Rigid Foam Above The Roof Sheathing

Over the foam you will need something to nail the roofing to usually a solid sheet of plywood or osb.

Rigid foam above the roof sheathing.

Avoids nails or screws which could penetrate the metal roof and cause leaks. Apply a continuous air control membrane to existing roof sheathing or tape all joints between roof sheathing panels with an acrylic or butyl adhesive. However for the best performance and for when space comes at a premium it s tough to beat rigid foam insulation. 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.

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. The exposed edges of the 3 layers of rigid insulation are no longer exposed. Of cellulose insulation in the rafter cavities the roof shown here has an r value of approximately 63. The nailbase panels are installed with long screws that have flat heads.

Then a second layer of plywood sheathing and asphalt shingles following the steps explained in this article. 4 of polyiso under 4 of eps all above the original roof sheathing. High moisture areas may benefit from leaving a small air space between roof sheathing and the rigid foam insulation panels. Box beam gable framing new gable overhang is framed in the form of a box beam hanging off of a new layer of roof plywood roof sheathing cantilevered over the top of the 3 layers of rigid insulation.

Rigid foam may shrink so some builders are wary of depending on the rigid foam to be your air barrier. Here is the controversy. Cut each sheet to the desired size using a utility knife then fasten it in place using staples or adhesive. How to retrofit a roof to a high performance roof with insulating sheathing above the roof deck plus cavity insulation in roof framing remove existing roofing and non adhered roof underlayment.

The main issue people encounter with insulation above sheathing is when it expands. Deck for a thick roof rigid foam insulation installed above the roof assembly can create an energy smart roof. In any case the foam needs to be thick enough to keep the interior of the foam above the dew point in your climate and the seams need to be carefully taped with a high quality compatible tape. Plywood has an r value of 1 25 and fiberglass batts have an r value of 3 14.

With three layers of 11 2 in. To save a step i would recommend using a nailbase panel foam preadhered to osb or sometimes plywood on one side only see photo facing page. Polyisocyanurate insulation above the sheathing and approximately 91 4 in. Nail it around the edges with roofing nails.