need attic insulation advice
December 16, 2009 6:40 PM   Subscribe

attic insulation question: I live in a split level home that has two separate attics. Is it counterproductive to have one attic better insulated than the other? (more specifics inside)

Both attics were already insulated with fibreglass batting rated at around R-12 when I moved in. I recently added R-40 batting to one of the attics, and am contemplating leaving the other attic as is at R-12. I have heard from non-professionals that I should not leave it this way, as it's doing more harm than good; I would have been better off leaving both at R-12 rather than insulating one attic only.

Anybody with any know-how or previous experience care to share their thoughts? Basically I'm trying to decide if I should insulate the other attic, or if it's ok to leave it as is.

Possibly relevant info:

-the attic that I have already added insulation to is slightly lower than the other attic
-location is Vancouver, BC (temperatures range from roughly -5 to +30 degrees celcius)
-cost is not an issue
-house is approxmately 30 years old
-I did not enjoy adding insulation to the first attic

thank you!
posted by cheemee to Home & Garden (4 answers total)
I don't think it's thermodynamically possible that insulating one part of your house would make other parts lose heat faster.
posted by 0xFCAF at 6:56 PM on December 16, 2009

If you were going to insulate only one, it would have made sense to insulate the one that's higher. Hot air rises.

I can't see how it can be worse to insulate one attic than to insulate no attics. But generally it is a bit of a waste. Think about the clothes you wear: you feel cold according to the parts of the body that are most exposed, not the most insulated. If you wear snow pants and a t-shirt, you're not a lot better off than with just pants and a t-shirt. But if you have snow pants and a parka, you're good down to -10C. The house will lose heat from the uninsulated attic, and the good work you've done on the other attic won't matter that much.

But if you're looking to avoid more insulating, maybe check out your window seals. A window that is even slightly ajar or doesn't seal completely is going to bleed heat much faster than any attic.
posted by musofire at 7:05 PM on December 16, 2009

It shouldn't cause any harm.

As mentioned above, it also is unlikely to develop the benefits you might have hoped when installing that insulation. If you do go back to insulate in the future I'd recommend using loose fill cellulose instead, but you can research that further when you come back o it.
posted by meinvt at 7:55 PM on December 16, 2009

R-12 is ridiculous, you should double that at least. R-40 would of course be better though R-52 is approaching the limit of diminishing returns with the degree heating days you have. Blown insulation can be put over fiberglas batts and tends to be cheaper than batt, even when you pay someone to do it, just watch that it won't contact your roof decking.

It's tough to talk specifically about the performance of your attic without knowing more details. Generally though if your attics have about the same square footage you can split the difference and get an average. So 50% R-52 and 50% R-12 gives you about R-30 overall. In practice it's less than this because a) R-Values are nominal and stuff like ceiling joists and electrical protrusions have a greater effect in the R-12 attic than the R-50 and b) the temperature gradient in the R-12 attic is steeper which adversely effects performance at the surfaces. Still you can cause worse performance by only insulating half the attics. People may think that because the lower insulated space may feel colder compared to the higher insulated space.
posted by Mitheral at 8:55 PM on December 16, 2009

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