Rigid Foam Above The 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 above the roof sheathing.

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. 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. This was then foamed to complete the air seal. Rigid insulation sheathing is made of a rigid plastic foam that is typically sold in 4 8 or 4 10 foot boards.

Go back and check out photograph 10. Johns manville polyiso foam sheathing insulation boards provide a continuous layer of insulation to reduce thermal bridging and improve energy efficiency. On the front of the roof edges the original roof deck sheathing was cut back flush with the wall and the fully adhered membrane folded down around the rafter tails to catch the wall water control layer and air control layer. Over the foam you will need something to nail the roofing to usually a solid sheet of plywood or osb.

Polyiso consists of a uniform closed cell polyisocyanurate foam core and provides one of the highest r values per inch of any rigid insulation. Continuous rigid insulation is a construction solution that provides a thermally efficient building enclosure. Apply a continuous air control membrane to existing roof sheathing or tape all joints between roof sheathing panels with an acrylic or butyl adhesive. How to retrofit a roof to a high performance roof with insulating sheathing above the roof deck plus cavity insulation in roof framing remove existing roofing and non adhered roof underlayment.

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. Installing rigid foam insulation on top of roof sheathing is a great option for commercial buildings and homes with limited space. Nail it around the edges with roofing nails. The nailbase panels are installed with long screws that have flat heads.