A properly vented roof is vital to the performance of the insulation and the roofing system itself. An improperly or inadequately vented roof can lead to condensation on the underside of the roof. This condensation can cause the roof structure to rot and can drip into the insulation, ruining it and causing mould.
The best installations consist of vents placed in the soffits and vents placed high on the roof; this assures a continuous flow of air along the underside of the roof.
Calculate the area of the vents. Determine the area of your attic floor, in square feet (length times width) and divide by 300; this will give you the area of the vents. Half of the vent area should be provided by soffit vents and half by roof vents. Vents are rated by their net free area, which is usually expressed in square inches; 144 square inches equals one square foot.
Install the soffit vents. Using the reciprocating saw, cut a 14-by-6-inch hole in the soffit, centred between the trusses or rafters. Nail a 16-by-8-inch soffit vent over the hole.
Install the roof vents. Measure down 16 inches from the top of the roof, this is where the top of the roof vent goes. Centred between the rafters or trusses, cut a hole for the roof vent; use the reciprocating saw to cut through the roof sheathing and shingles. The size and shape of the required hole varies, but an 8½-inch round hole is common.
Slide the square flange on the roof vent under the shingles and centre the vent over the hole. It may be necessary to pry the shingle nails out of the way. Lift up the shingles on each side and attach the roof vent by nailing through the flange with roofing nails. When properly installed, all but the bottom one or two inches of the flange should be under the shingles.
If you are building a new house, or re-roofing an existing house, consider using a ridge vent in place of roof vents. Don't trim the size of the roof vent flange, as this can cause leaks. If your soffits aren't wide enough to accommodate 16-by-8-inch vents, use round vents that snap into a two-inch hole. If you can't install soffit vents, double the square footage of the roof vents.
Always wear eye protection when using power tools. Use caution when working on a roof; fall protection is advised.