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Installing Polyiso in attic living space

I am working on a renovation of a 1940's house in Norfolk, VA. I want to properly install the recycled 2.5" polyiso insulation I have in my attic. The attic is used as living space. I have 2 x 6 rafters and want to increase the R value as much as possible by cutting sheets to fit between the rafters and then sealing them with spray foam to replace the R 19 bat that was up there.

It seems there is an ongoing discussion on the use of vents or not. My soffits do not have vents, and I would only have 1/2" left over after I put the polyiso up. My reading of the IRC code (2009 IRC 806.4) is that the boards could be installed as an unvented roof assembly if the polyiso boards were pressed tight against the underside of the roof sheathing and that no ventilation would be used, as well as no vapor barrier on the interior side. I am wondering if I am correct in this interpretation, and if anyone knows the up to date specifications on this issue?

Thanks,

Mark

Asked by Mark Imbrie
Posted Sun, 09/30/2012 - 15:00

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5 Answers

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1.
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Mark,
The only person who can answer your question is your local code official. My opinion is of no importance whatsoever.

On this matter, the code is ambiguous.

Answered by Martin Holladay, GBA Advisor
Posted Sun, 09/30/2012 - 15:53

2.
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Mark,

I know that headroom is a problem with finished attics, but don't forget to add at least an inch of foam between the drywall and the rafters to eliminate that significant thermal bridge.

Answered by Kevin Dickson, MSME
Posted Mon, 10/01/2012 - 01:35

3.
Helpful? 0

Thanks to both Martin and Kevin, as I have limited head room, I think the plan is for 1/2" to partially take care of the bridging but I will reconsider to get to an inch. Nothing is perfect in the world much less in a remodel. Ha! Yes, I will be working with the building inspectors but it sounds like this issue is still not settled, so the more thoughtful I can be the better. I believe in a diversity of opinion as the best option in such cases even if there can't be a consensus. Do either of you feel like having the limited vent space of 1/2 inch below the roof sheathing then the 2 offset layers of 2.5" polyiso sealed within reason on the edges with "great stuff spray foam", then the 1/2" layer covering the rafters for the bridging before the sheet rock would lead to some kind of disaster?

Answered by Mark Imbrie
Posted Mon, 10/01/2012 - 10:05

4.
Helpful? 0

Mark,
The best place to install rigid foam insulation is on top of your roof sheathing.

The second best place to install rigid foam insulation is under your rafters.

Between your rafters? I'm not a fan of that technique.

Will you have some kind of disaster? Probably not. But a lot depends on the quality of your work and the durability of your air sealing technique.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 10/01/2012 - 10:56

5.
Helpful? 0

Thanks for the dialogue. I have the time to be careful, but as I have read, careful only goes so far. Then I guess we are in a cost benefit situation, as is always the case it seems. I suppose I could just eat the cost of the polyiso I have and toss up some R 19 Fiberglass, that was what was up there, and leave well enough alone. Sometimes the best laid plans of mice and men need to be put aside? So close and yet so far.........Ha! But not getting the R value up would hurt after all this lack of planning...hahahhahha. Gotta love a self created challenge. Thoughts?

Answered by Mark Imbrie
Posted Mon, 10/01/2012 - 16:50

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