You can forcibly exhaust heat and humidity from your home's attic, even on calm days with no wind, simply by installing a power vent. The air enters through intake vents and is pulled through and exhausted from the attic with electric fans.
Determine the Proper Eave Vent Size
Power ventilators come in a number of sizes and are rated by cubic feet of air moved per minute (CFM).
- Determine the attic's square footage.
- Multiply this number by .7 to give you the minimum CFM required for your ventilator.
Example: An attic with 1,500 square feet x .7 = 1,050 CFM
Make sure you also have adequate air intake (typically through the undereave vents). A rule of thumb: 1 square foot of intake for every 300 CFM of ventilator.
Example: 1,170 CFM ventilator / 300 = 3.9 square feet of intake ventilation
This is the minimum intake required.
Power vents can be mounted on the roof or on the gable.
Before you begin, make sure all factory-installed bolts are tightened. Consider two things for maximum performance when you choose the location of the vent.
- Mount the vent as close to the center of the house as possible.
- Mount it as near to the roof ridge without it being visible from the front of the house.
To install the vent:Step 1
Measure the distance from the ridge and the edge of the roof to the desired location for your vent.
Transfer these dimensions to the inside of the attic. Measure an equal distance between the rafters at the selected location and mark the point. Drill a nail hole through the roof on the mark.
Cut out the circle template on the box and place it on the roof using the drilled hole as the center.
Trace around the template and cut through the shingles and decking with a jigsaw.
Remove the vent's dome. It makes installation easier.
Center the fan over the hole, making sure the up side of the base flange is pointed toward the ridge under the shingles.
Use caulk between the roof and fan.
With a utility knife, cut the shingles at the top of the fan to accept the fan's throat.
Secure the power vent firmly by nailing the base flange to the roof with 2 inch roofing nails. Replace the dome.
Set the thermostat at 90ºF degrees. The power ventilator will automatically start at approximately 102ºF degrees. When the temperature drops to 90ºF degrees, the fan will automatically shut off. You can test this with a work light or hair dryer.
For wiring, use a qualified electrician who is familiar with local and national electrical codes.