Rigid Foam Under Roof Deck

When installing a foil faced foam board insulation.

Rigid foam under roof deck.

Lstiburek takes blame for bad language. Install nailbase for cladding. All three types of rigid foam insulation expanded polystyrene eps extruded polystyrene xps and polyisocyanurate are suitable for use on the exterior of walls and roofs though they do not perform equally. Where rigid insulation is installed above the roof deck or air impermeable spray foam insulation is installed under the roof deck condensing surface temperatures are said to be controlled.

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. Install roofing underlayment flashing and cladding as per best practices. For a roof you can always install 2x4 sleepers on top of the foil faced foam followed by osb or plywood. In addition to installing the foam board on the underside of the roof deck you can attach it to the bottom edges of the.

Rigid insulation layers exterior to existing roof sheathing must provide sufficient thermal insulation r value to control condensation risk see climate specific guidance. Nailbase must be vented in high snow load areas. That way you get an air space and an additional r 2 or r 3 for the assembly not a lot but something. If you want to install rigid foam above your roof sheathing in climate zone 3 then 1 2 inch of rigid foam board is not ideal this rigid foam layer needs a minimum r value of r 5 and that s impossible with 1 2 inch of foam.

No air space no benefit. At r 4 it also has the lowest r value per inch. Moisture generated indoors or that infiltrates into the home is responsible for the bulk of the moisture in an attic insulated with spray foam on the underside of the roof sheathing. The 6 inches of rigid insulation added to the top of the existing roof deck needed to be done in a manner that was invisible.

They had to look original. When installing rigid insulation in an attic to be finished as a living space you must provide a route by which hot air. The trim the frieze boards the fascia and the overhangs all had to look like they belonged. Rigid foam under roof sheathing alaskan andy is right.