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Closed or open cell foam in our to-be-finished attic in Maine? And what kind of venting is necessary?

by LiveModern Webmaster last modified Jan 04, 2012 02:13 AM
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by last modified Feb 21, 2011

We have a 1914 foursquare in Maine and are planning spray foam inside rafters to finish the attic for more living space. The knee walls are not in yet: Does the foam have to stop before the sill plate? How does venting work? Are soffit vents necessary? Given these parameters, is open or closed cell preferable?



Given the Maine climate, you will want to try and maximize your R-value.

  • Guessing that the roof rafters are only 2x8 based on the 1914 construction date, I would highly recommend using about 5-6" of closed cell spray foam.
  • If you have 2x10 then you can just use 10" of open cell spray foam.

Venting is not necessary, but if the rest of the house is airtight you will want to install some sort of air exchanger to bring fresh air into the house.

But you will not need to vent your roof any longer as long as the foam is installed properly from the top of the sill plate to the ridge.



For more information:

Read "Should we insulate the attic of our 1926 bungalow with spray foam insulation?" a Q&A answered by Anthony Addesso.

Also, read "I'm converting my attic into a non-vented conditioned space. Should I use open cell or closed cell foam to insulate?" a Q&A answered by Steve Saunders.




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