Erik Olofsson is planning a small house in the Rocky Mountains of British Columbia. Ideally, he’d like to get the walls close to R-40. The question is how.
“Seeing that the received opinion around GBA is the tandem of polyethylene sheeting and exterior rigid foam is not ideal, what do the builders on this site recommend?” he asks in a post at the GBA Q&A forum. “Larsen trusses seem fairly labor-intensive and rigid foam is expensive … Is a double-stud wall the answer?”
A complication is a local building code that apparently calls for a polyethylene vapor barrier on the warm side of the insulation. Although once a common building technique, it’s no longer universally accepted by building scientists as the best practice in all climates. Many builders have abandoned the use of interior polyethylene, even as some building inspectors continue to insist on it.
Olofsson’s quest for high performance at a reasonable cost, while solving the riddle of air and vapor barriers, is the topic of this month’s Q&A Spotlight.
Double-stud walls a good option
Double-stud walls are designed to provide lots of exterior wall volume for insulation while sharply reducing thermal bridging. John Klingel and Albert Rooks are among those who think that building double-stud walls is a good approach.
“I built a double-stud in 1980 and have never regretted it,” Klingel writes. “New house will be the same, but thicker, and with dense-packed cellulose instead of fiberglass.”
“A double stud with a plywood exterior and interior poly and ADA [the Airtight Drywall Approach] will work,” Rooks says.
GBA senior editor Martin Holladay doesn’t push the double-stud option, but he does point Olofsson toward a number of GBA articles that have been written on the topic (see the “Related Articles” sidebar).
Or consider 2×6 framing
Rooks also offers an alternative for Olofsson to consider. “Another good quality wall is a 2×6 standard frame with taped plywood or OSB exterior + WRB [water-resistant barrier] + 4 in. to 6 in. of high-density mineral wool,” he says. “It will eliminate cold sheathing and rim-joist bridging while allowing the use of a service cavity in the stud bays.”
A key detail of all of these recommendations is that a polyethylene vapor barrier isn’t necessary. In the case of the wall assembly suggested by Rooks, the OSB or plywood sheathing becomes the “air/vapor control layer.”
“Use a rainscreen detail, good air sealing, and ventilation,” Rooks adds. “I’m a fan of mineral wool because it doesn’t settle, doesn’t rot even if continually wetted, is fireproof, won’t support mold or bugs. It’s like a little piece if the Canadian Rockies (since it’s made of Canadian Basalt) covering your house, and near as durable.”
Holladay, too, think there are alternatives to the polyethylene required by local codes. “You might want to negotiate with your local building inspector,” he says. “Many inspectors will accept MemBrain or vapor-retarder paint as an alternative to interior poly.”
Deciphering Canadian codes
Malcolm Taylor writes that the British Columbia building code allows an “airtight drywall air barrier” instead of polyethylene. “Illustrated details covering all aspects of the approach can be found in the Building Envelope Guide for Houses published by the Homeowner Protection Office, which is a Provincial Government department,” he says.
But to Holladay, there seems to be some confusion in the codes over the roles that air and vapor barriers play in a wall assembly. “I can’t help but shake my head at the inconsistency in the BC code,” Holladay says. “Do code officials think that polyethylene is a vapor retarder or an air barrier? If they think it is a vapor retarder, then the Airtight Drywall Approach is no substitute. To retard the flow of vapor, you need vapor-retarder paint. The airtightness of the drywall is irrelevant.
“If they think it is an air barrier, I wonder whether they require airtight installation details for all installations of poly in the province? For example, do they verify that the poly seams are sealed over framing members with Tremco acoustical sealant? Do they verify that all electrical boxes are airtight boxes? If they do, bless them. However, I doubt that they do. I think it’s far more likely that the building inspectors have no idea whether the poly they insist on is supposed to be a vapor retarder or an air barrier.”
Taylor says he can’t speak to how the code is enforced in the province as a whole, but at least on Vancouver Island and the lower mainland, inspectors “insist on meticulously sealed poly.”
“All seams, the joint between poly and lower plate and plate and subfloor all must have generous amounts of acoustic sealant,” he says. “Gasketed electrical boxes are also required.”
In Ontario, adds Lucas Durand, there’s apparently a little more flexibility. “For the house that I am building for myself,” he says, “I have gone with taped 1/2-in. plywood for both air barrier and vapor retarder (and racking resistance). There was some initial skepticism from my inspector but it didn’t take that long to convince her — I am lucky she has been open-minded.”
Creating sensible regulations
Some of the material in the Canadian publication gives Holladay reason to wonder whether government officials really understand the science behind code requirements. “As is often the case,” he writes, “this is an example of government bureaucrats enforcing regulations that are based on an incomplete understanding of the relevant scientific principles.”
While that could very well be true, Olofsson’s potential tussle with local officials could have an upside. “The nice thing about your questions is that it is exposing that you can move the envelope quality in your area forward,” Rooks says. “The code seems to allow enough room for well planned modification. Lucas points out a few more methods towards improved assemblies. They are all great when executed correctly.
“There is no shortcut to a ‘quality envelope.’ Plan on it being more cost and work. I think it’s the only sensible thing to do.”
Our expert’s opinion
Here’s how GBA technical director Peter Yost sees it:
The reason Martin Holladay and I nearly always agree on building science questions is that we both have studied under/worked with some of the best: Joe Lstiburek, John Straube, Terry Brennan, Bill Rose, Anton TenWolde.
And one of the reasons there is still such confusion on basic heat transfer and moisture flow is that too many building inspectors and code officials have not.
Is this silly or what?
- The highest priority in moisture management is bulk water: how many building inspectors check the connections between the water-resistive barrier and flashings at penetrations for continuity?
- The next highest priority in moisture management is capillary water: how many building inspectors check for capillary breaks between porous building components?
- The next priority is air-transported moisture: how many building inspectors require qualitative and quantitative information from blower-door tests?
- But by gosh, almost every building inspector insists on and inspects the “warm-in-winter-side” vapor retarder, by far the least important wetting mechanism in nearly all buildings and climates.
And we should not be worried about the vapor permeability of just that one dedicated layer in terms of wetting, but the vapor permeability of ALL layers in terms of drying. (For more information, see my Vapor Profile blog.)
GBA has a series of great resources on vapor retarders and air barriers and their differences. Use your GBA project folders, make one for your building inspector(s), and fill it up with building science reading for them!