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123ozzie 12-10-2011 12:55 PM

Foil faced rigid insulation question
 
I'm converting attic space to a living area. Because the roof rafters are 2x6, I'm considering using ridig XPS or ISO to increase the R value.Aside from cost issues, are there any advantages or disadvantages to using these products? Also, foil face boards offer a higher R value. Lastly, if I use the foil faced, do I still need a vapor retarder? Thanks in advance

Windows on Wash 12-10-2011 05:22 PM

Where is the home located?

The benefit to the boards is a much higher R-Value per inch and their lack of sensitivity to wind currents.

Post up a picture of the attic and you will get more direct feedback.

123ozzie 12-11-2011 11:24 AM

Quote:

Originally Posted by Windows on Wash (Post 790693)
Where is the home located?

The benefit to the boards is a much higher R-Value per inch and their lack of sensitivity to wind currents.

Post up a picture of the attic and you will get more direct feedback.


I'm located in zone 5. I'm confused mostly on wether or not to use the foil faced foam board. Does the asphalt paper between the shingles and the roof sheathing come into play? I know there isn't supposed to be a vapor retarder on both sides of the insulation. Also, can I double up two sheets of foil faced or not. Having trouble downloading pics. The computer and I sometimes don't see eye to eye.

Windows on Wash 12-11-2011 01:18 PM

Your finished drywall will be your vapor retarded in this case.

http://www.buildingscience.com/documents/published-articles/pa-crash-course-in-roof-venting/view


At 2x6, you are only going to get R-35 at max into that assembly and that does not count on the R-Value drop as a result of the thermal bridging.

Vapor retarder is going to go the warm side (i.e. inside space) in this application. You are going to need to furr out the structure in order to get really good insulation depth and proper R-Value out of that roof.


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