One problem area in many roof designs occurs at the eave, where there is often insufficient space for full insulation without blocking air flow from the soffit vents. Often the insulation is compressed to fit the space, diminishing its R-value.
For a truss roof, consider raised heel or oversized (cantilevered) trusses that form elevated overhangs in combination with rafter baffles and soffit dams. These should provide clearance for both ventilation and full-height insulation. Use of 2to 2!-foot overhangs also provides more room for insulation at the wall junction and additional window shading.
In stick-built roofs, where rafters and ceiling joists are cut and installed on the construction site, laying an additional top plate across the top of the ceiling joists at the eave will raise the roof height, prevent compression of the attic insulation, and permit ventilation. When installing a raised top plate, place a band joist at the open joist cavities of the roof framing. The band joist helps prevent windwashing of the attic insulation—where air entering the soffit vents flows through the attic insulation—which can reduce attic insulation R-values on extremely cold days or add moisture to the insulation. The band joist also serves as a soffit dam for the insulation.