Unvented Attic Open Cell Foam

Test with a blower door to commission it.

Unvented attic open cell foam.

Monitor the attic s relative humidity and temperature. This is done by using what is referred to as air impermeable insulation such as rigid foam board or spray foam. Thanks for sharing the language of your proposed code change proposal concerning unvented attics insulated with open cell spray foam. An alternate to spray foam insulation in an unvented attic is plenum trusses which allow you create a reverse bulkhead above the ceiling plane that is insulated.

Open cell foam has an r value of about 3 5 to 3 7 per inch while closed cell foam has an r value of about 6 per inch. The myth that open cell is more economical is bullshit. Air impermeable insulation typically spray foam installed to the underside of the roof sheathing. Open cell foam weighs 1 2 pound per cubic foot and closed cell weighs 2 lbs per cubic foot.

Don t install closed cell foam. Lately discussions related to open cell spray foam in unvented attics vapor drives and humidity levels have appeared. If you want to install rigid foam on top of the roof sheathing it s ok to install open cell spray foam underneath because open cell spray foam is vapor permeable. Make sure the installers get the attic sealed airtight.

The proposal you shared doesn t address the fire safety concerns that have been raised relating to the installation of forced air registers or return air grilles in attics with spray foam that lacks a layer of 1 2 inch drywall. Open cell spray foam can also be used to create an unvented attic in cold climate zones provided code requirements for a vapor retarder are followed. Dupont roofliner is a vapor barrier and therefore cannot be used on an unvented roof. There is 300 difference there but the cost they give you is about a third difference and open cell foam at 12 inches will not give you the performance of 2 inches of closed.

Don t use open cell spray foam in cold climates iecc climate zones 5 and higher without a vapor retarder. Two acceptable methods for insulating an unvented attic assembly in all climates are as follows. Another study s computer model indicated that open cell foam insulation is riskier than closed cell foam in all us climate zones.