Rigid Foam Under Roof Sheathing

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.

Rigid foam under roof sheathing.

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. You can even place it under the foundation slab. Plywood has an r value of 1 25 and fiberglass batts have an r value of 3 14. Installed on the roof exterior foam makes the living space below more comfortable and reduces the likelihood of ice dams.

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. Almost every house i see with exterior rigid foam in my online wonderings is covered with blue and pink xps the osb sheathing is nailed directly to the studs and is completely covered with the rigid foam. Nail it around the edges with roofing nails. Install it anywhere from the roof to the foundation.

We live in zone 5 suburb of chicago and are hopefully about to put rigid foam above the roof sheathing on just the northeast roof portion of our roof. 4 of polyiso under 4 of eps all above the original roof sheathing. Install rigid insulation board in multiple layers with joints offset vertically and horizontally between adjacent layers. The illustration shows two layers of osb or plywood roof sheathing.

The way this is installed would be the osb to the exterior. A lower layer conventionally nailed to the rafters and an upper layer installed as a nailing surface for the roofing material on top of the rigid foam. Apply continuous air control membrane to existing roof sheathing or implement continuous air control at roof sheathing. Remove existing roofing and non adhered underlayment to expose roof sheathing.

In my local area there is a green building consultant that recommends a ci panel which is basically eps foam glued to osb. Rigid insulation is one of the best tools to reduce your energy costs. Rigid foam insulation also known as insulating boards provide high r values. Start at the bottom of the roof and place the first piece of rigid foam insulation against the metal flashing starting at the center line.