Rigid Foam Under Roof Deck

When installing rigid insulation in an attic to be finished as a living space you must provide a route by which hot air.

Rigid foam under roof deck.

It s not coming from above the roof and it s not some new moisture source resulting from the spray foam. 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. Install roofing underlayment flashing and cladding as per best practices. Install nailbase for cladding.

Rigid insulation layers exterior to existing roof sheathing must provide sufficient thermal insulation r value to control condensation risk see climate specific guidance. In addition to installing the foam board on the underside of the roof deck you can attach it to the bottom edges of the. 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. Rigid foam under roof sheathing alaskan andy is right.

At r 4 it also has the lowest r value per inch. Eps is the most vapor permeable of the three types. That way you get an air space and an additional r 2 or r 3 for the assembly not a lot but something. 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.

When installing a foil faced foam board insulation. 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. For a roof you can always install 2x4 sleepers on top of the foil faced foam followed by osb or plywood.

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 trim the frieze boards the fascia and the overhangs all had to look like they belonged. Nailbase must be vented in high snow load areas. Lstiburek takes blame for bad language.