growth. Similarly, the preference of Anobium punctatum for particular species of timber may be due to this. However, as with other organisms specialising in decaying timber, partial decay and digestion of the chemicals that might otherwise limit infection by fungi may allow Anobium punctatum to infect and consume timbers that would otherwise be relatively indigestible and inhospitable to growth.

Timber decay to staircase structural timbers

Timbers supporting staircase within an under-stair cupboard: structurally significant decay has occurred to the sapwood band due to chronic problems with poor ventilation and long-term high relative humidity.

Woodworm activity to timber

This oak beam shows evidence of extensive decay of original sapwood band by Anobium punctatum and other related wood-boring beetles.

In the wild, after growing for about a year, the larva of Anobium punctatum forms a cell just below the surface of the wood where it pupates into an adult in approximately two to three weeks. The size of the larva when it pupates and the size of the adult and the resultant emergence hole will vary depending on the size of the larva at that time, and presumably on the relative suitability of the food and environment available. Anobium punctatum appears to have a preference for dead standing timber with the bark removed and only thrives under the conditions produced by the temperate climate of northern Europe. It therefore does not tolerate relative humidity below 60 per cent or timber moisture equivalents below 14 per cent, nor will it tolerate saturated timber and it will not thrive in temperatures much above 30°C.

THE CONDITIONS REQUIRED IN BUILDINGS

The environmental conditions within an occupied building are generally unsuitable for Anobium punctatum to lay its eggs, consume timber and complete its lifecycle. This is because it generally requires a relative humidity above 60 per cent for the eggs to hatch or for pupation to its adult form to occur. The sharply fluctuating and relatively low moisture contents of timber elements in an occupied building and the intermittent high temperatures that occur in many structures also prevent or restrict the growth and development of Anobium punctatum. For this reason, the insect generally requires at least three years to complete its lifecycle, not one, and the conditions required for it to flourish are only found in external structures such as outhouses and agricultural buildings, or in parts of a structure subject to chronic damp problems. However, it should be expected that at least 50 per cent of buildings in the UK have had some prior infection and decay by Anobium punctatum, and it is believed that nearly every house in New Zealand which is more than 15 years old has been subjected to some prior infection or decay. It has also been noted that almost every building in Germany has been infected.

Woodworm attach to chair leg

This antique chair joint was decayed by Anobium punctatum because of the use of poor quality sapwood timber and animal-based glue.

Timber structures in buildings in the UK likely to have been infected and partially decayed by Anobium punctatum at some time are those which have been subjected to damp conditions persisting for over five years, but not subject to liquid water penetration. Typical causes include condensation and/or high relative humidity, generally as a result of inadequate ventilation and cold-bridge condensation. Poorly ventilated basement and sub-floor structures, particularly the cupboards and voids beneath staircases, and timbers in poorly ventilated roof voids are therefore often found to have been infected at some time. The latter may be a particular problem in the north and west of the UK due to the relatively high moisture levels and reduced summer temperatures in roof structures compared to the south and east. Similarly, it is not unusual to find evidence of past woodworm infection and decay around poorly ventilated and insulated skylights or roof hatches, and in floor structures of bathrooms and kitchens subject to intermittent water penetration and/or high relative humidity levels. Despite the above, infection by Anobium punctatum today is rarely active or structurally significant, and heating and ventilation on occupancy will generally prevent further infection or decay.

Factors preventing infection and decay by Anobium punctatum in buildings are generally the absence of suitable sapwood timber in persistently damp conditions, and the absence of suitable cracks, crevices or holes for deposition of eggs on finished timber surfaces. Historically, the most significant damage by Anobium punctatum was perceived as being the decay of furniture, hence its common name, the furniture beetle. This is probably because in the past furniture was commonly made of cheaper and less durable local timber such as beech. The relatively high proportion of sapwood in country-made or ‘bodged’ timber furniture would also be vulnerable to Anobium punctatum, and the cracks and crevices formed at the joints in furniture also make it vulnerable to infection and failure at these points. It is not unusual to find decay to the bottom of the legs of poorer quality antique furniture due to the relatively high proportion of sapwood on turned elements in these areas, and because legs were often in contact with damp solid floor structures.

As a food source, timber is generally deficient in available nitrogen and this is often a major restraint on the growth of organisms relying on timber as a primary food source. It is for this reason that pre-digestion by fungi or bacteria often makes timbers more vulnerable to decay by other organisms and why contamination with highly nitrogenous materials makes timber more vulnerable to decay. However, the glues used for the construction of furniture in the past were often based on animal products such as horn and contained high proportions of proteins and other nitrogenous materials. Their use in the joints of furniture therefore made the glued timber particularly attractive and vulnerable to infection and decay. More valuable furniture was often made with the heartwood of durable timber such as oak or, later, tropical hardwoods. These are generally resistant to infection and decay by the larvae and may represent the majority of antique furniture surviving today.

Woodworm flight holes & frass
This timber was infected by active Anobium punctatum and shows the typical holes and deposits of gritty yellow frass, which can be shaken out of old emergence holes by vibration from road traffic or building works long after infection has ceased to be active.

IDENTIFYING ACTIVITY

In most cases, infection and decay by Anobium punctatum is first suspected due to the discovery of typical small emergence holes in vulnerable timber elements and this is often the only symptom, resulting in unnecessary treatment. Diagnosis of Anobium punctatum infection has even been mistakenly made on the basis of holes made by drawing pins or from other causes. With experience, it is possible to distinguish emergence holes of Anobium punctatum from those of other woodboring beetles and from other causes. However, even when emergence holes are correctly identified, these are by definition the result of past infection and decay, as they are made by the adults emerging and leaving, so may no longer be active. More recent emergence holes can be distinguished by the sharpness of the edges of the holes and the differential colour between the interiors and exteriors of holes, as these may soon become contaminated by dust or the surface application of paints and other materials.

Paint finishes or special paper strips may be applied over suspected areas of Anobium punctatum infection to identify new emergence holes as these appear. Activity may also be monitored by trapping emerging adults with electric UV flying insect traps, and by checking cobwebs, particularly around window openings, for caught adults. Similarly, pheromone traps are widely available commercially to allow emerging adult males to be trapped. All of these techniques may be useful for general monitoring of activity and may also help reduce the risk of re-infection. However, it may not be possible to determine where the adults have been emerging from.

Paper used to monitor woodworm activity showing flight holes
Paper patch fixed to timber to detect fresh emergence holes: this can be a cost-effective way of monitoring activity by Anobium punctatum and the efficacy of measures to dry the structure.

The deposition of quantities of fresh gritty frass from the emergence holes may sometimes indicate active infection. However, frass may often be found coming out of emergence holes in previously affected timbers many years after active infection has ceased. This may be due to vibration caused by heavy traffic on adjacent roads or building works elsewhere on the structure. Again, the appearance of freshly deposited frass around emergence holes has often been the justification for extensive remedial treatments in the past, even when the infection by Anobium punctatum has been dead or inactive for many years.

Searching for live Anobium punctatum larvae within timber is generally destructive, and surprisingly few larvae may actually be found. It is possible to use highly sensitive piezoelectric microphones embedded in the timbers to monitor activity, but this is not yet the basis of an effective diagnostic technique for use in the field. Similarly, it is possible to identify recently produced frass using immunological or genetic techniques. Again, this is not yet the basis of a cost-effective field identification technique.

In practical terms, the likelihood of significant Anobium punctatum infection is relatively easy to assess, in that if the deep moisture content of the timber is below 12 per cent, it is too dry for infection and decay to occur, while if the moisture content is between approximately 16 and 30 per cent it is possible, even if infection and decay is not present at the time of investigation. If a deep moisture content of 16-30 per cent is found in the sapwood of vulnerable timber, then an assessment has to be made whether this moisture content is likely to persist for over two years. If this is the case, then appropriate remedial measures should be considered.

In all cases, a risk assessment of the significance of active or past Anobium punctatum infection must be made; for example, there may be a high risk that active Anobium punctatum may be present or may occur, but a low risk of structurally or aesthetically significant damage occurring given the low significance of the vulnerable sapwood component of the affected timber. Alternatively, there may be a very low risk of continuing active Anobium punctatum infection, but a high risk of structurally significant decay having occurred in the past, for example, to joints in vulnerable timber structures or to timber supporting a valuable finish.

In the last 100 years, infection and decay of new furniture by Anobium punctatum has become less common. This is probably due to the increased use of tropical hardwoods and the application of solvent based varnishes and finishes which prevent the deposition of eggs in suitable materials. Active infection and decay is therefore generally confined to older furniture particularly that which has been stored for at least part of its life in damp, poorly ventilated and unheated conditions. In this context, it should be realised that a localised low level of Anobium punctatum infection may persist in infected timbers for many years after original infection, particularly under conditions which are generally unsuitable for the beetle to complete its life cycle. Adults may therefore eventually emerge from previously infected timber many years after original infection, with little or no risk of further infection or decay. This should not be mistaken for evidence of a sudden outbreak of active infection and decay.

TREATMENT

The management of decay to timber by Anobium punctatum should be considered in two parts. Firstly, it is necessary to consider the extent of decay and its structural significance. This may require the testing of suspect materials so as to determine their adequacy to carry the loads expected. In buildings, drilling and probing are usually cost-effective for this purpose, although it is possible that x-ray or other non-destructive imaging techniques may be necessary when examining particularly valuable or vulnerable furniture or historic items. Ultrasound and other techniques have been tried but the results have generally proved hard to interpret. When the extent and significance of any damage has been determined, it then remains necessary to carry out appropriate repair. In buildings this generally involves replacement or partnering of affected structures. Although resin consolidation has sometimes been proposed, this is rarely cost-effective in buildings, although it may be applicable to valuable historic artefacts or furnishings.

Secondly, consideration should also be given to control of any existing residual active infection by larvae and to minimising the risk of infection and decay in future. This can almost always be done b insuring that the moisture content of timbers is not allowed to remain at over 16 per cent for more than a year. This is usually easy to achieve within the built environment by the application of standard techniques for controlling moisture penetration and providing through-ventilation and drying. In modern occupied and heated buildings, the moisture content of timbers is generally well below 12 per cent, particularly with the use of central heating systems. In the experience of Hutton and Rostron, this is usually all that is required to control infection and decay by Anobium punctatum, although it may sometimes take a year of two for an area of active infection to finally die out and for pupation and emergence of adults to stop. In this context, it should be realised that the actual decay caused by the larvae is relatively slow and it would usually take an infection by Anobium punctatum many years to cause any further structurally significant decay. Drying may be supplemented with measures to control emerging adults such as UV and pheromone traps.


Building pathologist using piezoelectric microphones to detect larvae actively eating through infected floorboards. Like many specialist techniques, this is a useful scientific tool for monitoring activity, but not a cost-effective field technique.

Other treatment techniques may be considered if it is necessary to control an active infection by Anobium punctatum in the short term, for example to prevent the emergence of adults through a valuable decorative surface, or for management and contractual reasons, such as the sale of a property or a piece of furniture. Unfortunately, experience over the last 50 years has shown that the use of insecticides or chemical remedial timber treatments in buildings has not generally been cost-effective. This is because insecticides retrospectively applied to timbers generally only penetrate a few millimetres below the surface, and may therefore not affect the larvae causing the decay deep within the timber. It is also difficult if not impossible to ensure levels of insecticides that are toxic to the larvae in all parts of the vulnerable structure, particularly given the restraints of health and safety, and the environmental risks inherent in using insecticides or other potentially toxic chemicals. The environmental impact of some of the treatments achieving more effective penetration into timbers such as methyl bromide also preclude their extensive use. The use of insecticides may also represent a potential hazard to those occupying or coming into contact with the treated materials. Although the penetration of toxic levels of insecticides into the superficial layers of timber may be thought to prevent the emergence of adult beetles and restrict the development of new eggs, in practice Anobium punctatum seems to be adept at finding gaps or cracks in treated materials, allowing continued infection and decay, particularly if further water penetration occurs.

In recent years, more localised deep treatment using products such as organoboron timber treatments have been increasingly recommended. These may have the advantage of penetrating deeper into the timber, particularly under damp conditions, and may have a more persistent effect by killing the larvae over a longer period of time, possibly by killing or otherwise affecting the commensal organisms in their gut which allow them to digest cellulose. However, it can be hard to achieve or maintain a toxic level of these chemicals within the treated timbers under field conditions, and adults may continue to emerge after treatment. Because of these limitations, Hutton and Rostron has found chemical remedial timber treatments for Anobium punctatum to be rarely cost-effective.

Fortunately, other more effective techniques for controlling active infection by Anobium punctatum have been developed, generally by those involved with the conservation of museum artefacts. These treatments are generally based on environmental manipulation so as to create an environment that results in the early death of any Anobium punctatum larvae within the material. The most generally useful technique involves raising the temperature of the infected material to above 50°C. This may be easily achieved with furnishings or relatively small objects, but becomes much more difficult with larger more complex structures, such as a building. Special measures have to be taken to ensure that vulnerable materials are not damaged by excessive changes in relative temperature or relative humidity. This may be particularly problematic where vulnerable objects include other materials and finishes of a very different nature, such as oil paints and glues, which may have different responses to temperature and humidity. Raising the temperature will also significantly affect the relative humidity of the environment. The resultant drying may be a contributory factor in the killing of the Anobium punctatum larvae, but differential drying may also cause unacceptable cracking and damage to vulnerable materials. As a result, relative humidity must be carefully monitored and controlled during the heating process. These problems are generally now well understood and reputable firms exist with extensive experience of effectively treating structures and objects using these techniques. Despite this, heat treatment of a structure may be relatively expensive.

The international agreements preventing or restricting the use of methyl bromide or other similar compounds for the fumigation and the control of insect infestations has increased research into the use of inert gases for oxygen deprivation and the killing of insects. Carbon dioxide has been used in this way for many years and, more recently, nitrogen has been used. This is generally achieved by enclosing the objects or structures to be treated in a gas-proof container or enclosure, and pumping in the inert gas until the oxygen content of air has been reduced to below 0.2 per cent. These conditions may then have to be maintained for at least two weeks to ensure the suffocation of the insect larvae. However, it should be noted that damp conditions within the materials may protect the larvae from oxygen deprivation. These techniques may therefore be cost-effective for treating furniture or art objects but are unlikely to be cost-effective for treating buildings.

It is also possible to kill Anobium punctatum larvae by freezing. Obviously Anobium punctatum is able to survive at temperatures below freezing point in the wild and, if given enough time, the larvae are able to adapt to cold conditions. In order to kill them it is therefore necessary to subject objects as quickly as possible to a ‘deep freeze’ temperature lower than -20°C. Repeated cycles of freezing and thawing are also more likely to kill any remaining live larvae within timbers. However, as with heat treatments, it is important to consider the effect of variation in temperature and consequent variations in relative humidity on vulnerable materials.

In conclusion, proper maintenance and management should control Anobium punctatum infection and decay in buildings and furniture in most cases, without recourse to specialist remedial treatments. Unfortunately, a misunderstanding of the cause and effect of Anobium punctatum infection and decay in the past, and the inappropriate use and marketing of potentially environmentally harmful treatments, has resulted in the accumulation of potentially hazardous residues in the built environment. These factors have also resulted in a perception that any evidence of Anobium punctatum activity requires expensive and potentially destructive interventions and has resulted in enormous expenditure on unnecessary treatment – sometimes resulting in damage to original materials – that might have been spent on more cost-effective conservation measures. A better understanding of Anobium punctatum and the recent development of more cost-effective remedial measures renders more traditional treatments not only redundant but unacceptable.

 

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Damp Timber Article 3.

TIMBER DECAY – Dr Jagjit Singh

This article is reproduced from The Building Conservation Directory, 1996 (where it appeared as ‘Environmental Monitoring and Control’)

Sporophores of the dry rot fungus affecting floor joists
Sporophores of the dry rot fungus; Serpula lacrymans affecting floor joists

Building materials are decayed by the effects of adverse environmental conditions and the extent of damage depends on both the materials and the conditions. Among the most vulnerable materials are timber, paint, textiles and paper. Timber remains one of the most useful in a world of diminishing resources and is a major component in most historic buildings. It has many positive structural and aesthetic properties as well as being an energy-efficient and renewable resource. However, timber provides specialised ecological niches and many organisms have evolved to use it as a food. The most common and destructive to timber are dry rot, wet rot, common furniture beetle, and death watch beetle.

Orthodox remedial treatments often entail the loss of irreplaceable decorative finishes, floors and ceilings. Furthermore, treatment of the infestations with insecticidal fungicidal chemicals is not only expensive, inconvenient, hazardous to the operatives and occupants but also environmentally unacceptable and usually unnecessary. Environmental control and preventative maintenance provide an alternative, less destructive solution, and remain the most widely used methods for preventing biological decay.

BIOLOGICAL DECAY MECHANISMS

Biodeterioration of materials was defined by Hueck in 1968 as ‘any undesirable change in the properties of material of economic importance brought about by the activities of living organisms’. A wide range of materials are subject to microbiological deterioration, which are caused by a broad spectrum of micro-organisms.

Beetles responsible for the decay of timber principally include woodworm (Anobium punctatum), death watch beetle (Xestobium rufovillosum), powder post beetle (Lyctus spp), and house longhorn beetle (Hylotrapes bajulus). It is their larvae which cause most damage as they bore through the wood, feeding off it and causing damage to the structure and strength of the timber.

Decay fungi are capable of enzymatically degrading complex cellulosic materials, such as wood, into simple digestible products. The decay of wood cells by these fungi results in the loss of weight and strength of the wood. There are two main types of wood-rotting fungi found in buildings; wet rot and dry rot (see Table 2).

The principal environmental factors favouring the biodeterioration of building materials are temperature, humidity and a lack of ventilation. Moisture may be contributed by penetrating or rising damp; condensation; building defects and disasters such as leaks; and from construction moisture introduced in mortar, concrete and plaster for example.

ENVIRONMENTAL CONTROL – THE GREENER APPROACH

Environmental control relies on controlling the cause of the problem by controlling the environment. It is designed to ensure the future health of the building and its occupants by avoiding the unnecessary use of potentially hazardous and environmentally damaging chemical pesticides where possible and their consequential legal and management complications. Eradication of dry rot spores or insect pests in an historic building and its contents is in practice, impossible. The volumes of chemicals necessary and the toxicity required would be damaging both for the buildings and all its occupants. Where chemical treatment cannot be avoided materials and techniques should be used which have minimum adverse environmental effect.

By reducing the need to expose and cut out infected material, environmental control also reduces damage to the fabric and the finishes of a building. Where an historic building is concerned, this is particularly important, and the specification should ensure maximum conservation of existing materials to maintain the historic integrity of the fabric, as well as to avoid unnecessary expenditure. Its success depends on a thorough investigation of cause and effect. Through a methodical approach such as this, it is possible to decrease the cost of remedial timber works significantly or in some cases eliminate it altogether.

First the building should be thoroughly inspected using non-destructive techniques to locate and identify all the significant decay organisms within it. In cases of actual or suspected problems of woodrot or wood boring insects in buildings, investigation should be by an independent specialist consultant, architect or surveyor to establish the cause and extent of the damp timber decay, including the potential risk to the health of occupants before specification or remedial work. Correct identification of the fungi and insect material is important as not all fungi are equally destructive. Some rots are present in timber when it is cut or are acquired in storage. Fungal material may also be dead or dormant, the product of conditions now past.

Having identified the nature of decay, the environmental conditions which are required to support it should be considered (see Table 1). Only then will it be possible to devise a scheme to deal with the problem.

The aim of remedial building works is to control the timber decay, to prevent further decay and to correct any significant building defects resulting in conditions of high moisture content or poor ventilation of timber. In particular, it is important to reduce sub-surface moisture content of all timber to below 16-18 per cent. Timber should be isolated from damp masonry by air space or damp proof membrane, and free air movement should be allowed around timber in walls, roofs and suspended floors. All other sources of water should also be eliminated, such as overflowing gutters, leaking plumbing, condensation and rising or penetrating damp. Humidity in voids should not exceed an average relative humidity of 65 per cent. In addition, all active fungal material should be removed together with all rotten wood, and the structural strength of the remaining timber and fabric construction should be assessed to determine whether reinforcement or renewal is required. In the case of insect infestation, measures should also be introduced to avoid recontamination. Dirt, dust and builders’ rubbish provide a haven for insects and fungi. Voids and cavities should be cleared and the areas cleaned with a vacuum cleaner to remove dust. A programme of building maintenance and monitoring may then be instigated to prevent any future problems.

MONITORING SYSTEMS

Remote monitoring systems can be very useful in increasing the efficiency and decreasing the cost of maintenance programmes. They can be especially useful for checking the moisture content of inaccessible timbers in roof spaces, behind decorative finishes and in walls.

Sensors can be placed at all critical points after the investigation or after remedial building works. Areas can then be closed up and finishes reapplied; for example sensors may be placed in lintels, joist ends, valley gutter soles or in damp walls to monitor drying. It is important to use enough sensors and to place them with an understanding of the moisture distribution processes, because conditions can vary even in a small area. It is these local variations in conditions that produce the environmental niches which decay organisms exploit.

If more than 30 sensors are deployed, taking the readings can become onerous and this may result in human error or negligence. In these situations automatic monitoring systems become desirable and a number of specialised systems have been developed. With larger systems the wiring of sensors can also become a problem. For systems requiring 100 or more sensors, a computerised unit is used, working via a single four-core mains cable connecting up any number of nodes, each supporting four sensors. This system can be programmed to record and log data at regular intervals with alarm limits for each sensor. The data is then transmitted to a remote computer via a modem connected to a telephone line. Data from the system can then be analysed using CAD and programs for statistical interpretation.

TIMBER MOISTURE MONITORING SENSORS

The water content of building materials can be determined through a range of direct and indirect methods. Direct methods involve removal of a sample of the material to be tested which is weighed and then dried to determine its water content. This has the disadvantage of being destructive and it cannot be used for remote monitoring.

Indirect methods are based on measurement of characteristics related to the moisture level in the testing material. These involve thermal conductance, electrical capacitance and resistivity. Measurement of a surrogate material in equilibrium with the first material is another method. The use of electrical resistance moisture meters provides a quick and relatively accurate method of determining the moisture content of wood if a knowledge of their limitations is taken into account. Moisture meters measure the changes in resistance, due to changes in moisture content, between two electrodes placed in the timber. Increasing moisture content results in a reduction in electrical resistance.

Miniature sensors are fabricated from hygroscopic material which has been calibrated to match the moisture content changes in timber. They are encased in a protective shell. The sensor is then inserted into a previously drilled hole to the required depth and the hole sealed. In most instances the sensor cable seals the hole to the outside. The sensor will fairly rapidly come into equilibrium with the atmosphere within the hole. Due to their small size the sensors can be inserted into the centre or ends of large dimension timbers allowing the best chance of detecting defects early.

MASONRY MOISTURE MONITORING SYSTEMS

Systems for use with masonry can be based on the direct measurement of the material’s moisture content or by the use of a surrogate material which changes in moisture content in a similar way to the host material. This material may be of any hygroscopic type which, providing it has been calibrated correctly, can be used as the basis of a remote sensing system.

The sensors are placed in the material to be tested at the required depth, or in an array and the hole sealed to the external atmosphere. Sensors will come to equilibrium with the relative humidity within the cavity or drilled hole and hence with the surrounding material. Single sensors can be placed at varying depths but must be sealed within the area to be measured. A series of sensors individually sealed within the drilled hole can provide a profile of readings across the material. These can either be wired up and resistance measured remotely or can be removed, weighed and oven dried to calculate their water contents. Changes in the water content of masonry can be rapid when wetted so that it could provide an early warning of building defects leading to water penetration. However, drying down can take many weeks or years.

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 Damp Timber Article 4.

Condensation – Tim Hutton

This article is reproduced from The Building Conservation Directory, 2004

Mould growth due to damp and condensation
Mould growth can occur in carpet and underlay in both bathrooms and showers with inadequate ventilation. This results in moisture-laden air pulsing into adjacent areas and provides the conditions for condensation, mould growth and damage to finishes.

Despite the best efforts of the ‘damp-proofing industry’ and the proliferation of ‘waterproof’ products in the second-half of the 20th century, it is not possible to keep water out of buildings. Our grandfathers knew this and relied instead on drainage details and breathable materials, so as to allow any water entering the structure to dissipate. Failure to maintain these systems or the inappropriate introduction of ‘waterproof’ materials or ‘damp-proofing systems’ will result in the build-up of moisture and damp problems in both historic and new buildings. This is well illustrated by considering the phenomena of condensation in buildings.

Strictly speaking, ‘condensation’ describes the physical process by which substances change from a gas or a vapour to a liquid phase, usually as a result of a drop in temperature. However, the term is commonly used to describe the process when moisture in the air condenses out to form liquid water as fine droplets in the air, or on a relatively colder material. Common examples of the former in the natural environment are the formation of clouds when warmer moisture-laden air mixes with colder air above, and fog, where this occurs at ground level. Similarly mist forms when warm moisture laden air is cooled by heat loss over night. Examples of the latter include the misting up of car windows when the warm moisture-laden air within cools on the surface of the window screen, and the misting on the surface of a mirror when held in the moist air exhaled from the mouth. This occurs because reducing the temperature of the gases that make up air reduces the energy available to keep the molecules whizzing around randomly within the available space, and lets a proportion of the molecules settle down into a less mobile liquid phase, in which the motion is more limited. Conversely, molecules in the liquid phase may pick-up enough energy to leave the liquid and ‘evaporate’ off to join the other gas molecules randomly moving around the available space once more. In fact, at any time molecules will be ‘condensing’ and ‘evaporating’ from any liquid water. The more active and energetic the molecules are, the greater ‘pressure’ they exert. This is described as ‘partial vapour pressure’. If the energy and hence the partial vapour pressure of the molecule in the liquid is higher than those in the air, then there will be a net movement of water into the air resulting from net evaporation or drying. Conversely, if the temperature and hence partial vapour pressure of the water molecules in the air is higher than that in the liquid or other adjacent material, there will be net condensation.

RELATIVE HUMIDITY AND DEW POINT IN THEORY…

Bathroom extractor fans ducting into roof will cause condensation and timber decay
An exhaust duct from an extractor fan in a shower unit installed in an 18th century country house, is shown discharging into the roof void and providing the conditions for condensation, mould growth and insect decay in roof timbers. It also shows an uninsulated cold water tank which resulted in cold bridge condensation of moisture laden air.

At any given temperature and pressure there is a limit to the amount of water molecules that a given volume of air can hold. From the above it will be appreciated that this will rise and fall with the temperature. When a given volume of air contains the maximum amount of water possible at any given temperature it is described as ‘saturated’, and the moisture content of air at any given temperature is often described as a percentage of the maximum amount of water it could hold if saturated at that temperature. This is the ‘relative humidity’ (RH) percentage. Conversely, it will be appreciated for any given amount of moisture in a given volume of air, there will be a temperature at which the air would be ‘saturated’ and that any further drop in temperature could result in net condensation. This is called the ‘dew point’ temperature. Although the relative humidity of air is often measured and discussed, it will be appreciated from the above that it is not really a useful figure unless the temperature is also considered. Because the factors affecting condensation are so complex, specialists concerned with moisture movement will refer to tables or psychrometric charts’ to determine the relationship between the moisture content of air, the temperature, the partial vapour pressure, the dew point and the specific ‘enthalpy’ – the latter may be considered as representing the energy available within the system.

… AND IN PRACTICE

This can all seem very confusing even for specialists with a scientific background. However, when considering moisture movement and condensation in buildings, there are a few simple rules of thumb that are adequate for most practical purposes. Firstly, moisture can generally be thought of as moving from relatively wet to relatively dry areas or structures, and from relatively warm to relatively cold areas in buildings. Secondly, it is generally only necessary to determine and consider the dew point of the air, and the probable temperature of the fabric, in order to identify where condensation may occur. It should also be remembered that this is a dynamic process with continuous fluctuations in the temperatures and moisture content of air as a result of annual and diurnal changes, as well as the result of local heating and ventilation. Because of these factors it would be necessary to measure the temperature and moisture content of air over time and in a large number of representative locations to determine if net condensation was occurring. This is why the common practice of referring to individual RH percentage readings is often confusing and counter-productive; and why in most cases it is better to focus on possible moisture sources, and to look for evidence of moisture accumulating in vulnerable materials or on vulnerable surfaces, when investigating possible condensation-related problems.

Mould growth indicative of condensation

This mould growth on contaminated wallpaper represents a potential environmental health hazard to those occupying the building.

Damp perished wall plaster due to condensation

At the Monument in the City of London we see corrosion of railings and spalling of the staircase and spiral stair, due to water draining down the inside of the tower and staircase as a result of warm front condensation.

Timber decay to window frame cause by condensation

Decay to window frame is evident from condensation on the glazed surface, resulting in water penetration to timber elements which are prevented from drying due to the application of relatively impermeable gloss paint finishes. This had resulted in wet rot decay to the timbers. Original breathable paint finishes had previously allowed drying, preventing decay in the past.

CONDENSATION IN BUILDINGS

The air in occupied buildings will always contain moisture. This is because we are all mostly made up of water, and add water to the environment at every breath. Occupancy will also introduce water into the built environment with activities such as bathing, washing and cooking. In modern and refurbished buildings the installation of shower units, Jacuzzis, swimming pools and saunas in particular can add significant quantities of water to the internal air. Moisture will also enter the air within structures due to the evaporation of water penetrating from the exterior. This occurs mostly from ground and surface drainage via the foundations, and through walls and roofs due to defective roof drainage. Moisture-laden air may also enter the structure from the exterior when it is warm and wet outside relative to the interior environment. Any sources of moisture into the internal environment may result in moisture-laden air being cooled to below its dew point at relatively cool surfaces or within relatively cool materials within the building structures; resulting in net condensation and the accumulation of liquid water causing localised damp conditions. This localised accumulation of moisture as a result of condensation can result in a number of damp-related problems in buildings, including the decay or damage of building materials or contents, and affecting the health and comfort of occupants.

Cold-bridge condensation occurs when relatively warm moisture laden air comes into contact with surfaces, at or below its dew point, which are relatively cold as a result of locally reduced insulation values between the warm air and a relatively cold area. Typical examples of this process are condensation at the base of external walls, where it may be confused with rising damp, condensation on window panes where it often results in accelerated decay to the lower parts of window frames, and condensation to the undersides of roof surfaces. The latter may result in accelerated corrosion of lead roof surfaces. Liquid water penetration into structures will usually degrade their insulating properties and may therefore form a ‘cold bridge‘, resulting in further condensation. Because of this it is not unusual to find water penetration at the base of walls or through roofs also causing local condensation. Cold bridge condensation can also occur on relatively cold internal structures, such as inadequately insulated cold water tanks or refrigeration units.

Warm front condensation occurs when relatively warm moisture-laden air from the exterior enters into a relatively cold building, following a change in weather from cold to warm. This usually occurs in the UK with a ‘warm front’ arriving from the Atlantic from November through to February, and can result in water running down the interior walls of massive masonry structures under reduced occupancy, especially in the towers of churches or castles, and in subterranean structures.

Interstitial condensation occurs when relatively warm moisture-laden air diffuses into a vapour-permeable material or structure such as fibrous insulation or a porous brick wall. If it is relatively warm on one side and below the dew point temperature on the other; this can result in the moisture-laden air reaching ‘dew point’ within the material and depositing liquid water at this point. This becomes a particular problem if the diffusion of the moisture vapour through the material is restricted towards the cold side of the structure and if the insulation or thermal conductivity of the structure is such that the temperature profile is skewed towards the relatively warm side. The risk of condensation in these circumstances can be calculated using graphs and formulae, or using specialist computer programmes, and it can become a particular problem in heavily insulated or air-conditioned buildings. This is especially important when dealing with the conservation of buildings in extreme environments such as the conservation of buildings in the tropics, which tend to be air conditioned on refurbishment. In this situation, interstitial condensation can be a significant problem; and it is necessary to turn the usual calculations back to front, as conditions will be warm and wet on the outside and cold and dry on the inside of the structure. Similarly, extreme conditions can occur in very cold environments, and when refrigeration units are introduced, without adequate ‘vapour checks’ or insulation.

CONTROL OF CONDENSATION

Historically these problems have been controlled by ensuring that moisture laden air can exit to the exterior, and by controlling the effect of fluctuating air temperature by a continuous low level of structural heating. For example, historically buildings had relatively gappy structures and through ventilation was ensured, particularly in cellars and roof voids. Buildings were also ventilated by the passive stack effect via chimneys and staircases. Historically materials used in buildings such as thatch, lime plaster and traditional paints, were microporous or permeable allowing the movement of moisture vapour and drying. Low level structural and radiant heating was also provided in the past by the use of fires or stoves situated in massive chimney breasts and walls, or lately by the installation of massive hot water low level central heating systems, or in classical times by the hypocaust. More recently in new buildings reliance has been placed on insulation and ‘vapour barriers’. Unfortunately, these are generally imperfect, especially when retrofitted to existing structures, resulting in localised cold bridging or interstitial condensation. From the above it will be noted that the key factors in controlling condensation are ventilation, heating and insulation. These are the factors that usually require modification when dealing with an apparent problem with condensation in an existing building, often because they have been compromised by a previous refurbishment or change in occupancy.

DEFECTS CAN CAUSE CONDENSATION

Defects introduced in the refurbishment of older buildings or during the construction of new extensions may cause damp problems as a result of condensation. Some common examples are listed below:

  • the sealing of gaps around windows without provision of appropriate supplementary ‘trickle’ ventilation
  • the introduction of showers, jacuzzis, saunas or swimming pools with insufficient provision of extractor fans or passive stack ventilation
  • the installation of laundry units without proper installation of exhaust vents to the exterior
  • the installation of broken or crushed ducts from extractor fans in showers or bathrooms
  • the installation of extractor fan ducts exhausting into building voids such as roof spaces, rather than to the exterior
  • the failure to provide adequate ‘makeup’ air or trickle ventilation into areas fitted with extractor fans, to allow proper through ventilation
  • the blocking of existing flues and chimneys preventing passive stack ventilation
  • the blocking of existing vents or plenums designed to vent air to the exterior, in particular through the ceilings and roofs over function rooms, or at the skylights over staircases
  • the installation of intermittent heating, especially hot air heating systems, allowing warm moist air to ‘pulse’ into unheated areas under reduced occupancy – for example, in churches in reduced or intermittent use
  • the inadequate provision of low level structural heating to massive structures under reduced occupancy, such as churches or castles, allowing cold front condensation.
  • the provision of inadequate through ventilation to rooms under reduced occupancy
  • the introduction of security locked windows with no provision for locking in a partially opened position
  • the sealing of roof voids by the installation of insulation or sarking felts, preventing adequate through-ventilation
  • the sealing of floor voids by the blocking of airbricks, and the installation of fitted carpets or other impermeable floor coverings
  • the blocking of windows, hatches or other vents to cellar or basement areas, preventing adequate through-ventilation
  • the introduction of defective insulation and ‘vapour barriers’ or ‘vapour checks’; especially in extremely hot or cold environments, or around cold structures within buildings, such as cold water tanks or refrigeration systems.

REMEDIAL MEASURES

Sensor probe

This sensor probe is designed to monitor temperature and moisture profiles through a wall to identify and resolve problems of interstitial condensation as part of an H+R Curator building monitoring system.

 

From the above it can be seen that in most cases the appropriate remedial measures to control condensation are often directly related to correcting defects previously introduced. This should be done as far as possible by putting the building back to the way it was originally designed, with the use of original materials and detailing. This is not always possible given the changes in use and modern styles of occupancy. However, in nearly all cases problems can be reduced by reference to the recommendations for new buildings found in Building Regulations and the associated British Standards. These are especially useful in specifying remedial works and in providing ‘comfort’ to organisations such as building guarantors, mortgage lenders and official bodies such as Building Control. In these circumstances it is often cost effective to seek advise from an independent specialist.

MOULD AND EFFLORESCENCE

As the temperature of moisture-laden air approaches dew point and the relative humidity rises, a number of moisture associated problems can become apparent, even before condensation occurs. In particular, superficial and interstitial mould growth can occur, especially on surfaces or in materials contaminated by dust or other organic materials. This typically occurs in poorly ventilated areas such as behind furniture and pictures, behind the glazing of pictures, in soft furnishings or carpets, and in poorly ventilated cupboards or corners of rooms, both at ceiling and floor level. This can cause serious damage to decorative and historically important finishes, as well as representing a significant health hazard, especially to sensitive individuals. Interstitial mould growth in contaminated carpets, soft furnishings or insulation materials due to this raised moisture content and poor ventilation is a particular health hazard, and appropriate respiratory protection should be used in affected areas. Hygroscopic salts can also cause significant damage as moisture from the air is absorbed and evaporated from affected plaster, masonry or brickwork, with fluctuating temperature and air moisture contents above dew point. This can cause damage to decorative stonework and plaster finishes, even at relative humidities fluctuating around 75 per cent or less. Although these problems are not caused by condensation, in the way the term is usually used; they can be understood and managed using methodologies similar to those discussed above.

DEHUMIDIFIERS

In recent years it has become more common to try and control condensation in buildings using dehumidifiers. These may be useful in unoccupied buildings where environmental control can be achieved, such as in storerooms or museums, where it is the contents rather than the structure that is thought to be at risk. However, the use of dehumidifiers in occupied historic buildings is rarely cost-effective. This is because of the difficulty of effectively controlling air movement, and the high management and maintenance input required to ensure the efficient operation of the dehumidifiers themselves. It is common to find dehumidifiers installed in such circumstances vainly trying to dehumidify the entire external environment of the United Kingdom, or merrily extracting water from the air which is then allowed to recycle into the environment that is being attempted to be controlled. Efficient and cost effective use of dehumidifiers requires a high level of technical input, long-term monitoring and control. This usually requires independent specialist specification and supervision. Where possible it is therefore generally better to rely on the ‘fail safe’ and buffered systems inherent in the natural ventilation and structural heating of historic buildings.

In conclusion, historic buildings with their original materials and detailing should not generally suffer from problems resulting from condensation. Where there have been changes in occupancy, and where new materials or new extensions have been added, problems resulting from condensation can usually be easily and cost-effectively controlled by an understanding of the control mechanisms inherent in the original structure. Remedial measures are then generally associated with passive ventilation and structural heating systems. Where new materials or structures are introduced or problems persist, the information contained in Building Regulations and British Standards can be used to solve most problems. In these cases an holistic investigation of the building structure, materials and occupancy should be carried out, and short- or long-term monitoring may be required to ensure that the most cost-effective remedial measures are undertaken. This will ensure the conservation of the maximum amount of original materials and detailing.

Damp survey, timber survey for house & flat purchase, combined damp and timber survey, specialist timber and damp reports for mortgage providers.  Independent advice from one of our specialist damp proofing and timber decay surveyors.Damp-proofing quotation, woodworm treatment, & rising damp surveys cost are provided by Damp Aid UK in the following North West, Midlands and South East regions / areas
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