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tpagel 11-18-2009 06:59 PM

batt insulation installation question
I have seen photos of batt installations where the flanges of the kraft paper get stapled to the faces of the studs and rafters facing inward, toward the cavity, not the outer face, facing the room. My house attic has insulation with the flanges stapled to the bottom of the rafters, overlapping, and not actually a bad job. But I want to finish off this space. Is this "unique" flange method OK? I would really rather not rip out 1700 SF of insulation and start over! If I put drywall over the flanges, would there be any adverse effects?


Gary in WA 11-18-2009 08:52 PM

It is installed the best way:

"Even tiny voids can cause heat loss, because they allow air to move around inside the stud cavity. If a batt is stapled to the sides of the studs, crushed or cut short, air can rise up the warm side of the cavity and fall down the cool side. Heat rides on this convective loop, actually traveling around the insulation."

I would pull some out to make sure you have a continuous air flow, 1-2", or more on low sloped roof, to remove the moist conditioned air leaking from in the room. Soffit and ridge vents are equally important. Use a vapor barrier paint primer as well.
Be safe, Gary

tpagel 11-18-2009 09:51 PM

Thanks. I know for a fact that there is molded foam channels in each rafter bay of the cathedral ceiling, so air flow should be OK. Would you go ahead and tape all the seams, just to make it that much more monolithic a membrane? there are some tears and some gaps in flange coverage. I figure tape is cheap, and I'm not paying for my own time. ;-)

Gary in WA 11-18-2009 11:08 PM

The paper is a vapor retarder, that lets some moisture through to vent out the ridge.Even the tears would seal mostly alone when the ceiling drywall is applied. The seams do not need it. The insulation gaps I would fix. It has to be flush with the ceiling drywall, no air spaces. Here is some info on insulation gaps:
The R-value of the rafters:

Figures 3 + 7, seal all holes: I would use rigid foam board on the rafter bottoms to isolate the cold rafters from the warm space, as mentioned (only if you did not have a vapor retarder).

Read this one carefully: foam blocks under, in line with knee walls -
Be safe, Gary

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