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Attic - rigid foam insulation on top of blown insulation; is it bad?

Attic - rigid foam insulation on top of blown insulation; is it bad?


Old 01-22-13, 12:06 PM
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Question Attic - rigid foam insulation on top of blown insulation; is it bad?

Hello, I have recently become a home owner about 6 months ago, and thus found these forums for all the reasons you might expect

So first time post here, and I'm definitely hoping someone can offer some assistance...

I encapsulated my crawl space recently, which involved insulating the walls with rigid foam. I subsequently have some left over rigid foam, which I would like to use in the attic to further insulate my home.

My attic has blown insulation between the joists, and was curious if I could cut some of the rigid foam to size and lay it on top of the existing blown insulation. I don't know if putting rigid foam on top of the blown insulation will impact the effectiveness of it. The rigid foam, of course, is extremely light, and I don't think it would compact the blown insulation all that much, but I hope someone here has some helpful insight here.

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Old 01-22-13, 03:51 PM
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Welcome to the forums! How thick is the insulation? If it is only 1/2 to 3/4" you don't have enough r value to matter. I would not waste time putting it in the attic, as it won't add to what you have. If you feel the blown in is inadequate, blow more in.
Old 01-22-13, 04:12 PM
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Definitely do not add foam to the top of your insulation. The foam is not vapor permeable, and in the winter it could possibly trap moisture on its underside, cause condensation and wet your insulation.
Old 01-22-13, 09:57 PM
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Bad idea. Much better to use the rigid foam in another area where it could do more good. If nothing obvious comes to mind, just double up what you already did in the crawl space.
Old 01-23-13, 07:38 AM
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Thanks for the feedback everyone. I'm not sure if the blown insulation is adequate, but I guess I should get someone to look at it who can let me know. The rigid foam is 1 inch thick. I can't really double it up in the crawl because I have a 16 mil moisture barrier on the ground and up the walls too, covering the foam.

I guess I'll see if I can donate it somewhere. Sad to see it go to waste.
Old 01-23-13, 07:53 AM
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If the crawl is still a bit cool, kitchen or bathroom floors could use that insulation. Did you detail the rim joist above the foundation walls?

I don't like to bother selling my odds and ends so my left over rigid foam will be bundled or boxed and set out by the side of the road with a free sign. I doubt it will be there an hour and it gives me a good feeling that someone will make use of it.

Old 01-24-13, 05:34 AM
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Although it appears the point has been driven home, I thought I would mention that placing the ridgid foam on top of the blow-in can and probably will compact the blow-in insulation which will reduce it's R value a bit.
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