Rigid Foam Above The Roof Sheathing

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.

Rigid foam above the roof sheathing.

Installing rigid foam insulation on top of roof sheathing is a great option for commercial buildings and homes with limited space. Apply a continuous air control membrane to existing roof sheathing or tape all joints between roof sheathing panels with an acrylic or butyl adhesive. Rigid foam sheathing is a form of insulation continuous insulation to be exact that s applied to the exterior of the building. This was then foamed to complete the air seal.

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. 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. Go back and check out photograph 10. 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.

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. 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.

Rigid insulation sheathing is made of a rigid plastic foam that is typically sold in 4 8 or 4 10 foot boards. Johns manville polyiso foam sheathing insulation boards provide a continuous layer of insulation to reduce thermal bridging and improve energy efficiency. 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. Over the foam you will need something to nail the roofing to usually a solid sheet of plywood or osb.