August 30, 2007


Unvented cathedralized attic (UCA) assemblies, created by eliminating ventilation and by moving the thermal insulation and air barrier from the ceiling plane to the rafters, immediately below the roof deck, are increasingly common in low-rise residential construction in the hot-humid and hot-dry southern United States. Unvented cathedral ceilings (UCCs) are similar to UCAs with the exception that the interior finish is also installed on the underside of or between the rafters rather than on the underside of the ceiling joists or collar ties.  The test program described in this paper sets out to determine whether or not an assembly that meets the new IRC code requirements but is constructed without a vapor barrier and using an air impermeable, vapor permeable, low-density, open-cell sprayed polyurethane foam insulation can perform satisfactorily in the cold wet climates of Seattle, WA and Vancouver, BC (Zone 4C).