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New materials and techniques are prompting many builders to re-examine how they ensure the airtightness of building envelopes—and literally turning this important work inside out. In this video, Building Knowledge Canada leaders Gord Cooke and Andrew Oding visit a home under construction and discuss the advantages of securing airtightness from the building exterior. 

Gord: So, in my history of airtightness testing since 1987, in the early days, we were always focused on interior air barriers, ergo for example polyethylene tape and caulking. But now we have these choices of simplified exterior air barriers, which have really been your history.

Andy: We’ve seen, in particular, over the last five to 10 years, a real move from interior air barriers to exterior air barriers. It’s something we saw in the commercial sector a number of years ago, and are now seeing in residential. So, there are two ways that you can do an exterior air barrier.  One way is to use a product like a continuous insulated sheathing with approved sealants and or tapes, which has a Canadian certification as a system, not just as a product (CCMC). It's the insulation board plus the approved tapes and details that make it a good air barrier system. We've seen exterior air barriers applied that get 1 air change or less before any poly vapour barrier is put into the assembly. 

The advantage of exterior AB is partly because there are fewer intersections and penetrations on the outside as opposed to penetrations on the interior wall. You’ll notice that tapes and sealants used on exterior air barriers are not your ordinary tapes. The tape must be approved by the insulation or sheathing AB manufacturer as part of the AB system. Approved tapes will expand and contract with the thermal properties of the attached insulation or sheathing. The same is true with the water-resistant barriers (second plane of protection). There’s a nice building science where you can get approved water resistance barrier systems and materials as exterior AB systems. In other words, you’re getting two control layers for the price of one.  

Andy: Often, a builder will tell us, “Well, I know I could do an exterior air barrier with a foam product like this or with a house wrap. But it's different trades that are controlling the air barrier.” We remind them that it does take a little bit of change and education but the beautiful thing for builders is that, when you're not relying on the traditional interior polyethylene as the primary AB (it may still be installed as the vapour control layer), you don't have to worry about making it continuous for example caulking, clamping or overlapping. I remember there was a study that was done by a large insulation manufacturer that quantified that there is less than half the number of penetrations on the outside AB vs an inside AB. 

Gord: For AB we need to think about all the transition details like the transition from the wall to the foundation, from the wall to the attic space, the AB continuity around vent penetration, and flashing penetrations. So, there are different strategies and different materials that we can use to create and connect air barriers. We can do it from the outside or the inside as long as they're continuous and connected. 

Andy: That's right. Watertight and airtight with one set of materials. That's pretty cool.

According to Gord and Andy, it’s an exciting time to be working on building airtightness. Building Knowledge Canada’s Builder 360° program is a way for builders to learn about innovative and emerging materials and systems that can simplify their work while adding to the effectiveness of their housing designs.

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Connect with us anytime, and let us know what outstanding questions you might have on this subject.