Where should the air barrier go?
My local paper has a very informative column written by a local person. He does a nice job educating people about the need for tight houses with good insulation and has lots of energy saving suggestions.
Recently he wrote a column about the importance of limiting air flow and the impact of air leaks. he said:
“It is better not to have an air/vapor barrier than to have one on the cold side of an assembly.”
I sent him an email taking issue with the comment:
“I think we need to separate air barriers (necessary everywhere) from vapor barriers, which are not really necessary and can cause more problems that they are worth in most climates. Current building science concepts allow, and even encourage, air barriers on the outside of the wall assembly. Taped Zip sheathing is a popular example. Another is using rigid foam sheets outside the sheathing. Once a good air barrier is in place, wherever it is, it stops air flow through the assembly.”
He responded:
“A heated house is pressurized by warming (and expanding) air, which will escape into the structure, unless it is stopped by an air barrier. So if the barrier is on the outside, the air-flow will be stopped there — next to the coldest surface and drop its moisture. This is just plain physics. If I had a dollar for every moisture damaged house I saw for this very reason, I would be a rich man. During the post-WWII rebuilding boom in Northern Europe hundreds of newly insulated buildings had to be torn apart and re-done due to this sort of moisture damage. People (including architects of the day) simply didn’t know better. It was a pricey learning curve.
To avoid cumulative moisture damage inside the outer wall we must stop the air-flow before it enters the wall — not on the cold side of the assembly.”
My response:
“With a good air barrier, there isn’t any flow of interior air into the wall assembly because nothing is driving the airflow.”
So, am I missing something or is he?
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Replies
Stephen,
No, you're not missing anything. The writer you had the email exchange with is misinformed.
In a cold climate, an exterior vapor barrier is a bad idea. An exterior air barrier is fine.
Back in the 1980s, the phrase used by this author -- "air/vapor barrier" -- was used to describe polyethylene. His statement is true for polyethylene.
But these days, builders need to distinguish between air barriers and vapor barriers, since they perform different functions.
Tyvek is an air barrier but not a vapor barrier. Steel roofing is a vapor barrier but (usually) not an air barrier.
-- Martin Holladay
Jon,
Assuming that the exterior air barrier is well-installed, not defective, and that the fiberglass batts or cellulose insulation aren't installed with wanton disregard for standard installation instructions, the type of convective looping you describe is quite rare.
You'd need to have a significant air channel adjacent to the cold sheathing, or stud bays that were totally uninsulated, to get enough convective looping to lead to worrisome moisture accumulation.
Lots of homes have been built with exterior air barriers (for example, taped Zip sheathing), and these homes are working well.
-- Martin Holladay
You are missing the fact that interior air might flow to an external side air barrier, drop it's moisture, then flow back to the interior. There might be things driving this air flow (eg, convective loops or room to room pressure differentials) and insufficient resistance (eg, loosely fit fiberglass insulation). Where should the air barrier go? Ideally on both sides. But barring an air and vapor barrier on the cold side, "must" is too strong.
https://buildingscience.com/documents/information-sheets/installation-of-cavity-insulation