Prepare the roof. Build the rafters, and secure the 5/8-inch CDX sheathing material. Leave a gap on both sides of the roof near the ridge so air can escape. Place a ridge cap over the ridge to hide the gaps.
Temporarily secure long plastic forms between the rafters on the inside of the ceiling. These forms are only 1 inch thick and can be purchased at any hardware store. They run from the bottom of the rafters up to the ridge.
Spray the foam insulation between the rafters on the inside of the house. Start slowly. The plastic forms provide spacers for ventilation.
Cut off any excess insulation with a large saw. A utility knife is too small and would take forever to cut away at insulation.
Remove the plastic forms from between the rafters. There is now ample space available for air to travel up through the rafters, out through the ridge beam or down through the eave vents.