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yaemish 01-18-2012 09:18 PM

Hybrid Spray/Rigid Foam Insulation
I live in upstate NY (not sure what climate zone that is) and need to insulate the knee walls in my finished attic. I've been doing research and know that I have to use foam in my situation. Going full spray foam between the 2x4s is going to be expensive and I'm not sure I will realize an ROI in the next few years.

I was thinking that I could used closed cell spray foam in combination with rigid foam board. I know that it wouldn't be as perfect as a full spray foam application, but it should be close. How should I do it? Should I put the rigid foam directly on the back of the drywall and foam over it? Or should I do an inch of closed cell spray and then put the foam board over it?

Windows on Wash 01-19-2012 06:25 AM

Using spray foam between the studs is probably overkill if you are going with a rigid foam topper.

Use a good fiberglass batt, cover it with rigid foam, and then check code and make sure the foam is approved for exposure use. If not, you will need to cover it with an ignition barrier.

Make sure you have enough foam depth to prevent condensation on the framing and/or be careful to make sure you vapor retarder details are correct.

yaemish 01-19-2012 08:16 AM

Right now I have fiberglass that is exposed and subject to wind washing. I feel like the stuff is next to useless in this type of installation. I would just like to know from someone with experience if this is something that will work effectively.

Windows on Wash 01-20-2012 08:13 AM

Covering the FG with a rigid foam or other air barrier will prevent the wind degradation of the R-Value.

CC foam definitely has a higher R-Value than FG but comes at a price. If you are using rigid foam, fill the bays with FG or other more batt type insulation, seal all the seams in the rigid and be done with it.

Make sure the rigid foam is covered with the code required ignition barrier or get approval.

Gary in WA 01-20-2012 11:55 PM

F.g. is good:

You have to maintain that air barrier:


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