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Old 09-21-2010, 04:55 PM   #1
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Increasing effectiveness of attic insulation

Hey there,

Pretty much a newbie to this sort of thing, so your patience is appreciated.

Last winter (in Michigan) I noticed that some interior walls were freezing cold - literally, one morning when it was -2F outside, there was frosted condensation of the interior wall where sunlight was being focused by a hanging chandelier. I assumed the other side of the walls (which is inside of the attic) weren't insulated or that it had fallen down. To my surprise, they're insulated with batt insulation and it all seems to be in order, placement-wise.

Now a friend of mine who's an excellent handyman and contractor said it's likely due to air getting right in between the insulation and the studs and that I could cover it all with rigid foam sheets to prevent airflow and increase the effectiveness. I'm a total newbie, but doesn't that retard moisture exchange and cause all sorts of long-term problems with moisture/mold control?

I haven't confirmed the R-value of the batt insulation yet, so I plan on doing that, but what's a good way to fix this problem? Is the proposed fix an acceptable one? Would it be better to remove the batt, put in foam and seal it, and then cover the foam with the batt? Or any other suggestion for something completely different?

Much appreciated!


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Old 09-21-2010, 05:39 PM   #2
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Are the walls themselves insulated?

Maybe there is insulation but a granulated form that has fallen down inside the walls.


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Old 09-21-2010, 08:06 PM   #3
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Hey Allan,

Thanks for the reply - not that I could see. Just seems to be your typical drywall with batt insulation on it. Any other thoughts on what the problem might be? Thanks!
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Old 09-21-2010, 10:08 PM   #4
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Flux: Any pics? I'm not sure I am 100% w/ your description. Batt insulation (fiberglass) is pretty poor stuff. If you can, get some better insulation in there; cellulose batts or blown in, if possible. If I were going to put up foam boards like your friend said, I'd first make sure that the drywall on the inside is air tight, which may mean tearing the old stuff off and re-doing it. Look up the Airtight Drywall Approach on, etc.
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Old 09-21-2010, 11:21 PM   #5
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Do you have ice dams?

Like all insulation, fiberglass only works if it is installed right.

I like what your buddy said, I think you have some cold air circulating behind the drywall. Try going in the attic, remove what's there, and cut 2" of rigid foam board to fit between the studs. Use GreatStuff to seal it in place, right up to the drywall. When you're done with that, use whatever insulation you like to increase the Rvalue of the wall to at least R20 and more is better. Only do NOT use any faced insulation. The foam is supposed to stop moisture from inside passing through, but if any does get past the foam you don't want facing to stop the moisture from escaping.

When I've insulated kneewalls I've filled the stud cavities with foam, and then run fiberglass batts the long way holding them in place with chicken wire and a staple gun. Its hard to fill the voids without compressing the stuff but if you're careful its cheap and works.
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Old 09-22-2010, 01:49 PM   #6
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"Performance of cavity insulations are degraded primarily by two factors:
  1. Absence of insulation
  2. Convective looping in and around the insulation
The fist factor is a contributor to the second. Gaps, voids, and incomplete cavity fills leave spaces in which convective looping can occur.
An omission or degradation in any type of insulation will reduce the insulating performance of the overall assembly.
One of the heat exchange mechanisms that cavity insulations are intended to control is convection. When convective looping is able to occur in or around the insulation, its function is subverted. Also, air moving through insulation because of wind or other pressures can also subvert its performance. Therefore, low density fibrous insulations should be covered by air barriers in areas subject to air movement." from:
Your Contractor friend is smart. Run the foam board in the joist bay and up into the rafter bay as well, to air seal the bottom and prevent wind-washing of the rafter insulation. Don’t close off the baffle into the sloped ceiling there.

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Old 09-22-2010, 04:26 PM   #7
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Thanks for the input everyone, and many thanks to you, GBR! Excellent PDF link and exactly the problem I've got. Now that I know what I've got to do, it's time to get to it.

Thanks again!


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