Air and Vapour Barrier for Double Stud Wall in Cold, Dry Climate
Greetings from your neighbors to the north!
We’re looking at building a 10 or 12″ thick, double stud wall with either blown-in cellulose or batt insulation. We are in a dry, cold climate (Regina, Saskatchewan) From reading Riversong’s various blogs and techniques, I’ve noticed he typically uses the air-tight drywall approach and a paint as his vapor barrier with no poly in the system.
I do realize that this debate has went on here for quite some time, however, I still have questions. Since we’re not doing the work ourselves, we do not feel as though the air-tight drywall approach will be possible in our situation because of unfamiliarity and the inability to continually inspect progress.
I am looking for the most suitable double stud wall assembly which starts on the inside with drywall and finishes off with hardi-board siding.
Thank you for your input.
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Replies
Logan,
Suggest you take a look at the houses Thorsen Chlupp recent built... Sunset or Sunrise, i forget which now had the external truss/cellulose walls. Barring that, you have some good resources in your area... Talk to the guys over at greenedmonton.ca for some more contacts.
I would use 8 mil Teno under the drywall and a high perm exterior sheathing to allow drying to the cold side. Get a case of Tremco accoustical sealant to seal laps and you will be snug as a bug.
Logan: If you are worried about crappy work, then nothing you plan will work. I would suggest that you get ahold of a crew familiar w/ building tight houses and talk your plans over with them. There are various ways to get a tight wall, one of which is putting plywood on the outside of your inner wall and taping/gooping the plywood to the studs. But, like the ADA, it takes careful workers. Search for Thorsten's Sunrise house here to get an idea of what he does, as one, good example.
Logan,
John Klingel's solution above is good, as Is Doug McEvers who has long experience with ADA.
The plywood on the exterior of the interior frame will offer the added benefit of placing the air barrier away from the finish layer of drywall. In this way the air barrier is perhaps less prone to penetrations during construction and then the next 200 years (we hope) as things change such as wiring, pictures and who knows what from future occupants.
Taping that same exterior sheeting on the interior wall with a suitable tape is really not to difficult. The trick is transitioning to the floor (more tape or a gasket) and then to the ceiling or "lid". That depends on whats going on there. Think of adding a membrane to span inaccessible joints (transitioning from that plywood layer, around the interior framing to the lid.) that you can tape on either end.
Doug McEvers suggestion of an exterior sheeting on the final exterior wall is, as always, right on (in my opinion) for your climate assuming that you'll add a rainscreen to make the whole system work properly.
Btw... In case it's not clear. In the above assembly, the plywood is also the vapor control layer (taking the place of poly). 1/2" plywood has a perm rating of 0.6 to 0.7 perms (typically). That's a good placement position and perm rating for your wall. Placing it well away from the finished drywall again improves the likelihood of a continuous barrier being maintained and not tampered with as the years pass.
Goo luck!
Logan,
You might enjoy reading the report by the CCHRC in Alaska. Their REMOTE Manual is extremely informative and addresses many of the questions you would have. The URL is :
http://www.cchrc.org/docs/best_practices/REMOTE_Manual.pdf
Thank you all for your responses. While the REMOTE wall is well chronicled and detailed, I still prefer blown-in cellulose insulation because of its non-petrochemical base (personal environmentalist preference). I think we'll end up using Albert's option (which was based partially on both Doug and John's) as you can achieve a tight house without trapping wetness inside of the cavity. As for the rainscreen, that's a must, do you have any experience with selecting one product over another?
My resource has 1/2" 5-lam plywood as having a perm of 0.36. I think visqueen is 0.06, so, either way, plywood is far more permeable than visqueen. Thorsten (at least in his Arctic wall) runs the plywood right over the top of that inner wall, then across the lid; one, continuous barrier that way. He then has a 2x4 "wall" under that (trusses on top) for a utility chase. I am thinking seriously of suspended sheet rock instead of the 2x4s, as it may (??) be easier and at least I can get a larger chase for the HRV ducts and a place to hide guns from Big Brother.