Rigid Foam Insulation Above The Roof Sheathing

Deck for a thick roof rigid foam insulation installed above the roof assembly can create an energy smart roof.

Rigid foam insulation above the roof sheathing. 3 benefits of rigid foam sheathing. 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. Foamular extruded polystyrene xps rigid foam insulation offers foamular extruded polystyrene xps rigid foam insulation offers a comprehensive line of products choose the strength you need from 15 to 60 psi and exceptional moisture resistance. Gable end detail overhang cut and then rebuilt to hide the thickness of the over roofing rigid insulation.

Polyisocyanurate insulation above the sheathing and approximately 91 4 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. With three layers of 11 2 in. The following measures are necessary to ensure high levels of performance for this roof retrofit strategy.

Plywood has an r value of 1 25 and fiberglass batts have an r value of 3 14. Products provide superior insulating properties. Thermax sheathing is a non structural rigid board insulation consisting of a glass fiber infused polyisocyanurate foam core laminated between 1 0 mil smooth reflective aluminum facers on both sides. More insulation on the exterior is good.

Of cellulose insulation in the rafter cavities the roof shown here has an r value of approximately 63. Installed a new fully adhered membrane to the top of the old board sheathing prior to adding all that rigid insulation. The main issue people encounter with insulation above sheathing is when it expands. High moisture areas may benefit from leaving a small air space between roof sheathing and the rigid foam insulation panels.

Performance imperatives for high r value roof retrofit using insulating sheathing above the roof deck plus cavity insulation in roof framing. There are at least two good reasons why this approach makes more sense than installing the insulation under the roof sheathing. The glass fiber reinforcement contributes to improved fire performance and dimensional stability. Effective water control is essential to the performance of any attic roof system.

Gba taught you that it s a great idea to install rigid foam on the exterior side of your wall sheathing as long as a the rigid foam is thick enough to keep your sheathing above the dew point during the winter and b your walls don t have any interior polyethylene. 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. However for the best performance and for when space comes at a premium it s tough to beat rigid foam insulation.