Air barrier discussion

Nathan Sikes
Consultant at Allana Buick & Bers, Inc.

Do air barriers affect the "Breathability" of buildings?

Simon Bowkett
Principal at Bowkett Williams Consulting
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Yes. See Construct Ireland Issue 6 volume 4 article by Joseph Little containing report of 4 Wufi simulations that prove the point ... pp. 79 ff seem relevant. Not only do they adversely affect vapour transmission, they adversely effect it so badly that mould is going to grow ...

'Simon Bowkett'

Yes. See Construct Ireland Issue 6 volume 4 article by Joseph Little containing report of 4 Wufi simulations that prove the point ... pp....'

Scott Wood
President of Scott Wood Associates, Llc.

Little's article discusses vapor barriers and smart retarders not air barriers. It seems in my experience that air transported moisture vapor (especially zones 1-4) is by far the largest contributor to condensation issues within the envelope. I’ve seen it with vapor barrier installations that redirect or channel air movement to gaps in the vapor barrier. You can find a high probability of moisture issues in these areas due to air transported vapor. The question posed is do air barriers affect the breathability of building. In terms of air movement, yes the air barrier reduces the “breathing” of a building. The whole point of an air barrier is to stop heat loss due to convection and the gross movement of air transported water vapor. If the air barrier is not a vapor barrier and materials are in place to manage (not block) vapor driven moisture, the building is able to “breath” in terms of vapor drive. So to answer the question on "breathing", no to air, yes to vapor IF the air barrier in not also a vapor barrier.
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Ryan Dalgleish
Manager at Air Barrier Association of America

Moisture transport by air leakage can transport anywhere between 60 - 100 times more moisture that by diffusion (which is the vapour barrier's main purpose). The biggest confusion in the marketplace is what the difference is between the air barrier and vapour barrier and after talking with people, most times you find that they say "vapour barrier" and then explain the functions of an air barrier. From a moisture management standpoint within a building enclosure, stop exterior water penetration, stop air movement, then worry about diffusion. Unless you have a swimming pool in an Artic climate, I would guess that any building failures that occur are rarely a result of vapour diffusion. Buildings breathe by their mechanical systems (which you can control) and we should not rely on our building enclosure to breathe (which we cannot control)

Wagdy Anis
Principal, WJE
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When you say a building "breathes" surely you don't mean infiltration and exfiltration of air through the enclosure. Building assemblies "breathe" i.e. have the ability to dry out and control condensation due to diffusion. Infiltration and exfiltration is a really bad thing because it promotes gross condensation, loss of durability and mold growth. It also creates loss of control over the HVAC system. Ventilation of fresh "breathable" air is provided by the HVAC system according to ASHRAE standards 62.1 and 62.2. Air barriers to prevent air movement through the enclosure are a good thing. Whether they are vapor permeable or not depends on their position relative to the insulation and the climate involved. Stop confusing vapor retarders and air barriers please. Go back and read Quirouette's paper "the difference between an air barrier and a vapour barrier".

Nathan Sikes
Consultant at Allana Buick & Bers, Inc.


I carefully used the term "breathability" because I've heard people use that from a position authority as a positive benefit from not using an air barrier. Judging from the responses I received to the question there is some confusion in the community regarding vapor barriers and air barriers and breathability. There is no positive benefit from not using an air barrier. The effects of not using an air barrier is air leakage either into the wall or out depending on air movement and temperature differences on the wall. Localized air leakage will bring warm moist air through the hole or air leakage path and cause localized damage due to the elevated moisture in that area. The other effect is poor energy performance. Warm air is moving through the wall where and when it can, and not as intended when designed. Will your building fail if you don't use an air barrier? NO. The question is: how is your building going to perform? If you use an air barrier it will perform as intended, if the air barrier is designed properly in conjunction with the vapor barrier if needed, your building will breath ie allow vapor transfer safely without condensation