Image Credit: National Renewable Energy Laboratory
More Q&A Spotlight
Adam Peterson is building a house with double-stud exterior walls, and he’s run into a problem.
“Blame it on lack of clarity on my part,” Peterson writes in a post at GBA’s Q&A forum, “but when my framer built my double-stud walls he didn’t oversize the window rough openings to account for 1/2-inch plywood sheathing connecting the inner to the outer walls. He figured that this gap could be covered solely with drywall.”
The back of the inside stud walls have been covered with MemBrain, an air barrier and “smart” vapor retarder, which has been bedded in acoustical sealant and stapled in place. This is in advance of blowing cellulose into the wall cavities. Later, Peterson will insulate the inner 2×4 wall, which is the service cavity for plumbing and wiring.
But what should he do about the lack of a solid connection between inner and outer stud walls at the window openings?
“I’m wondering how the lack of plywood here will impact airtightness,” Peterson writes. “Should I take extra measure to apply acoustic sealant to the drywall window return?
“I will need to blow in the insulation prior to drywall, of course, so I will need some sort of blocker at the rough openings in order to prevent the cellulose from flying out of this gap. Perhaps just some more MemBrain stapled wall-to-wall? Just trying to think of the implications this will have on my build.”
Peterson’s dilemma is the topic for this Q&A Spotlight.
Installation of MemBrain is a problem
Having the MemBrain installed now is not ideal, says GBA senior editor Marin Holladay.
Holladay agrees with earlier comments from Michael Maines and Dana Dorsett that gussets should be installed to connect the two stud walls.
“You certainly need some…
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7 Comments
Router?
It didn't occur to me at the time this issue first arose, but using a router with a fence and just routing a rabbet around the inside of the framing and letting in plywood might work. I don't think skipping routing the corners would make much difference.
plus sheetrockers need a heads up
As an architect this is a standard conversation I need to cover with every builder just in case they forget. I also like to be on hand during initial window installations. Also (very important) make sure the sheetrocker knows about the sheetrock returns when estimating!
Blocking and long screws
what should he do about the lack of a solid connection between inner and outer stud walls at the window openings?
I think Solid 2x blocking ripped and placed in the gap then secured using structural screws like a Simpson SDWS screw. Which has a flat head that counter sinks and is available in long lengths. It looks like a 8 to 10" screw thru the inner sill then the block then into the outer wall would tie it all together.
Double Stud Wall
Double stud walls have become my "go to" framing package. It has proven to be a less expensive way to achieve a high performance insulation package. I'm not an engineer but a "smart" vapor barrier seems to be the proper procedure. However I find it a bit impractical in application. It seems the installer in the article has the same problems as I have encountered. It is somewhat of a delicate product and nobody on the job has any respect for it or the intense amount of time it takes to install it correctly. Time is money and I can't see any practical and/or feasible way of using it for an air barrier that actually works by the end of the job. I have found it much more practical to maintain my air barrier at the exterior sheathing, whether it be with a high grade house wrap or sheathing and tape and use the Membrain behind the drywall. This removes any worry for me with air infiltration at the window openings since they are sealed tight on the exterior. The first home I did this way, I too forgot to make the openings large enough for plywood. I have had no issues so I continue to not use plywood (that and I'm OCD about the 1/2" of thermal bridging). This may cause trouble with drywall returns but I don't see any issues with wooden jamb extensions. There are a few other issues that I have had to adjust for but nothing major. The more we do the better our process. This (plus other things) gives me great door blower numbers and a HERS index in the low 40's.
Interesting.
Is Membrain a much more delicate product than Intello Plus? I've found Intello Plus pretty durable to incidental damage. (The plain Intello product, without the Intello Plus grid, is not so resistant and not something I'd recommend).
Keith,
Q. "Is Membrain a much more delicate product than Intello Plus?"
A. Yes.
You'd have to check with the architects about sheathing the around the opening from the other side of the studs. sheathing the king stud connected to the jack instead of the jamb side can add rack strength and serve as a fire block if you do it top to bottom. the sill will need to be notched to fit around the cripples and be a pain to get into place but is doable, then touch up with some acoustical sealant or foam around the studs. The header is solid on the ext... so no good way to attach, Id check measurements if that can be sheathed on the window side and still allow for expansion. then add an additional layer of membrain over the area that was cut into.
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