Airtightness – Part 3

Airtightness Part 3: Reducing condensation risk within the building fabric

Improving energy efficiency and considering indoor air quality

Image 1: Trapped moisture within an insulated pitched roof and mould growth on the inside of the external lining due to interstitial condensation

In the previous two airtightness articles I provided an overview of the basic principles of the subject. As building regulations demand increased levels of energy efficiency it is important to ensure that health, Indoor Air Quality (IAQ) and building durability is not compromised, be it in a new building or particularly in retrofitted existing building. Increasing thermal performance will naturally require much high levels of build quality which in turn will lead to a significant reduction in gaps and cracks in the external building envelope. In this way, as buildings become more airtight, it is essential to ensure adequate ventilation is provided to meet the requirements of the building inhabitants. This may be attained by mechanical or natural ventilation systems. One of the key indicators of an uncomfortable, potentially unhealthy living atmosphere is the relative humidity within the living space.

It is widely known that airtightness is essential for a thermal insulation layer to perform to its optimum efficiency. Airtight buildings result in a comfortable indoor environment and help to offset the risk off structural damage due to moisture penetration   into constructions from the interior, resulting in interstitial condensation.

Moist convection currents (i.e. air leakage), can result in large amounts of moisture penetrating the thermal insulation layer within a short period of time, thus   potentially effecting the structural integrity of timber or steel structural layers within buildings as well as degrading   the thermal insulation’s effectiveness over time (as shown in image 1). Air leakage into building elements and the resulting moisture penetration frequently results in mould growth and impairs the integrity of the structure. The question that arises for new and particular existing buildings is, by what means can we improve an initially poor airtightness layer when performing energy-saving renovations, while at the same time, increase the thickness of the insulation layer without introducing interstitial condensation risk?

Indoor Air Quality (IAQ)
It is generally recommended that in order to attain optimum levels of comfort and health within the living space internal humidity should be maintained at between 40% and 60%. The potential for mould growth, dust mites and other factors which contribute to an unhealthy living atmosphere is dramatically reduced, once the humidity is maintained within these parameters. The most common pollutants which we are exposed to within buildings are:

•           Radon

•           Moulds and other allergens

•           Carbon monoxide

•           Volatile organic compounds

•           Legionella

•           Asbestos fibres

•           Carbon dioxide

•           Ozone

We now spend up to 90% of our time living within buildings. Therefore it is essential to appreciate the quality of the air we breathe every day.

The relationship between humidity levels and the potential for each of these pollutants to increase is summarised on figure one below.

Figure 1: Maintaining the internal humidity at between 40 and 60% contributes to a healthy living atmosphere

An effective ventilation strategy by natural or mechanical means can assist in ensuring the relative humidity is maintained at a healthy level. This will also reduce the risk of mould formation on the surface of internal walls or on cool areas within a home. It should be noted that if the humidity is maintained above 80% for a prolonged period of time this can lead to mould growth.

This is why one may often find mould growth in bathrooms, behind wardrobes in bedrooms, particularly on cool north facing walls or at the junction between a window and a wall, primarily in areas where a lot of moisture is been generated (e.g. a kitchen or bathroom). An example of this is shown on the images below.


Image 2: Mould growth at a window to wall junction in a child’s bedroom and in a kitchen

Mould is a naturally occurring fungus, something which we are exposed to and whose spores we breathe in everyday. Therefore moulds are ubiquitous, and mold spores are a common component of household and workplace dust and in the general environment.

There are many species of mould and most mould types are generally harmless provided they are maintained at a low level. However, it should be avoided and removed if it occurs and steps should be taken to ensure it does not return.

Prolonged exposure to high levels of mould has been shown to contribute to respiratory disorders, skin and other allergies.   Some molds also produce mycotoxins which can pose serious health risks to humans and animals. Some studies claim that exposure to high levels of mycotoxins can lead to neurological problems and in some cases death. Prolonged exposure, e.g. daily workplace exposure, may be particularly harmful. Therefore, it is essential to take steps to limit mould growth and to remove it where it occurs as quickly as possible.

The link between air leakage and interstitial condensation

While condensation occurring on internal surface linings within buildings may lead to mould growth, surface damage and deterioration, at least this is visible and can be acted upon relatively quickly. When vapour penetrates the building envelope this may lead to significant issues over a longer period of time, which may only materialise at the surface when considerable damage has occurred within the construction.
It is often the case that the airtightness layer in a building also acts as vapour control layer. A vapour control layer (e.g. INTELLO PLUS) is applied on the warm side (internal side) of an insulation layer, and this is generally applied in timber frame buildings or timber parts of block construction, (e.g. to separate the cold attic space from the warm living space below), or where solid masonry walls are insulated internally. The vapour control layer retards the passage of warm heated vapour held within the air in the living space, as it naturally migrates to the cold side of the building envelope. This in turn reduces the risk of condensation within the building envelope as it cools, referred to as interstitial condensation.   Good practice details and guidance in relation to the application of a airtightness and vapour control layer are referred to in the previous airtightness articles.

Air leakage and moisture management

In lightweight construction, particularly timber elements of masonry and timberframe constructions, the airtightness layer (i.e. the vapour barrier/ check) typically has a dual function. A vapour barrier acts a vapour control layer which retards the transfer of the warm, potentially moisture laden air from the habitable space, penetrating the external fabric and coming into contact with cooler elements within the construction.

With increased insulation standards in modern buildings the average internal temperature has risen over the last 10 years by 3.50C. This temperature rise means that air in the living space can maintain more moisture than ever before. In this way the vapour control layer has a critical role. A leak in the VCL therefore becomes important, not only from an energy perspective, but also from a moisture management perspective.

Figure 2: even a 1mm tear in a VCL can significantly increase the risk of condensation within buildings

A study by the renowned Fraunhofer Institute of Building Physics in Germany measured moisture penetration into the structure of a building as described above. The VCL had a diffusion resistance Sd of 30-m (Vapour Resistance of 150 MNs/g).The measurement confirmed that only   0.5 g of moisture per m2, per day penetrated into the structure (See figure 2) when the membrane was perfectly sealed.

This volume of moisture poses no problems for buildings. The moisture penetration through a leak was determined in the second test. The results were alarming and explained many cases of structural and insulation damage in buildings. The moisture penetration by convection (air flow) was 800 g of moisture per m2, per day with the smallest slit of only 1-mm.

Therefore, the airtightness of the VCL has a critical role to play not only from a thermal perspective, but also from a moisture management perspective, to offset the risk of condensation within walls and roofs in buildings.

Condensation resulting from air leakage in cold attics

A  perfect example of an area where condensation often arises within buildings is in cold attics. This may have been noticed over recent months as homeowners placed their Christmas decorations back in the attic! As the attic is a cold space it is essential to minimise the amount of warm heated air which may penetrate into this void from the heated space below. Otherwise, considerable amounts of condensation will occur.

Traditionally, an effective strategy to offset this risk, is to introduce cross ventilation in the attic at eaves and ridge level, and to also apply a modern “breathable” roofing underlay on the roof as oppose to the traditional vapour tight bitumen roofing felts. While these strategies can assist in offsetting the risk of condensation in attics, addressing the source of moisture entry in the first place (i.e. from the living space), and limiting it, is a far more effective strategy combined with applying a diffusion open roofing underlay on the roof.

This may be achieved by applying a continuously sealed airtight vapour control layer below the insulation. Even when breathable roofing underlays are applied on roofs, it has been shown that condensation can still occur if the insulation is not adequately airtightly sealed on the warm side with a suitable vapour control layer (VCL).   Figure 3 below illustrates the penetration of warm air from the living space through leaks and tears in the VCL into the construction.

Figure 3: Air leakage/Convection into an insulation in a roof leading to interstitial condensation

Such a scenario is illustrated in the images below where condensate has occurred behind a breathable roofing underlay due to excessive moisture vapour penetration into the attic space. If the moisture penetration is too high, this may lead to a film of water eventually building up behind the breathable membrane, which in turn blocks the pores of the membrane, leading to a reduction in the ability for vapour to migrate through the material and the inevitable condensation follows.


Image 3: High levels of condensation in an attic with a breathable external roofing underlay due to poor airtightness detailing in the vapour control layer

Figure 4 illustrates two cold attics where the vapour control layer has been airtightly sealed and a high performance breathable roofing underlay has been applied externally.


Figure 4: A vapour control layer continuously sealed on the warm side of the insulation and a high performance breathable underlay on the outside of the roof.


The objective of building is not only to build energy-efficient buildings with a comfortable living environment, but in particular to construct buildings with a healthy living environment which is durable and requires minimal maintenance. While one should consider the emissions from chemicals (e.g. such a cleaning agents), or furnishings brought into a building, first and foremost, the prevention of mould both on and in the structure should be a priority.

Mould spores damage the immune system and promote/ cause allergies and the microbial volatile organic compound (MVOCs) produced by mould can cause physical and psychological health problems. If mould is in a dry environment it presents a much lower risk to health. If the mould becomes damp again, however, the hazards are just as great as they were before.

If mould grows on an inner surface inside a building (e.g. due to thermal bridges or surface condensation) it is visible and can be removed as required. If mould grows within a structure, it can go unnoticed and be reactivated by increased humidity within constructions or condensation on an annual basis – posing a permanent health hazard to the occupants. The objective when building should be to achieve the highest possible levels of safety, and health for the occupants and of course, to minimise heat loss while at the same time maintain high levels of comfort and building durability. Such a strategy lends itself to a sustainable building in every sense of the word.

Combining high levels of airtightness with an efficient ventilation strategy are key aspects to attaining this objective.


Niall Crosson Technical Engineer BTech, MEngSc, MIEI

Pictures and graphics provided courtesy of Ecological Building Systems