Rigid Foam Under Roof Sheathing

Rigid insulation layers exterior to existing roof sheathing must provide sufficient thermal insulation r value to control condensation risk see climate specific guidance.

Rigid foam under roof sheathing.

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 insulation also known as insulating boards provide high r values. Install it anywhere from the roof to the foundation. Rigid insulation is one of the best tools to reduce your energy costs.

Nail it around the edges with roofing nails. Install nailbase for cladding. In my local area there is a green building consultant that recommends a ci panel which is basically eps foam glued to osb. They keep your home at a comfortable temperature without the need to crank your heat.

1 inch and 2 inch thicknesses are common. 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 way this is installed would be the osb to the exterior. You can even place it under the foundation slab.

Install rigid insulation board in multiple layers with joints offset vertically and horizontally between adjacent layers. Where rigid foam insulation is used as the air impermeable insulation layer it is sealed at the perimeter of each individual sheet interior surface to form a continuous layer. 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. Most unvented attics incorporate spray polyurethane foam spf on the underside of the roof deck and attic walls.

A leak free roof membrane that is fully adhered to the roof sheathing should be installed. The boards are available in several thicknesses and r values. 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.