Standard framing assumes tapering of insulation depth around the perimeter with resultant decrease in thermal resistance . An increased R value is assumed in the center of the ceiling due to the effect of piling leftover insulation. Advanced framing assumes full and even depth of insulation extending to the outside edge of exterior walls . Advanced framing does not change from the default value. 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.