Can I use fanfold insulation under housewrap?


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Old 01-17-17, 08:13 PM
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Can I use fanfold insulation under housewrap?

I am insulating a knee wall, re insulating actually since it was done poorly originally, It is a 2x4 knee wall with drywall, poly then fiberglass batts, with the "exterior" in an unconditioned space. I am going to re insulate with roxul and I was going to put house wrap on the unconditioned side and was wondering if it would hurt to put 1/4 inch perforated fan fold insulation and then house wrap.

I am then going to lay the old fiberglass batts next to the knee wall for extra insulation.

The wall has always seemed cold and now I know why, They just ran the batts around the electrical outlets instead of trimming them properly and there are gaps at the bottom and top of each stud cavity.

Thanks
 
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Old 01-18-17, 03:07 AM
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House wrap should only be used on the exterior of the house. The perforated fanfold will be a waste of money since you are using Roxul cut properly and tight. Can you post pictures of the knee wall on both sides so we can see what you see??
 
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Old 01-18-17, 07:19 AM
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A kneewall is essentially an exterior surface, just doesn't get rained on, hopefully.
Quote from Dr Bailes: "I've also seen attic kneewalls sheathed with non-rigid materials, mainly housewrap. (See Figure 6.) It's possible to use housewrap on kneewalls and to do it well."
Solving Comfort Problems Caused by Attic Kneewalls | GreenBuildingAdvisor.com

The purpose of covering the back of the exposed Roxul insulation is to reduce the air infiltration. The Roxul is already going to do most of that job.

The key here is air sealing and defining where that will happen and keeping it as a continuous boundary, below the kneewall to its top.

To your question, I would want something covering the back side but would not bother with the fanfold. Plywood or osb would make the assembly similar to an exterior wall. Thermax foil faced rigid foam board would add significant insulation value and can be left exposed, where most rigid foam board must be covered with a thermal barrier.

Air sealing plus a neat installation of the Roxul will make a big difference. Adding the housewrap would not be bad.

Bud
 
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Old 01-18-17, 01:39 PM
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I would go with the Thermax and tape all seams to keep the infiltration down. House wrap is semi permeable, and that is why I would not use it on knee walls. I think a more solid approach is better.
 
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Old 01-18-17, 02:39 PM
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The thing with thermax is it seems it is impermeable, On the existing kneewall there is poly behind the drywall so I don't want to make two vapor barriers.

I have the fan fold from years ago and its perforated so it can breath so I was thinking if i could use it there I would.

Ill post a pic.
 
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Old 01-18-17, 05:26 PM
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It would be more rigid than the housewrap and basically a vapor diffusion retarder. Since there should be minimal moisture moving through that wall you should be fine. Seal it where you can to improve the air barrier performance of the wall. Never a problem having more than one air barrier.

Bud
 
 

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