Iso Foam In Unvented Attic Ceiling

If you don t want to use sips or nailbase there are four basic approaches.

Iso foam in unvented attic ceiling.

Unvented roof systems where the insulation and air barrier are applied directly to the underside of the roof deck are common in applications where designers are seeking cost effective and energy efficient roof assemblies. Foam board insulation would be a more economical especially if you are supplying the labor see photo. Most unvented attics incorporate spray polyurethane foam spf on the underside of the roof deck and attic walls. Johns manville aptm foil faced insulation sheathing board is an excellent choice for insulating attics and cathedral vaulted ceilings.

When applied to the. In the normal vented attic areas flat ceiling but very limited head room accessibility in attic is there any advantage to sheathing the ceiling with rigid foam board below the ceiling joists infilled with batt insulation and overlaying with drywall as opposed a thicker layer of more inexpensive attic batt insulation no rigid foam overlay. Spray foam is a very expensive option however. However designers of compact roof assemblies must consider vapor permeance and arrangement of the roofing materials to produce acceptable hygrothermal performance and low.

Ill use rockwool batts to fill the rest of the cavity 3 5 and do a 1 foam board unfaced along the underside of the rafters for thermal bridging. This is done by using what is referred to as air impermeable insulation such as rigid foam board or spray foam. Polyiso provides one of the highest r values per inch of any rigid insulation r 6 at 1 inch. Unvented roof assemblies should be insulated either with air impermeable insulation rigid foam or spray polyurethane foam or with a combination of foam and air permeable insulation.

Two acceptable methods for insulating an unvented attic assembly in all climates are as follows. 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. Im using 2 iso foam board foil faced against my shiplap with one part can foam around the edges to seal it up. In general closed cell foam is the best choice for unvented cathedral ceilings.

Attics and cathedral vaulted ceilings. Foam filled cathedral ceilings may make sense in some cases. Foil faced iso board can be cost effective insulation for finished attics and cathedral ceilings. Vented ceiling across rafters.

An alternate to spray foam insulation in an unvented attic is plenum trusses which allow you create a reverse bulkhead above the ceiling plane that is insulated. This allows you to install your bulkheads above the ceiling plane without the need for spray foam insulation. It is also one of the most expensive and most building inspectors will not allow it to be left uncovered for fire code reasons.