Why is an exterior air barrier superior to an interior air barrier?
andyfrog
| Posted in Energy Efficiency and Durability on
Lstiburek recently discussed his old airtight drywall assembly:
But in the clip he remarks that it’s better to locate the air barrier on the outside–why is this?
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Replies
I found that interior and exterior barrier are confusing terms.
What he's probably saying is that it's easier to wrap a house with a membrane from the outside because then you don't have to deal with intersecting walls and floors. But his air barrier is under exterior insulation on the warm side to avoid any condensation so it could still qualify as an "interior" air barrier :-) See why it's confusing.
So in short, because of the way we frame house in North America, it's easier to install it that way. In some European countries, they have a barrier on both side. See swedish platform framing for instance.
Andy,
As Alex said: Practically it may be easier and more effective to install an exterior air-barrier. The advantage of an interior air-barrier is it stops the cycling of air in and out of the wall cavity driven by pressure differences. An air-barrier placed mid-way through the wall makes it harder to puncture, but also harder to repair or modify.
Houses are regularly built with air-barriers in all of those locations and perform very well. I'm not sure the location is that important - certainly not anywhere near as important as it's integrity and continuity.
From the perspective of a remodeling project, I chose to go with an exterior air barrier (actually mid-way through the wall, under new exterior foam) from an ease of assembly standpoint. It's not too hard to wrap something around all the outside surfaces, vs. basically impossible to detail all the little corners and transitions from the interior.
From a long-term durability perspective, I'd guess an exterior air barrier is easier to keep intact. People tend to only infrequently make holes in the outside of their houses or all the way through the wall, but every picture hung and drywall crack on the interior is potentially degrading an interior air barrier.
Jonny,
One problem with the air-barrier moving around is the chance it isn't recognized as such later on. Whatever the disadvantages of interior poly vapour-barriers, (at least here in Canada) they were understood by most people who built or remodeled. I think it is Hammer and Hand who put stickers on every sheet of sheathing identifying it as an air-barrier for future workers.
>put stickers on every sheet of sheathing
I've been thinking that there should be some a version of plastic "caution" tape that says "Warning: This is a bearing wall!" I could use it in a few places where we've designed an interior partition that wouldn't doesn't necessarily look like a bearing wall to be an important structural element!
Yeah, I've had to identify bearing walls during renovations and it's sometimes remarkable difficult.
I think that's the big knock on using box-beams as lintels too. Once in I doubt anyone would recognize them as load-bearing.
Exterior is easier for a given level of air sealing.
More from Joe L:
An advantage of interior air barriers over exterior systems is that they control the entry of interior moisture-laden air into assembly cavities during heating periods.
Installing both interior and exterior air barriers addresses the weakness of each.
Other sources:
ENERGY STAR requires that an air barrier be installed at the exterior vertical surface of the wall insulation in all climate zones and also that an air barrier be installed at the interior vertical surface in Climate Zones 4-8.