As buildings have become better insulated, the amount of heat they lose has fallen dramatically – this is true even for dwellings that are constructed to the minimum standards of the Building Regulations.
What this means, in turn, is that the proportion of heat that’s lost through uncontrolled air leakage has become relatively higher. So for all self-builders who want to be energy efficient, airtightness merits close attention.
This reflects a key issue in construction: namely that many modern houses do not perform as well as they are designed to do. This is known as the performance gap, and poor airtightness is one of the major causes.
Most people think of air leakage in terms of the wind blowing into our houses. While this is partly true, in reality the situation is more complex.
Wind creates zones of positive and negative pressure around our homes, which both pushes air in and sucks air out. In addition, as we all know, heat rises and this can also generate positive and negative pressure zones within a building.
Air leakage occurs mainly at the junctions between different elements of the building and particularly between materials. Some components are naturally airtight, while others need to have an additional layer applied to achieve the desired performance.
Any open chimneys or ventilation bricks are of course free air passages to the exterior. Leakage also occurs through penetrations – a typical example would be from services, such as drainage pipes and cables – and even from light fittings if the airtight layer is on the inside of the structure.
We all need fresh air to breathe, of course, as well as to push unwanted toxins and odours out of the house. A completely airtight building would not be healthy and in fact would contravene the Building Regulations (Approved Document F), which sets out a minimum air change rate of 0.5ACH@50Pa.
Many people ask: why should we go to such a lot of effort to seal up our buildings, if we then have to cut holes in them in order to provide adequate ventilation?
This is a perfectly reasonable question, and the answer is that what we are seeking to eliminate (or at least minimise) is uncontrolled air leakage. Once the building is airtight, a fresh supply can thus be provided through controlled pathways to all rooms (note that simply opening a window can effectively be a controlled pathway).
In a super-efficient and airtight building, such as a Passivhaus, the amount of heat lost through controlled ventilation becomes an issue; however, if the pathways are known, heat can be recovered from the expelled air and used to pre-warm the incoming fresh supply.
This is achieved with mechanical ventilation and heat recovery (MVHR) technology. Ventilation aside, it’s worth noting that the heat recovery won’t begin to save energy until airtightness is better than 3ACH@50Pa, as electricity is needed to run the system and this energy will outweigh the gains in situations where there’s too much leakage.
The basic principle is to establish an air barrier line all the way around the building, including its walls, floors and roof. This is similar to the thermal envelope that is used to establish the boundary for insulation.
A holistic approach is required – in other words, every aspect of the house needs to be considered. The recommendation in Passivhaus guidelines is to draw a red line on the detailed plans to show where the barrier will lie.
This barrier line can be drawn either inside the structural members of the building or outside of it, but cannot be a mixture of both, as this introduces great complexity and is a recipe for failure.
In the UK, internal air barriers are more common, but these require specialist components and have multiple penetrations for services. In North America, external air barriers are generally preferred, as they are considered simpler to install correctly, with fewer components and fewer penetrations to worry about.
The barrier may require an additional layer to be added to the construction in some places, but in other areas the existing material may be sufficient. For example, oriented strand board (OSB) is relatively airtight so it may only be strictly necessary to provide an effective seal at the joints between the boards.
However, some OSB products are more airtight than others, so it’s important to take care
with their specification. There are now also some specialist OSB components that are designed to be used as airtight layers.
Concrete is another product that’s effectively airtight, so solid floors should not need an additional layer. Some masonry can be made airtight with the addition of a parging coat (a thin layer of mortar), while monolithic systems such as insulating concrete formwork (ICF) perform well.
Glass is also airtight, so in most cases it’s the junctions between dissimilar materials that need to be focused on.
Buildings are complex and have all kinds of necessary structural connections between elements, such as at wall/floor junctions or wall/roof junctions.
Careful detailing is required to achieve good performance at these points but, increasingly, products are becoming available that help designers and installers deal with these challenges.
For example, where a soil pipe exits the airtightness layer, a purpose-made flange is fitted to the pipe and the horizontal section is then sealed to the barrier through which it penetrates to attain the required result.
The most important thing in achieving good airtightness is to plan the barrier line fully at design stage, then to have a process in place to ensure that every detail of the design is carried through during the construction phase.
Getting this right requires cooperation between the design team and the main contractor. It is much cheaper and less stressful to do this at the early stages than to have to take sections of a building apart and retrofit a barrier where air leakage has been identified by post-construction testing.
A particular challenge is to think ahead and get the sequencing correct. For example, achieving airtightness where rafters sit on a ridge beam is difficult.
Placing a strip of membrane over the beam before the rafters are put in place makes it much easier and quicker later on, but needs to be done as the roof is put in place – much earlier than we would normally think about airtightness and well before the finishing trades are on site.
Some tradespeople may not have worked to these standards before, so will need to go through training to fully understand their role in the process and learn the sequencing of applying what to many will be new products.
This is best addressed by appointing an airtightness champion – someone who will understand exactly where the barrier lies and communicate this to all trades on site, supervise all relevant work and make sure that later trades do not damage the layer as they work.
Some sites also put in place a ‘no blame’ reporting mechanism where any breaches in the airtightness layer can be detailed and thus made good before they get covered up.
In a word, no. Although vapour-control layers (VCLs) can be airtight, bizarrely, it is possible to use airtight materials that still allow water vapour to pass through.
This can be important for the control of moisture and other pollutants – a true eco building will be able to adsorb these into the structure.
A VCL prevents this but it is not necessary to use one in all timber frames, depending on how they are constructed and insulated, and on the local climate.
One of the key steps to getting a good airtightness result is forensic testing. This isn’t the final pass/fail test. Instead it is undertaken when the airtightness barrier is in place, but the finishing layers (plasterboard etc) have not been completed. This means that the barrier is accessible and remedial action can be taken if required.
The process is basically the same as for a final test (ie, pressurising the building) but involves the identification of leakage routes using smoke pens and thermal imaging cameras. The result should be a detailed report that shows the builder where there are issues in the fabric to resolve.