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Double-stud wall construction filled with Roxul

I am in discussions with a contractor on how to insulate a staggered double-stud wall, 2x6 on the outside and 2x4 on the inside.

We can save a fair amount of money by filling each stud bay with Roxul and a layer of poly between the studs. I see this detail all the time with blown-in cellulose. What is the downside to 2 layers of Roxul in a staggered stud wall assembly?

Asked by David Matero
Posted Tue, 11/27/2012 - 09:46
Edited Tue, 11/27/2012 - 11:59

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4 Answers

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1.
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David,
Dense-packed cellulose will do a better job of reducing air leakage than Roxul. But Roxul should perform well if the installers do a careful job. Whichever insulation material you use, pay close attention to air sealing details.

I don't know where you are located, but unless you are in a very cold climate, polyethylene shouldn't be included in your wall assembly.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 11/27/2012 - 09:56
Edited Tue, 11/27/2012 - 10:02.

2.
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Martin, thank you. I forgot to mention I am zone 6, Brunswick, Maine. Poly between the studs was recommended by an insulator. In lieu of that do you suggest we tape/seal the sheathing? I was worried about the potential movement of stick built framing and Roxul, but the staggered studs will help with that. The install savings is rather significant over blown in that will help us in other places.

Answered by David Matero
Posted Tue, 11/27/2012 - 10:36

3.
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Martin,

According to my understanding, for permeable insulation, the air sealing would be essential at the interior surface of the wall. Regarding the avoidance of using poly except for very cold cliamtes: Aside from the consideration of climate zones to define this advice, what is the outdoor temperature which differentiates whether to use poly or not use it?

Maybe this cannot be quantified because the decsion must factor how much time at the lowest temperature versus how much time at a higher temperature where drying can occur.

But, in any case, I would like to know exactly the conditions where poly cannot be used versus where it can be used.

Answered by Ron Keagle
Posted Tue, 11/27/2012 - 10:44

4.
Helpful? 0

Ron,
The best way to answer the question is with WUFI.

One big variable: your indoor RH.

Another big variable: your air leakage rate. The higher the air leakage rate, the riskier it is to use poly.

A third big variable: the details of your wall assembly. Obviously, if you have exterior rigid foam, you don't want interior poly.

A fourth big variable: whether or not the building will be air conditioned. Cooling the space in summer makes the use of poly more risky.

In general, if the wall has no exterior foam, and if the building is air conditioned, I usually advise builders to avoid the use of interior poly unless they are in the colder parts of climate zone 6 (or anywhere colder, including most of Canada). Even in those regions, it's usually possible to design a wall assembly that doesn't need poly.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 11/27/2012 - 11:57
Edited Tue, 11/27/2012 - 11:58.

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