Airtightness
Airtightness is an essential part of creating a healthy, comfortable, energy-efficient living environment.
Airtightness Part 1
Airtightness is an essential part of creating a healthy, comfortable, energy-efficient living environment. Air leakage is one of the most significant contributors to inefficiently heated buildings. Research confirms that air leakage can account for up to half of all heat losses in modern buildings and reduce insulation performance by as much as 480%. Considering Ireland is one of the dampest and windiest climates in Europe, it’s no surprise that airtightness is now seen as a fundamental element in the construction of low energy, healthy buildings.
Decreasing the air leakage rate in buildings is one of the most cost effective means of constructing low energy, sustainable buildings. It is essential of course that a very airtight building must have an effective ventilation system; be it by mechanical or natural means.
What is an airtight dwelling?
Air leakage is the uncontrolled flow of air through gaps and cracks in the fabric of a building (sometimes referred to as infiltration or draughts). This is not to be confused with ventilation, which is the controlled flow of air into and out of the building through a purpose built ventilation system that is required for the comfort and safety of the occupants.
The term airtightness refers to the minimisation of these gaps and cracks. Of course one can open and close windows or doors in an airtight house as they choose. The difference with an airtight house is that when the windows are closed, the curtains stop moving in front of it!
In the past the majority of heat loss from our buildings could be attributed to little or no insulation and a very low level of airtightness. As insulation levels have increased over the decades, the proportion of heat loss in buildings due to air leakage has increased dramatically. Whereas ventilation is intended, air leakage or draughts are not. The key is to “Build Tight and Ventilate Right”.
One of the most cost effective means of reducing heating bills, increasing the energy efficiency in buildings and ensuring that the insulation can perform effectively is to use quality insulation while making sure the building envelope is as airtight as possible. In this way addressing air leakage can be pinpointed as one of the most effective and cheapest means of increasing the energy performance of our buildings.
An insulation layer is only as effective as it is sealed. If the insulation is continuously exposed to air movement, this will lead to a significant reduction in performance. For example, if one were to wear a woolly jumper on a cold windy day it will not insulate effectively, whereas if one were to wear a light windshield over the jumper, then it actually insulates effectively. Insulation in our homes is very similar. Figure 1 highlights common areas in buildings where air leakage occurs (red arrows), and where intended openings may be placed (green arrows).
Figure 1: An leaky inefficient building and an airtight building (Red arrows indicate air leakage and green arrows intended openings)
Airtightness
Air leakage can occur due to a combination of poor building design, poor workmanship and inappropriate materials been used on site. It can account for up to 50% of all the heat losses through the external envelope of a building. An airtight dwelling does not mean it is hermetically sealed, rather that it has reduced air leakage to an absolute minimum and of course, has an effective means of ventilation. Ireland is not a country exposed to extremes in low temperatures; however we are exposed to extremes in wind pressure.
Airtightness is a precursor to maximising thermal performance both in winter and summer, optimising acoustic performance, improving comfort levels and reducing the risks associated with condensation within building elements, particularly in timber based constructions.
How do I measure airtightness?
The only way to measure the level of airtightness a building has achieved is by conducting a blower door test. A blower door test involves installing a large fan within a temporary door frame in an external door opening and inducing a pressure differential. The building may be depressurised or pressurised.
There are various units used to measure airtightness. The Irish building regulations dictate that buildings should be tested to EN Standard 13829:2001, to measure the leakage rate of the building envelope.
An airtightness measurement usually involves a combination of depressurising and/or pressuring a building to a pressure difference of 50 Pascal’s (50Pa). A pressure difference of 50 Pa can be compared with the equivalent pressure induced by a wind speed of approximately 22mph on a building.
Once the pressure differential reaches 50 Pa, leaks can be readily identified in the external envelope and an accurate measurement of the air leakage rate or permeability of the external envelope of the building can be calculated. The air permeability of a building at a pressure differential of 50Pa is referred to as the Q50 of a building and is measured in m3/hr (of airflow) per m2 (of total external envelope area).
The standard upper limit level of airtightness in buildings in Ireland and the UK at the moment, is to achieve a Q 50 of less than 10m3/hr/m2 for all new buildings. This literally means that when we exclude ventilation and design openings, and exert a pressure difference of 50Pa on the external envelope of the building, 10m3 of air per hour passes through every square meter of the external envelope…by no means energy efficient and certainly not airtight!
According to the Airtightness Testing and Measurement Association (ATTMA), for airtightness best practice, buildings which are mechanically or naturally ventilated should achieve an air permeability of less than 3m3/hr/m2 should be achieved. This is quite a high level of airtightness. However the most stringent requirement for airtightness can be found in Passiv Haus which requires a Q50 of less than 0.6m3/hr/m2. This indicates how leaky a building which records an air permeability of 10m3/hr/m2 is!
The cost for airtightness testing can vary depending on a number of factors such as:
- the size of the building,
- its location relative to the tester,
- and the number of buildings to be tested. If there are a number of buildings on one site the tester may provide a discounted price.
The costs for testing residential buildings can vary from about €500 to €1000. The cost for commercial buildings can be significantly higher.
Figure 2: A pro clima Wincon airtightness quality control unit and a blower door fan
As the blower door test is carried out when the building is complete, it is generally very difficult and much more costly to seal up leaks at this stage. Ideally, an intermediate airtightness quality assessment test should be conducted prior to the internal lining been applied. This can be conducted by using a blower door or pro clima Wincon airtightness quality control unit (see figure 2). At this stage areas where air leakage is occurring can be easily identified and sealed cost effectively.
How do I locate airleakage?
Air leakage points can be highlighted by using a fog emitting device, thermal imaging or, when depressurising the building, by simply using you hand in which case you may feel the air leak. When the climatic conditions are suitable (when the temperature difference between inside and outside is sufficient), thermal imaging may be used as means of highlighting air leakage.
The illustrations below demonstrate some thermal images which highlight air leakage paths. Blue coolers in the images highlight the cooling in the areas of airleakage and warmer areas area coloured yellow/red. A scale at the bottom left of each image provides an indication of the temperature at a specific location. The blue areas indicate where the majority of heat loss and potentially condensation may occur.
Figure 3: Air leakage at gable wall in dormer roof
Figure 4: A dormer roof before blower door test. Notice that the insulation layer highlighted in orange/yellow 
Figure 5: A dormer roof while a blower door test is running. Notice the effect of air leakage by the increase in blue areas of cooling. The insulation layers’ efficiency has been dramatically reduced.
What means of ventilation are there available?
An essential component in an airtight building is to ensure an effective controlled ventilation system is employed. There are various ventilation systems ranging from natural, to mechanical, a mixture of mechanical and natural or even mechanical ventilation with heat recovery. Whether buildings are naturally or mechanically ventilated a high level of airtightness ensures these systems perform to their highest efficiency.
While a natural ventilation system ensures sufficient fresh air is supplied to the living space, as the stale heated indoor air is replaced by fresh possibly cooler external air, the heat from the outgoing indoor air is lost. This is referred to as a "ventilation loss". A mechanical ventilation system with heat recovery not only exchanges the stale indoor air with fresh outdoor air, but it also recovers the heat from the outgoing stale air and exchanges this into the cool fresh incoming air. Hence the "ventilation losses" can be dramatically reduced. If a building is very "leaky", then cool external air will leak into the building, which will then dramatically reduce the efficiency of the heat exchange unit. In this way ventilation and airtightness should be considered collectively.
How do I achieve high levels of airtightness?
There are three key steps to maximising the airtightness of a building:
1. Design for Airtightness
The Architect designs the building bearing in mind key airtightness details.
2. Building for Airtightness
Both the installer and all personnel on site who interact with the airtightness layer must install the airtightness materials correctly. Coordination of work on site and communication between professions is essential. Examples of the airtightness layer may be a vapour check/barrier on the warm side of the insulation or the plaster onto the inside of a block work and all connections to structural components and service penetrations.
3. Test for Airtightness
The only way to confirm we have achieved the level of airtightness specified by the architect is to carry out a blower door test to measure airtightness.
Achieving high levels of airtightness is very much down to good building practice, and using the proper materials to achieve a durable airtight seal. Some of these details are highlighted in the selection of images below.
Figure 6: A pro clima service grummet sealing a pipe penetration of the airtightness layer
Figure 7: A "service batten" on the inside of the airtightness layer which ensures penetrations of the membrane are not damaged improves airtightness durability
Figure 8: Airtightness sealing of collar ties in roof with pro clima TESCON PROFIL corner jointing sealing tape
Figure 9: Overlaps of the Airtightness membrane (INTELLO PLUS), sealed with a suitable overlapping sealing tape (TESCON NO 1)
The next part of this feature will centre on detailing for airtightness and construction durability, and considering building health. With airtightness the devil is without doubt in the detail!
Niall Crosson
Technical Engineer BTech, MEng Sc, MIEI
Pictures and graphics provided courtesy of Ecological Building Systems
Airtightness Part 2
In the previous article I outlined the basic principles and advantages of Airtightness. These were primarily:
- Avoiding drafts and discomfort
- Avoidance of moisture related building damage
- Avoiding high heat losses due to air leakage
- Introducing a controlled ventilation system
- Improved acoustic performance
- Improved indoor air quality
While it is one thing writing about the principles of Airtightness, it is another thing achieving high levels of Airtightness practically, on site. The following are the primary steps to achieving higher levels of Airtightness in construction:
1. Forethought, planning of works schedule and clear simple design principles;
2. Quality control on site and good workmanship with the use of appropriate materials to achieve an airtight seal;
3. A final measurement which confirms we have achieved the required level of Airtightness which we were aiming for in the first place.
The glue that cements everything together is communication, coordination of works on site and a high level of consciousness and awareness regarding the importance of Airtightness from the design stage to the build process. Figure 1 summarises these key steps.
Figure 1: Design, Build and Test for Airtightness -the 3 key steps to achieving Airtightness on site
Designing for Airtightness:
To maximise the Airtightness of any building the Designer/Architect plays a pivotal role. The architect should select the level of Airtightness to be achieved from the outset. Table 1 taken from the Airtightness Testing and Measurement Association (ATTMA) in the UK, provides guidance regarding normal and best practice for Airtightness in various building types:
| Type | Air Permeability | |
| m3/(h*m2) @ 50Pa | ||
| Best Practice | Normal | |
| Offices | ||
| Naturally ventilated | 3 | 7 |
| Mixed Mode | 2.5 | 5 |
| Air conditioned/low energy | 2 | 5 |
| Factories/warehouses | 2 | 6 |
| Superstores | 1 | 5 |
| Schools | 3 | 9 |
| Cold Stores | 0.2 | 0.35 |
| Dwellings | ||
| Naturally ventilated | 3 | 9 |
| Mechanically ventilated | 3 | 5 |
Table 1: Normal and best practice Airtightness criteria for different building types (ref: ATTMA Technical Standard 1; Issue 2, 2007)
Airtightness sealing details for common leakage areas which are difficult for the builder to seal on site, can be greatly simplified at the design stage, leading to vast improvements in workmanship and assuring an airtight seal is practically achieved. The key is to keep the Airtightness detailing as simple as possible. Superior levels of Airtightness can be achieved and of course significant savings can also be made as a result of time savings on-site, at the installation stage.
The major cost in relation to Airtightness is generally not down to the material used, but the cost for labour and time on site sealing unnecessarily complex details. Designing effective, buildable Airtightness details does not add significant costs in terms of design, but can make dramatic savings in time, labour and cost on site. The more uncomplicated the detail, the more chance of success on site, the cheaper the cost and crucially the better the Airtightness result!
On the other hand, where details are complicated by unnecessarily intricate details, this leads to a requirement for more sealing material and more labour time on site, increased costs and may lead to a poorer Airtightness result.
From the outset the architect should clearly define the Airtightness layer on the building design in both a plan and elevation view. It is particularly important to clearly outline what forms the air barrier at each location and especially how various interfaces between different sections and materials are made (see figure 2 below).
If one can trace a continuous line defining the Airtightness layer around the external envelope, without lifting the pen once from the drawing, then we have perfect Airtightness layer. In reality this is impossible of course as we have to lift our pen at connections to windows, separating floors, service penetrations and so on. It is at these details that the designer has to clearly specify a buildable Airtightness solution.
The Architect should specify materials and propriety airtight sealing products which simplify the airtight sealing process for the installer and guarantee a clean, durable and lifelong seal. The Airtightness achieved at the build stage should last for the lifetime of the building, not just the day of the test. The following details provide a sample of some simple yet very clear and effective Airtightness details.
Figure 2: Some examples of effective Airtightness details
2 (a) Overlaps of a vapour control layer sealed with an Airtightness tape at overlaps (shown in red)
2 (b) Service penetration sealed with flexible service grommet
2 (c) Window frame sealed reliably to vapour control layer
2 (d) Separating floor detail sealed continuously
Building to achieve Airtightness:
There is no doubt that good workmanship and a high level of awareness and consciousness on site within various trades is a central part of achieving high levels of Airtightness on site. From the outset an "Airtightness Champion" should be appointed on site. An "out of sight out of mind" philosophy will lead to poor quality control and inevitably a very poor Airtightness result at the end of the project.
Typically this person will be on site for the length of the project and interact with the various trades on site at all times. The Airtightness Champion should have direct contact with the designer and the airtight system supplier to clarify any outstanding details over the length of the project. When the building is at the construction stage communication between trades is absolutely essential.
It is crucial to schedule a site meeting with key site personnel (i.e. plasterer, window installer, electrician, plumbers, etc) who will interact with the Airtightness layer as early as possible. It is also essential that the airtight installer maintains a high level of consistency and consciousness to continually seal the Airtightness layer on site. Scheduling of key steps in the build process and determining how the various trades interact with the Airtightness layer throughout the build process should also be planed early. Airtightness courses and training are now available from Airtightness system suppliers, such as Ecological Building Systems, or from an independent national training authority, such as FAS.
Intermediate Airtightness assessments are absolutely essential to ensure air leakage paths are identified as early as possible. This can be conducted by using a pro clima WINCON Airtightness quality control unit, or a conventional blower door fan, both of which are described in my previous article.
On site, potential air leakage paths can be categorised into 3 main areas:
1. Structural leaks
2. Service leaks
3. A combination of structural and service leaks
1. Structural Leaks: These can occur where two structural elements in the building fabric connect to each other. An example of a structural leak can be a leak between a window frame and a wall or where overlaps in a vapour check are ineffectively sealed, attic hatches or even cracks in walls.
2. Service Leaks: These can occur where pipes or electrical cables penetrate the external envelope and are ineffectively sealed. One of the worst offenders is the dreaded recessed light fittings. The simplest way to alleviate service leaks is to minimise the amount of such openings on the external envelope. Including a service zone on the inside of the Airtightness layer will also significantly reduce the risk of service leaks and lead to greater flexibility on site.
3. A combination of a structural and a service leak: This is where a structural leak interacts with a service leak leading to air leakage. These can often be the biggest contributor to air leakage and one of the most difficult to seal when the building is complete. An example of such a leak could be a separating floor joist which is poorly sealed at the connection to the external wall, leading to air leakage through the floor joist at light fittings on the ground floor ceiling, plug sockets in the internal stud partitions on the first floor and even air leakage throughout the first floor.
Finally, materials used to achieve an airtight seal throughout the external envelope should be designed to provide an airtight seal for the lifetime of the building, not just for the day of the test and be simple for the installer to apply. Expanding foam cannot be relied upon to provide a reliable durable airtight seal. Systems which have been independently assessed by recognised bodies, such as the Irish Agrément Board, should be specified (e.g. pro clima Intelligent airtight system), to guarantee performance.
Examples of poor and good practice details are illustrated in the following images:
Figure 3: Good Practice Airtightness detailing:
(a) Airtight sealing of Airtightness membrane and surrounding attic hatch opening; (b) Airtight sealing of surrounding timber rafter penetration and overlaps of Airtightness membrane; (c) Airtight sealing of ceiling joist in refurbished block construction;(d) Joints and service penetrations through hollow core slab sealed with proprietary Airtightness sealing tapes and glues
Figure 4: Poor Airtightness detailing:
(a) Tear in a substandard vapour barrier; (b) Poor sealing around gap between window frame and block wall; (c) and (d) Large openings in a hollow core concrete slab leading to air leakage; (e) Large gap to facilitate services in hollow core slab leading to air leakage Figure 5: Insulation does not provide a continuous airtight seal
(a) Gaps between timber structural elements and rigid insulation; (b) Poorly fitted over compressed fibreglass is not airtight
Testing to confirm Airtightness:
When the building is complete it must be tested to confirm that the level of Airtightness achieved is as specified by the architect, and also to identify leakages in the building envelope. An Airtightness test is carried out by a blower door tester. The only way to confirm we have achieved the specified level of Airtightness is with a blower door test. Over the last decade the number of testers providing this service has increased dramatically. The principles of blower door testing were outlined in the previous article.
Conclusion:
Good build practice on site is a precondition for high levels of Airtightness on site, however, good workmanship alone can only achieve a limited amount. The economics of achieving the highest levels depend on adopting a holistic approach from very early in the design process. The role of the architect is central in determining clear, concise, simple details, which the various trades can follow and carry out on site.
The Airtightness of the building is not just for the day of the test, but for the lifetime of building. Materials used to achieve Airtightness should be designed for that purpose and should have third party certification from an independent authority, such as the Irish Agrément Board.
To avoid costly delays and remedial action for sealing leaks both the architect and the builder must be aware of the importance of Airtightness and what is required. Reducing air leakage in buildings is one of the most cost effective means of conserving energy, improving building durability and workmanship standards on site and to minimise condensation risk within the building envelope. With Airtightness in practice there is no doubt that the devil is in the detail be it at the design stage or the sealing procedure on site!
Niall Crosson
Technical Engineer BTech, MEngSc, MIEI
Pictures and graphics provided courtesy of Ecological Building Systems
In the next article:
Avoiding condensation risk within constructions is a precondition for building durable, healthy, low energy constructions. Selecting materials which combine high levels of Airtightness with effective protection against condensation is desirable. In the next article I will focus on designing low energy, airtight, healthy constructions.
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.
Conclusion
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
Airtightness of Windows and Doors
It is now widely accepted that if we are going to be serious about designing and building truly low energy buildings, we have to prevent the uncontrolled leakage or infiltration of heated air or cool external air out of or into the living space. Previously I outlined the basic principles and advantages of airtightness. These were primarily:
- Avoidance of moisture related building damage
- Avoiding drafts and discomfort
- Avoiding high heat losses due to air leakage
- Introducing a controlled ventilation system
- Improved acoustic performance
- Improved indoor air quality
This article will focus on a particular detail that often leads to air leakage and the inevitable, whodunit dispute on site, the window or door junction to the building structure. The following are the primary steps to achieving higher levels of airtightness in construction particularly at window and door junctions:
1. Forethought, planning of works schedule and clear simple design principles
2. Quality control on site and good workmanship with the use of appropriate materials to achieve an airtight seal. This includes identifying at the beginning of the project, who is responsible for sealing the window to the buildings airtightness layer.
3. A final measurement which confirms we have achieved the required level of Airtightness which we were aiming for in the first place.
Communication and coordination of works on site and a high level of consciousness and awareness regarding the importance of airtightness from the design stage to the build process are essential components of attaining a positive result.
How to ensure that airtightness is maintained around the windows?
Attaining best practice airtightness on site encompasses the various trades which may interact with the external envelope of the building. This may be the electrician who fits the plug sockets, the plumber who installs the piping or the ventilation supplier who must install extract or intake vents. The window installer is also one of the key personnel on site which can make or break the airtightness strategy. Windows are one of the most common failure points when an airtightness test is been carried out on a building.
While a window may, in theory, have an excellent level of airtightness, it is essential to ensure the INSTALLED window attains an airtight seal to the existing structure. If this is not attained the integrity of the thermal performance of the window can be compromised. A Passive window in particular can be the most expensive component within a low energy building. There is little point in investing so much in a building component if it is not installed correctly. It therefore pays to ensure it is installed correctly. At the end of the day this is what airtight building is all about, good building practices and getting things done correctly.
One of the main reasons a window often proves to be one of the most common leakage points in a building is not just down to the complexity of the junction, but it is due to the transfer of responsibility on site in dealing with this detail. It is important from the outset to identify who is responsible for sealing the window to the buildings airtightness layer when it’s installed. Otherwise, the window installer and the internal membrane installer or plasterer will more than likely rely on each other to seal the junction which leads to a dilution of responsibility, and in many cases, to no one sealing it in the end and the inevitable leak which ensues.
While it is impossible to provide guidance in relation to airtight window installations on every form of construction within the limits of this article, the following sections will provide general guidelines to two of the most common forms of construction on the market today, Timber Frame and Masonry construction. The key to attaining an airtight seal on any construction is to try to ensure that the air barrier layer on the wall or roof connect to the window or door frame seamlessly. The air barrier layer on a timber frame building tends to be either a vapour control layer membrane or an OSB sheathing board. In masonry, this tends to be the plaster on the block. Plasterboard tends to provide an unreliable level of airtightness and certainly shouldn’t be relied upon for those seeking to attain best practice air tight results over the lifetime of the building.
Timber frame window junction:
In Timber frame construction the airtightness layer tends to also act as a vapour control layer on the internal side of the construction. More details regarding the role of a vapour control layer are available on my previous article Airtightness Part 3
I will now describe 2 different approaches to sealing a window airtightly in timber frame construction.
Applying airtight seal before the windows are installed:
Figure1: Applying airtightness tape to window prior to installation offers greater flexibly and efficiency
Applying airtightness tape before the window is installed is the preferable method of attaining airtightness on site. Pro clima Contega SL window sealing tape is presented in Figure 1.
Here we can see the window installer has pre-applied the tape on the edge of the window frame. This has the following advantages:
- It is very quick to apply
- The edges of the tape are easily concealed once the internal lining is applied
- It is easy to seal around tight corners
Once the window is installed the tape can be bonded directly to the internal OSB or vapour control with the integrated adhesive tape on the opposing side as shown in figure one. It is also important in all applications of such a tape, to ensure the window is sealed to the airtightness layer below the sill. One disadvantage of this approach is that if the tape is not bonded correctly to the window initially, then the installer has to try and seal the tape to the frame later. If the internal lining is not thick enough, this may lead to tape been exposed on the window frame when the building is complete.
Applying airtight seal after the windows are installed:
Figure2: Applying airtightness tape to window frame after the window is installed
Applying airtightness tape after the window is installed is also possible in cases where it is not possible to install the tape prior to the installation of the window. Figure 2 illustrates this approach. This method tends to be more time consuming as the installer has to ensure that excessive tape is not applied to the frame leading to it being exposed when the building is complete. This is particularly critical on expensive Passive House windows! Tight corners can also be cumbersome and time consuming.
Masonry window junction:
Masonry construction is the most common method of building in Ireland. The typical cavity block wall connection to the window frame can often lead to air leakage on site. This is due to the varying quality of the finish on the inside of the blocks been used which may of been damaged during installation, the skill of the builder to ensure the tolerance between the block window reveal and the window is not too high and the width of the cavity required to conform to the building regulations or to build low energy buildings means cavity’s have increased in size from 50mm to 300mm in some cases!
Bridging the gap between the window frame and the internal plaster on the block is the key to attaining a reliable seal. When one accounts for the range of factors outlined which may occur on masonry walls it is safe to say there is no single solution for every masonry wall. The following section will describe two ways of effectively sealing masonry walls to window frames airtightly.
Applying airtight seal before the windows are installed:
Figure 3: Applying airtightness tape to window before installation
Applying airtightness tape before the window is installed is the preferable method of attaining airtightness on site. Again, Pro clima Contega SL window sealing tape is presented in Figure 3.
Here we can see the window installer has pre-applied the tape on the edge of the window frame. This has the following advantages:
- It is very quick to apply
- The edges of the tape are easily concealed once the internal lining is applied
- There is easy to seal around tight corners
Once the window is installed the tape can be bonded directly to the masonry with a continuous bead of high performance acrylic glue. The tape should be bonded to the wall completely with the glue. Then the plaster can be applied directly to the tape, ensuring a reliable, durable airtight seal is attained. It is also important in all applications of such a tape, to ensure the window is sealed to the airtightness layer below the sill.
Applying airtightness tape to window after installation:
Figure 4: Applying airtightness tape to window after installation
Applying airtightness tape after the window is installed is also possible in cases where it is not possible to install the tape prior to the installation of the window. Figure 4 illustrates this approach. This method again tends to be more time consuming as the installer has to ensure excessive tape is not applied to the frame leading to it been exposed when the building is complete. Again the tape is then fully bonded to the surrounding block-work with suitable airtightness glue, such as pro clima Orcon F. Then the fleece can be fully plastered at a later stage.
How do I know if my windows leak?
Once a window is installed and sealed it is important to confirm there is no leakage occurring either through the window itself, or the connection between the window and the adjoining structure. It is best to ascertain this prior to installing the internal finish. This is because it is easier to locate the leak at this stage and if any adjustments or remedial work is required it can be carried out more cost effectively without damaging the finished product.
Otherwise, the phenomena of dancing curtains in front of closed windows may occur when the building is complete!
A leak can be identified by depressurising or pressuring the building with a blower door or a WINCON fan.
This induces a negative or positive pressure on the building envelope and allows leaks to be detected with ones hand, a smoke pencil or fog emitting device such as a wizard stick shown below, or by using a thermal imaging camera, if the conditions are suitable.
Figure 5: Fog emitting device, Wizard Stick, often used to highlight air leakage around windows
As with all buildings, when high levels of airtightness are attained this lends itself to a high standard of construction and as a result unintended gaps and cracks will be minimal. It is therefore essential to ensure that a clearly defined ventilation strategy is designed into the building from the outset by passive or mechanical means. The key is to build tight and ventilate right!
Conclusion
It is clear that in order to attain optimal airtightness in buildings, the junction between windows and the building structure must be sealed continuously. It is no longer acceptable to install windows and rely on temporary seals such as poorly fitted expanding foam or poor quality sealing glues and tapes. These provide at best, a short term seal. The airtight seal attained when the building is constructed shouldn’t just last for the day of the test, but for the lifetime of the build.
Ensuring windows are installed and sealed airtightly have the following benefits:
a. Improved thermal efficiency
b. Increased comfort levels (No moving curtains on windy nights!)
c. Improved build quality and less builder call backs due to poor installation
d. Reduced risk of condensation between the window and the building structure and hence less risk of mould
e. Improved durability
f. Superior acoustic performance, particularly in urban areas
It is clear that airtight windows should be a priority from the outset, but there is little point in specifying or procuring exceptionally high performing windows if they are not installed correctly.
Niall Crosson
Technical Engineer BTech, MEngSc, MIEI
Pictures and graphics provided courtesy of Ecological Building Systems Ltd.
