Unvented Attic Open Cell Foam

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.

Unvented attic open cell foam.

Make sure the installers get the attic sealed airtight. 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. 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. Two acceptable methods for insulating an unvented attic assembly in all climates are as follows.

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. Thanks for sharing the language of your proposed code change proposal concerning unvented attics insulated with open cell spray foam. Closed cell foam can also increase the racking strength of attic walls which. Air impermeable insulation typically spray foam installed to the underside of the roof sheathing.

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. 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. The myth that open cell is more economical is bullshit.

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. 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. This is done by using what is referred to as air impermeable insulation such as rigid foam board or spray foam.

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. 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.