Unbaffled standard framed scissors truss attics are assumed to have a void fraction of 0.016. Insulation is assumed to be fiberglass batts installed in roof joist cavities. In the vented case, at least 1.5 inches between the top of the batts and the underside of the roof sheathing is left open for ventilation in each cavity. A ventilation rate of 3 air changes per hour is assumed. In the unvented or dense pack case, the ceiling cavity is assumed to be fully packed with insulation, leaving no space for ventilation .
Rigid insulation is applied to the top of roof decking with no space left for ventilation .
Roofing materials are attached directly on top of the insulation. Framing members are often left exposed on the interior side. U o for metal framed truss assemblies from approved laboratories shall be used, when such data is acceptable to the building official. Ceiling sheathing installed between bottom chord and drywall. DCLU moved to Key Tower in Spring 2001.