Rigid Foam Above Roof Deck

Contributing editor michael maines.

Rigid foam above roof deck.

Most unvented attics incorporate spray polyurethane foam spf on the underside of the roof deck and attic walls. Included with the article was a drawing illustrating a method using layers of polyiso with stacked 2x4 s at the eave. With three layers of 11 2 in. You might be able to get away with keeping the existing roofing in place but i would recommend removing the shingles for a few reasons.

The rigid insulation over roof would have controlled the temperature of the condensing surface the underside of the roof deck so that i would not have had to use the spray foam except at the perimeter to provide the air seal to the tops of the walls. Deck for a thick roof rigid foam insulation installed above the roof assembly can create an energy smart roof. The roof deck can be plywood metal concrete or wood planks. Rigid foam insulation installation prepare roof deck.

Generally 40 of the total assembly r value should be above the roof deck. My question was prompted by your article about using rigid foam insulation above the roof deck. Then 2x4s usually installed on the flat are installed above the rigid foam. The first step with roof installation is to prepare the roof deck by getting rid of all dirt dust and contaminants.

Polyisocyanurate insulation above the sheathing and approximately 91 4 in. These 2x4s are extended at the eaves and rakes to create a roof overhang. Of cellulose insulation in the rafter cavities the roof shown here has an r value of approximately 63. 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.

Owens corning insulation products help improve thermal performance and control moisture in commercial institutional and. First removing the shingles allows you to inspect the roof deck and address any questionable areas before encasing the whole thing in foam. High r value unvented roof assemblies will require a proportionally larger amount of insulation above the roof deck for condensation control. The bottom layer of a single ply membrane roofing system is the roof deck or substrate.

The persist approach shown in that article includes rigid foam above the roof sheathing. However the drawing assumed an attic below with additional batt insulation.