Mainstream construction air-sealing
MALCOLM TAYLOR
| Posted in Energy Efficiency and Durability on
It’s pretty well established that alternate air-sealing and vapour diffusion strategies are preferable to interior poly. Here in Canada, except in high performance houses, the transition hasn’t been made, but I understand in most regions of the US poly is no longer used by mainstream builders. When they stopped using poly, did they adopt alternative strategies, or simply stop using the poly?
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Replies
Malcolm,
Your use of the term "alternative strategies" is loaded. The term implies that (a) in the past, outward vapor diffusion was a problem, and (b) if a builder doesn't use an interior vapor barrier to address outward diffusion, then another strategy to limit outward vapor diffusion is necessary.
In fact, outward vapor diffusion is almost never a problem. In the case of wet walls, the two most common causes are (a) Bad flashing details that allow the penetration of wind-driven rain, and (b) Air leaks that allow warm, moist interior air to enter walls and contact a cold surface.
Your question implies an unstated question: Are most U.S. builders doing a good job of wall design and construction? The answer to that is no. The two biggest flaws I see are bad details for managing wind-driven rain and too much air leakage.
Martin,
My post came from another question here about wet basement OSB, and in moving it here to avoid muddying the discussion there I think I moved the mud too, I'm really only interested in air-sealing.
The poster has batt insulated stud cavities and is worried about the moisture they contain, as drywall is about to be installed. But what struck me is that there doesn't appear to be any intermediate stage of air-sealing. No gaskets or caulking being talked about. So I wondered whether the most common air-sealing strategy in mainstream building is really just relying on a standard drywall installation, or whether anything more is done?
Malcolm,
The short answer: Every researcher who has looked into the matter has found that new homes in the U.S. are gradually getting tighter. I don't have these studies at my fingertips, but if needed I can look them up.
U.S. builders still have a long ways to go. As noted in a news story on GBA, the 2009 IECC set a very low bar -- only 7 ach50. One recent survey found that a "high percentage" of new home builders are getting blower door results of between 4 and 6 ach50. Nothing to write home about, but (believe it or not) better than ten years ago.
The most confusing thing about your question is the idea that when you include interior polyethylene, you have addressed air leaks -- and when you don't include interior polyethylene, you have a leaky house. Neither of these assumptions is true.
My state adheres to 2009 IECC. WRB is tapped and copious amounts of caulking/sealing is applied to the framing areas which are responsible for air leakage.
Martin,
The practice here in BC is to use the poly as an integral part of the air sealing strategy. It is continuous over the ceiling and walls, is caulked to the subfloor, window and door openings and all penetrations. The advantages are that it is inspect-able and yields fairly predictable results even without a blower door test. The disadvantages are that you have the poly.
So at present we have this detailed layer of air-sealing, and on top of that the drywall. What I wondered was: as builders in the US did away with the poly, did they continue any of the attendant air-sealing, or do you just end up with the standard drywall layer we have and an overlay? Maybe my assumption that poly used to be installed in the US with the same level of air-sealing is wrong?
I don't want to beat this to death, I'm just trying to understand how drywall, installed without other air-sealing strategies doesn't lead to problems, or if other air sealing is done, what trades are doing it when?
Malcolm,
When interior polyethylene is used in Vermont, no one caulks anything -- and few builders have heard of airtight electric boxes. So the polyethylene doesn't really contribute to airtightness. When Vermont builders use poly, their homes are leaky. When they stop using poly, their homes are still leaky.
The fact that you are caulking seams and caulking the poly to the subfloor and penetrations obviously makes a difference. If builders in BC wanted to use this approach when installing drywall instead of when installing polyethylene, the effects would probably be similar.
In other words, the key is the caulk.
Malcolm,
If you are seeking better alternatives than poly to air seal walls...
I'd consider taping ply wall sheathing, and taping and carefully sealing mud sills, windows and other penetrations (tape, gaskets, caulk). Or the airtight drywall approach, sealing boxes and the drywall/floor intersection, as well as the top of walls in the attic. There are also air sealing products that allow water vapor transmission (more expensive than poly for smart vapor barriers).
What are US builders doing today, that's somewhat improved over the past? As Martin says, in general, they probably aren't doing much. But maybe more carefully using Tyvek or similar bulk water and air barriers that allow vapor transmission, applied over wall sheathing. We probably don't have good data to approximate the percentage of air sealing practices performed for new construction in various climates in the US.
The problem with poly for air sealing is that it traps moisture in walls. And as Martin notes, installing poly is usually not stopping air leakage around windows/doors/electrical boxes/mudsills, or tops of walls in attics. That pretty much describes what I did 30 years ago.