Rigid Foam Under Roof Sheathing

If the sheathing is on the exterior of the foam then drying isn t a concern because interior vapor will never reach the sheathing and the sheathing will always dry to the exterior especially with any rainscreen.

Rigid foam under roof sheathing.

Install rigid insulation board in multiple layers with joints offset vertically and horizontally between adjacent layers. Rigid insulation sheathing is made of a rigid plastic foam that is typically sold in 4 8 or 4 10 foot boards. Continuous rigid insulation is a construction solution that provides a thermally efficient building enclosure. The purpose of the requirement that rigid foam installed above roof sheathing meet certain minimum r values is to ensure that the roof sheathing stays warm enough during the winter to avoid moisture accumulation and possible sheathing rot.

Rigid foam sheathing is a form of insulation continuous insulation to be exact that s applied to the exterior of the building. 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. Gable end detail overhang cut and then rebuilt to hide the thickness of the over roofing rigid insulation. Roof edge detail overhang extended to hide the thickness of the over roofing rigid insulation.

This construction approach was first used in the 1970s and it has been implemented much more widely since it was first included in the international code council s icc model building code in 2007 irc section 806 4. Exterior foam sheathing is often installed on existing homes as an energy saving retrofit when new roofing or siding is needed. Installing rigid foam insulation on top of roof sheathing is a great option for commercial buildings and homes with limited space. Installed a new fully adhered membrane to the top of the old board sheathing prior to adding all that rigid insulation.

Nail it around the edges with roofing nails. Rigid insulation layers exterior to existing roof sheathing must provide sufficient thermal insulation r value to control condensation risk see climate specific guidance. Install nailbase for cladding.