Rigid Foam Insulation Above The Roof Sheathing

Polyisocyanurate insulation above the sheathing and approximately 91 4 in.

Rigid foam insulation above the roof sheathing.

The following measures are necessary to ensure high levels of performance for this roof retrofit strategy. 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. Of cellulose insulation in the rafter cavities the roof shown here has an r value of approximately 63. However for the best performance and for when space comes at a premium it s tough to beat rigid foam insulation.

The main issue people encounter with insulation above sheathing is when it expands. Effective water control is essential to the performance of any attic roof system. Insulated roof assemblies can be vented or unvented. Over the foam you will need something to nail the roofing to usually a solid sheet of plywood or osb.

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. Roof edge detail overhang extended to hide the thickness of the over roofing rigid insulation. There are at least two good reasons why this approach makes more sense than installing the insulation under the roof sheathing. 3 benefits of rigid foam sheathing.

Gable end detail overhang cut and then rebuilt to hide the thickness of the over roofing rigid 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. Products provide superior insulating properties. 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.

Plywood has an r value of 1 25 and fiberglass batts have an r value of 3 14. 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. More insulation on the exterior is good.

Installed a new fully adhered membrane to the top of the old board sheathing prior to adding all that rigid insulation. 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. 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.