In the modern fast-paced construction industry, the quality of the materials and the construction methodologies are given prime importance to achieve higher energy efficiency standards. However, the Airtightness of the building envelope is often ignored due to the lack of awareness on the relationship between the Airtightness and the Energy Efficiency.
In the recent days, the buildings are provided with energy efficient insulation systems. However, because of the absence of airtight envelope, the air leakage nullifies the impact of the efficient envelope on the overall operation costs of the building.
So, in this article we talk about the importance of ‘Airtightness’ in the building envelope.
Let’s understand about the ‘Air Leakage’
In general, Air Leakage is assumed as 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. 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.
Air Barrier – the solution to achieve airtight envelope
The basic principle to optimize the Airtightness 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. ATTMA guidelines recommends drawing a red line on the detailed plans to show where the air 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.
Concrete is another product that’s effectively airtight, so solid floors should not need an additional layer. 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.
Challenges faced during the execution stage
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. Some project teams 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 the requirements.
This is best addressed by appointing an airtightness champion – someone who will understand exactly where the barrier lies and communicate this to all participants on site, supervise all relevant work and make sure that later participants do not damage the layer as they work. It is preferred that the airtightness champion shall be ATTMA certified building envelope professional or equivalent.
Let this be the final stage in ensuring the building achieves the desired Airtight Envelope. 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.
This process 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.
The below image shows the airtightness testing undertaken by our specialists: the front door is replaced by a temporary unit, with a fan that pumps air into the house to increase the pressure.
Image: Blower Door System used for airtightness testing