Stephen Gurney from Ecological Building Systems explains the main causes of poor airtightness test results.
Interview with Stephen Gurney
Stephen Gurney has worked in the building trade for many years but about six years ago he became more interested in ecological buildings and sustainability. Joining Ecological Building Systems he now deals with the technical side of airtightness and vapour control, how it's designed in buildings and then applied on-site.
How the Different Airtightness Elements are Joined is Very Important
Stephen explains that in order to create an effective airtightness layer, the designer must think carefully how each of the elements join together.
For example, how does a concrete floor slab (airtight in its own right) connect with the membrane on the wall? The strategy must make sure everything is sealed properly and that there is no air leakage.
Allowances must also be made for soil vent pipes and electrical cables, etc., perhaps using specially designed grommets.
The Airtightness Layer Often Uses Special Membranes, Boards and Tapes
Depending on the construction type, there are various products that will be used to achieve the airtightness layer. Pro Clima is one of the leading manufacturers of these high performance materials.
The Best Components Can Become Ineffective if They Haven't Been Installed Well
Getting the installation right is essential because otherwise energy efficiency will be compromised (and therefore running costs will increase, too).
So if installers bung a bit of foam and silicone sealer around very expensive triple-glazed windows, eventually there will be shrinkage and a bypass will be created (airflow to the outside).
Air Leaks Must Be ‘Patched Up' in Order to Meet the Passivhaus Standard
For low energy buildings, carrying out an air test once the windows have been installed is a way of identifying the leaks.
Small holes in the fabric may seem irrelevant but when they are repeated over the entire building they quickly add up to one big hole!
Dealing with these leaks early on is much better than when the finishes have been applied.
Vapour Control Prevents Moisture Gathering on the Structure
Vapour control and airtightness should work hand in hand. Both are trying to stop air moving from the inside of the building to the outside.
Stephen says: “If you are losing air and it's the air from inside the building, it's warm moist air but it's quite stable so humidity levels around 50%. But if that breaks through our envelope, what happens is it will hit a cold point at some point in your structure and condense there.”
When water gathers within a building, mould and rot can form (which in turn might trigger health issues for the occupants, such as asthma).
Invest in High Performance Airtightness Tapes and Materials
It is a false economy to save money by using cheap airtightness tapes. All that happens with inferior products is that over time the glues dry out, become brittle and then the tapes stop functioning as air barriers.
The glues in high performance tapes are similar to the glues that are used in aviation to hold sections of aeroplane wings together! Not only can they take a high variance of stress but they are very good with humidity as well.
Tradespeople Need to Understand Why They're Taping!
Stephen believes that airtightness is compromised when the people handling the materials don't understand what they're doing – they're just handed a roll of tape and told to get going!
So on-site education in the form of ‘toolbox talks' is imperative to get everyone to understand that the efficiency of the building relies on how well these tapes are applied.
An ‘Airtightness Champion' Should Take Responsibility for All Work on the Air Barrier
It's a good idea to have somebody on-site who's always on top of the airtightness. This ‘Airtightness Champion' is there from the beginning to the end and is always checking that the materials are being applied correctly and that subcontractors, such as electricians and plumbers, are not making holes in the envelope.
There Are Two Distinct Approaches to Retrofit
1) Seal completely on the inside with very good vapour control. This would be the best choice when a timber frame is being created inside a stone or brick building. If the vapour air is not sealed all the way through there will be a much greater risk of condensation gathering.
2) If insulation is being applied internally then a diffusion-open material that works with the building structure is a good option. This is capillary active and moves moisture away from the wall.
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