Rigid Foam Insulation Above The Roof Sheathing

Rigid foam insulation also known as insulating boards provide high r values.

Rigid foam insulation above the roof sheathing.

1 inch and 2 inch thicknesses are common. Rigid insulation sheathing is made of a rigid plastic foam that is typically sold in 4 8 or 4 10 foot boards. You can even place it under the foundation slab. Rigid insulation provides thermal protection and it can also serve as an air and moisture barrier.

The exposed edges of the 3 layers of rigid insulation are no longer exposed. Nail it around the edges with roofing nails. 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. Almost every house i see with exterior rigid foam in my online wonderings is covered with blue and pink xps the osb sheathing is nailed directly to the studs and is completely covered with the rigid foam.

Rigid foam boards used in ceiling insulation are often made of either cellulose fiber fiberglass polystyrene or polyurethanes. 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. Most local building codes require that rigid foam boards made from polyurethane or polystyrene be covered with a 15 minute fire rated thermal covering if they are used on the inside of the house. The way this is installed would be the osb to the exterior.

Benefits of rigid foam on top of sheathing. Install it anywhere from the roof to the foundation. This type of insulated roof assembly limits thermal bridging through rafters. More insulation on the exterior is good.

Nailbase must be vented in high snow load areas. Many homes in more urban areas or other places with dense populations are opting for this method because they can better optimize space. Install roofing underlayment flashing and cladding as per best practices. The exposed edges of the 3 layers of rigid insulation are to be sealed with another layer of membrane.

After installing several layers of rigid foam above the roof sheathing these workers installed plywood above the rigid foam. Rigid insulation layers exterior to existing roof sheathing must provide sufficient thermal insulation r value to control condensation risk see climate specific guidance. The boards are available in several thicknesses and r values. Box beam gable framing new gable overhang is framed in the form of a box beam hanging off of a new layer of roof plywood roof sheathing cantilevered over the top of the 3 layers of rigid insulation.

Installing rigid foam insulation on top of roof sheathing is a great option for commercial buildings and homes with limited space. Rigid insulation is one of the best tools to reduce your energy costs. Over the foam you will need something to nail the roofing to usually a solid sheet of plywood or osb.