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Insulating attic knee walls

Posted by kristok (My Page) on
Fri, Mar 28, 14 at 14:38

Our old house has a finished attic room that is not insulated. We are embarking on insulating the knee walls and the small bit of the ceiling we have access too from the rest of the unfinished attic. Our access to the back side of the knee walls is pretty good.

I know I need to seal the air gap between the floor joists. For that I'm planning to cut some rigid foam insulation to rough size and then seal it in place with some expanding foam.

What I can't figure out how to do is get the fiberglass batts to stay in the stud bays. Should I minimally staple them in place and then cover it all with more rigid foam insulation? Or just space some 1x2s across the studs? What I don't want happening is that in 3 years (or whenever) the fiberglass batts have slumped down into the bottom of the stud bays.

I also can't figure out if I should be using a vapor barrier. I live in the Los Angeles area, as I know that makes a difference. The best guess I have so far is that a vapor barrier is unnecessary in my area, and if I use one it should be next to they drywall.

I also realize I asked quite a few questions here. Thanks for any input!


Follow-Up Postings:

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RE: Insulating attic knee walls

Here on the truly left coast, (3rd & Crenshaw), we usi one of two methods, vapor barrier always to the conditioned side, spray adhesive in a wide "S" pattern, or set a couple of 2" drywall screws at the top into the drywall, from the back.


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RE: Insulating attic knee walls

The spray adhesive doesn't give out after a few years?

What should I do to insulate the sloped portion of the ceiling? The gap between the back of the ceiling and the bottom of the roof decking is only the thickness of the rafters and there are occasional nails sticking through from the roof. Should I block off the bottom of the sloped section with some rigid foam and try to fill the cavity with loose insulation? Or should I try to stuff some batts in there? I've also considered trying to fit a piece of solid foam with foil facing in there and sealing it to the rafters with some expanding foam.


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RE: Insulating attic knee walls

Foil faced is for sound, kraft is convectional, however, when insulating the sloped section it's actually easier to use unfaced batts. no adhesive and to not attempt to overfill, IE., R13 for 2x4, R19 for 2x6 & 2x8, R30 for 2x10.

The fibers of the insulation in contact with the framing members will usually act to resist slippage, but yes, you can afix a piece of 2" foam. Cut it for a tight fit.

Depending upon the OC spacing, 16" or 24" a modified full garden rake, a three tined cultivator or a broom stick with a large knob of masking tape wound onto the end.


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RE: Insulating attic knee walls

On a different forum someone suggested using the foil-faced rigid foam, with the foil facing back towards the roof, would act as a radiant barrier.

My adhesive question went back to your original suggestion of using spray adhesive to get the batts to stick to the backsides of the knee walls. Is there a brand you recommend, or just the normal 3M stuff you could get at home depot? I'll probably back it up with some rigid foam for a little extra R-value, so even if the adhesive gives out eventually it should stay in place.

Thanks for your help!


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RE: Insulating attic knee walls

"would act as a radiant barrier."

Since asphalt shingles absorb heat just as a function of their composition, I would think that the heat being reflected would shorten the life of the sheating.

The 3M from home desperado is a quality adhesive, and to aid in retaining an R19 or R30 in the knee wall, use some tie wire vertically for R19 and 3 runs horizontally for R30.


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RE: Insulating attic knee walls

radiant barriers don't shorten life of shingles.

check with southface inst website for
info on insulating kneewalls they have
excellent air sealing pdf's if you do a site search.
details are excellent.

staple batts in place, to faces of 2x's
use rigid foam sheathing on attic side of
kneewall. continue the sheathing to ceiling below
and air seal to joists.

you can also buy friction fit insulation
and wires that hold the batts in place.

best of luck.


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