Unvented Attic Open Cell Foam

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.

Unvented attic open cell foam.

Some spray foam manufactures have tested primer coating products that result in the formulation of a class ii vapor retarder directly on the surface of the open cell spray foam. Air impermeable insulation typically spray foam installed to the underside of the roof sheathing. Don t install closed cell foam. This is done by using what is referred to as air impermeable insulation such as rigid foam board or spray foam.

Two acceptable methods for insulating an unvented attic assembly in all climates are as follows. The myth that open cell is more economical is bullshit. 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.

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. It appears that many opinions presented are based on limited building science field research generalize a complicated building science situation and doesn t fully explain the continued successful long term performance of spray foam insulated unvented attics in hot climates. Another study s computer model indicated that open cell foam insulation is riskier than closed cell foam in all us climate zones. Thanks for sharing the language of your proposed code change proposal concerning unvented attics insulated with open cell spray foam.

Monitor the attic s relative humidity and temperature. Test with a blower door to commission it. 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. 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. Dupont roofliner is a vapor barrier and therefore cannot be used on an unvented roof. Don t use open cell spray foam in cold climates iecc climate zones 5 and higher without a vapor retarder.