Foam Insulation On Roof Sheathing

If installing closed cell spray foam or other low permeance insulation to the interior it is very important to verify that the roof sheathing and framing are dry e g.

Foam insulation on roof sheathing.

Insulated roof assemblies can be vented or unvented. 12 15 moisture content or less before installing cavity insulation. In warm climates zones 1 through 4 low density open cell spray foam insulation is the typical choice since it is vapor permeable and will allow drying of the exterior sheathing and roof framing if they happen to get wet via diffusion into the interior of the building. R 7 5 scored squared edge rigid foam board insulation sheathing.

Plywood has an r value of 1 25 and fiberglass batts have an r value of 3 14. Open or closed cell spray foam is applied to the underside of the roof sheathing and additional fiberglass or cellulose insulation is blown in as a cost saving method for meeting high insulation requirements and filling in the cavity space between the rafters to the ceiling deck. Open cell spray foam insulation is risky in all climate zones we already know open cell spray foam is risky in cold climates. 3 benefits of rigid foam sheathing.

Moisture from indoors can diffuse through the foam and find the cold roof sheathing where it accumulates and eventually rots the roof. Owens corning foamular 150 1 1 2 in. Featuring integrated moisture air and thermal protection zip system r sheathing completely reimagines traditional wall assemblies by streamlining exterior water air and thermal management. More effective insulation with r values ranging from 3 6 to 8 0 rigid foam sheathing has much better insulation per inch than other materials i e.

Both papers basically came to the same conclusion. Johns manville r panel rigid roof insulation board offers r values of up to r 23 6 providing high thermal efficiency. Unvented roof strategies with open cell and closed cell spray polyurethane foam insulation sprayed to the underside of roof sheathing have been used since the mid 1990 s to provide durable and efficient building enclosures. There are lots of different ways to insulate this type of roof but one of the best methods calls for the installation of rigid foam insulation above the roof sheathing.

There are at least two good reasons why this approach makes more sense than installing the insulation under the roof sheathing.