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View Poll Results: Most cost effective way to insulate a garage attic for R-40?
Spray Foam 0 0%
Ridgid Foam Board on ridgid foam board 0 0%
Fiberglass 0 0%
Blown in 1 25.00%
Combination Fiberglass & Foam board 0 0%
Ask Obama to create a subsidy package just for me, and get it for free 3 75.00%
Voters: 4. This poll is closed

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Old 03-02-2012, 07:32 PM   #1
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Ridged foam board in attic, going for R-40


I would like to use a cut to size 3" unfaced foam board between the 2 X 4 roof rafters (leaving a 1/2 air space next to the roof) and then a foil faced 4" foam board to go over the whole underside of the attic ceiling. I will spray foam and tape for air gaps. (Gabble ends will be done the same way)

The engineered trusses are all 2 x 4 and I would like to store stuff up there if possible, so insulating the roof instead of the ceiling is a more appealing option.

Are there any issues with this? The quotes from the spray foam guys was $6700 to $7000 and I can't afford that.

I'm insulating the attic space over my future man room above my garage (2 1/2 car garage) 24X36 so it's not a huge space.

Anyone got any other ideas?

Ken

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Old 03-03-2012, 12:46 AM   #2
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Ridged foam board in attic, going for R-40


I doubt very much that your trusses were originally engineered to have any sort of storage capacity on them.
So you will probably be compromising them by doing so.
But, you will do as you will I am sure.

Andy.

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Old 03-03-2012, 06:48 AM   #3
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Ridged foam board in attic, going for R-40


The last one might work if you kick some back to the DNC.

Where are you?? Insulation techniques change for different climates. Doing it right requires some knowledge of how and where. There was a good article in the Sept 2011 Fine Homebuilding magazine.
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Old 03-03-2012, 08:50 AM   #4
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Ridged foam board in attic, going for R-40


If the bottom cord is just 2 X 4's it's not made for storage, period.
Look at some of the older post with pictures showing what happens when people try it.
Broken rafters, sagging ceilings, sagging roofs.
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Old 03-03-2012, 11:58 AM   #5
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Ridged foam board in attic, going for R-40


Thanks for the replys, when I say storage, the home office space is small to begin with, so the attic storage would be a few boxes of holiday decorations, nothing with any real weight to it.

So is the consensus that I should skip all the insullation on the roof and just put R-49 in the ceiling Via fiberglass or blown in? and not store anything up there.
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Old 03-03-2012, 12:03 PM   #6
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Ridged foam board in attic, going for R-40


Since 2006 IRC code changes requiring attic storage in the design (even with a 2x4 bottom chord), you may be able to store there: http://publicecodes.citation.com/ico...001_par031.htm

Other local conditions may apply, check with AHJ: http://www.hcpdc.com/pdf/FAQ%20Resid...equiremtns.pdf

Gary

P.S. I would leave more than Code minimum 1" air space: http://www.buildingscience.com/docum...n-roof-venting
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Old 03-04-2012, 12:27 PM   #7
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Ridged foam board in attic, going for R-40


I"m 9 miles from the coast in Maine but only a mile from NH

Ken

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