Many attics, including those under low-sloped roofs, do not have enough space for adequate insulation at the edge of the attic floor. If soffit insulation requires a baffle to keep a ventilation opening against the sheathing, there may only be about 100 mm (4 in.) of space left for insulation. This restricted insulation level will allow the snow to melt just above the overhang and promote ice damming.

Try to put the best insulation possible at that edge to reduce heat loss. Spray foam is ideal as it air seals as well as insulates. Cut pieces of extruded polystyrene will help as well. Install a piece of extruded polystyrene 25 mm (1 in.) from the sheathing to maintain the ventilation air space and then fill between this foam board and the attic floor with insulation (see Figure 5-22).