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Friction Fit Foam Board in Vented Cathedral?

Towsonite | Posted in General Questions on

While I know the general topic has been beaten to death, I didn’t know if there would be an issue with just friction-fitting foam board in a vented cathedral. I’m assuming sprayfoaming in place would give some minimal benefit by eliminating convection on the sides of the boards, but the greater concern is obviously whether or not this could cause some type of moisture issue. It will be taped to the rafters at two or more layers of the foam board to minimize convection.  My thought is that spray foaming the boards in place would not be necessary, but I wanted some other opinions on this one.

General Info: I’m in Maryland, the rafters are 2×6, so I can get 4.5″ polyiso and have 1″ vent space. I plan to put a 1/2″ polyiso board on the underside of rafters before affixing drywall, so a grand total of 5″ of polyiso.  Air-sealing will be at drywall level.

Thanks for your help.

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  1. Expert Member
    Akos | | #1

    Because of the thermal bridging of the studs, you are not increasing the assembly much with the cut and cobble polyiso. Lot of work for not much gain. Since your assembly is vented, it will work, no need to foam anything in place.

    You can get the same assembly performance for much less work by installing R15 high density batts between the rafters and putting 1" of polyiso under the rafters. HD batts don't suffer from wind washing thus you can skip any baffles. Make sure to install them so they are flush on the inside to keep the vent space clear.

    Even better, increase the 1" under the rafters to 2.5" for a pretty high value roof. You can get 4" drywall screws from any commercial drywall place and hang the drywall directly over the foam.

    Don't forget to relocate any devices boxes to match the new finished ceiling.

  2. Towsonite | | #2

    Bumping up, does anyone else have any thoughts on whether or not the above can cause moisture issues?

    A note re insulating and thermal bridging that I should have included in the original, the spacing is usually around 24", so the thermal bridging will be approximately 1/3 less than usual. I would like to have added more, but I have 1/2" polyiso under the rafters gives me about r3.5 to take off a good chunk of the thermal bridging as well. This is for the area between kneewalls and ceilings so it's not a huge area in any case. It's a cape, but it needs to be vented for too many reasons to list here.

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