Rigid Foam Under Roof Sheathing

Roof edge detail overhang extended to hide the thickness of the over roofing rigid insulation.

Rigid foam under roof sheathing.

Gable end detail overhang cut and then rebuilt to hide the thickness of the over roofing rigid insulation. Most unvented attics incorporate spray polyurethane foam spf on the underside of the roof deck and attic walls. Exterior foam sheathing is often installed on existing homes as an energy saving retrofit when new roofing or siding is needed. Rigid insulation sheathing is made of a rigid plastic foam that is typically sold in 4 8 or 4 10 foot boards.

Rigid foam sheathing is a form of insulation continuous insulation to be exact that s applied to the exterior of the building. Nail it around the edges with roofing nails. 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. 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.

Many homes in more urban areas or other places with dense populations are opting for this method because they can better optimize space. Install rigid insulation board in multiple layers with joints offset vertically and horizontally between adjacent layers. Installing rigid foam insulation on top of roof sheathing is a great option for commercial buildings and homes with limited space. Continuous rigid insulation is a construction solution that provides a thermally efficient building enclosure.

Installed a new fully adhered membrane to the top of the old board sheathing prior to adding all that rigid insulation. Install nailbase for cladding. You can install rigid foam on the inside of a wall as well but adding exterior foam increases the r value of a wall or roof without eating up interior living space.