A cooler attic could mean a longer life for your roof. Installing soffit and ridge vents allows you to use natural air flow to cool your attic in the summertime. Hot air rises and escapes through the ridge vents while cooler air is drawn in through the lower soffit vents, creating a natural convection that circulates the air and cools the space. Measure your attic before buying and installing your soffit vents.
Calculate the total area of the attic space. Measure the length and the width of the space and multiply the two numbers to find the area.
Divide the area by 14 to determine the amount of space that requires venting in total square feet. Every 14 square metres (150 square feet) of attic space needs 0.09 square metres (1 square foot) of venting.
Divide the total metres of venting required by 2 as half of the venting is supplied by the ridge vent, and half is supplied by the soffit vents.
Calculate the area of each soffit vent. The formula for calculating the area of a circle is the radius squared times pi. The radius is half of the diameter. A 15 cm (6 inch) vent has a diameter of 15 cm (6 inches); therefore, the radius is 7.5 cm (3 inches). Square the radius by multiplying it by itself. In this example, 7.5 squared equals 56.25. Multiply that by pi, or 3.14. The area of a 15 cm (6 inch) round soffit vent is 176.625 cm squared (28.26 inches squared).
Convert the area of the soffit vents into square metres by dividing the area in square cm by 10,000. The area of a 15 cm (6 inch) vent in square metres is 0.0176 (0.196 square feet).
Calculate the total number of soffit vents needed by dividing the soffit vent area in square metres from Step 3 by the area in square metres of a single vent from Step 5. For example, 0.46 square metres (5 square feet) of soffit vent area divided by 0.0176 square metres (0.196 square feet) per vent equals about 26 15 cm (6 inch) round soffit vents.
Mark where each soffit vent is to be placed. Space the vents evenly along the soffit, placing them between joists and rafters. Ideally, the vents should line up vertically with existing or planned roof vents.
Inspect the soffit overhang area in the attic. Carefully cut away any insulation on the soffit overhang that would block the vents using a utility knife. Wear goggles, a dust mask and work gloves.
Cut a hole into the soffit at the mark using a hole saw bit and an electric drill. Choose a hole saw bit size that is 2.5 cm (1 inch) smaller than the vent, creating a 1.2 cm (1/2 inch) area for attaching the vent to the soffit.
Apply a thin bead of caulk to the flange or lip area of the soffit vent.
Center the soffit vent over the hole, stopping at the flange. Secure it into place using a hammer and the nails provided with the vent, or galvanised roofing nails. Alternately, attache the vent using 1.2 cm (1/2 inch) long number 4 sheet metal screws and an electric drill. Most round soffit vents have four to six pre-drilled holes, indicating nail or screw placement. If the vent does not already have holes, space four to six nails or screws evenly around the vent.