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I am reroofing my house. I am taking the opportunity to pull up the decking above the cathedral ceiling and adding air baffles to help cool the roof and reduce ice dams.

In order to accomplish this, I am using 1 inch rigid foam board pushed down two inches with spacers between board and roof deck. I am using the foam board to regain some of the reduced r value from compressing the fiberglass insulation, but also for the rigidity to help compress the fiberglass.

Should I air seal all the seams between sheets of foam board, or leave them open to allow moisture in the fiberglass a path to escape? The fiberglass is foil backed with the foil against the ceiling. Ceiling is tongue and groove wood, so probably pretty permeable.

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    Is there a vapor barrier at ceiling level? – isherwood Sep 9 at 16:36
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    Have you considered using vents made for a cathedral roof? I couldn't find the link to the exact product I used (pink foam, but the same idea), but thses ought to do the trick: sprayfoamsys.com/product/… – Ring Sep 9 at 16:53
  • Vapor barrier at ceiling is foil faced fiberglass insulation, which is probably not terribly air tight. – Frank f Sep 9 at 17:20
  • if those accuvent baffles are anything like the baffles locally available at the box stores, they will be tough to install because they are pretty flimsy. – Frank f Sep 9 at 17:21

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