Installing Gable Vents

In an unvented attic, the temperature can rise past 150 degrees when it's only in the mid-90s outside. What's more, that heat will radiate down from the ceiling, making you droop and driving up the cost of cooling your house. In fact, heat coming down through ceilings can account for 12 to 30 percent of your total cooling bill.

The cure for this steamy situation is ventilation and gable vents are the easiest way to provide it. They're not just summertime allies, either. In winter, gable vents get rid of water vapor that works its way from the living space to the attic, where it can condense, soaking your insulation and rotting your rafters and top plates.

Bottom line, gable vents are a great idea. Here's how to select, size and install them.

It's best to install two gable vents, one at each end of your house. They need to provide a square inch of inlet and outlet vent area for each square foot of attic area. In other words, you'll need 1 square foot of vent for every 144 square feet of attic floor. Typically, manufacturers specify the area of the vent on the vent packaging, usually calling it "net free area."

Choosing a material is mostly a matter of durability and looks. Copolymer vents are the most durable, but the range of colors is rather limited. Metal vents are also quite durable, but they need to be primed and painted. (Rust-Oleum® has a good line of rust-resistant paints.) If you have a clapboard or wood shingle exterior and only a wood vent will do, go with redwood it'll last longer. Once you have your vents, you're ready to go.

Step by Step
1. Install vent framing. Following the manufacturer's template, construct and install a rough 2-by-4 frame between gable-end studs, as shown. Use 16-penny common nails or 2 1/2-inch screws. Make sure the frame is square and allows your vent 1/8-inch clearance on all sides. Next, using a long bit or bit extension, drill 3/8-inch holes through both the sheathing and the siding. This will enable you to locate the corners of the rough opening from the outside of the house.

2. Cut the rough opening. Outside, snap chalk lines to connect the holes. This will outline the rough opening for the vent. Using a reciprocating saw equipped with a bimetal blade (which can cut through the occasional nail), cut out the rough opening. What you do next depends on what kind of siding you have and whether the vent is made with integral trim trim that's already attached to the vent and that doubles as a nailing flange. If it has no integral trim, plumb and shim the vent and nail it to the rough opening. You'll add trim after the vent is in.

Next, hold the body of the vent in place in the rough opening, check that it's plumb, and then draw the outline of the nailing flange with a pencil. (If your vent has no nailing flange which may be the case with some wooden vents omit this step, and instead draw an outline equivalent in width to the trim you intend to use.) With a circular saw set just to the depth of the siding, cut along the lines, and then remove the bits of siding. Use a saw blade that's appropriate for your siding. (You may want to check out our article on choosing blades for circular saws.) Check to be sure the vent flange lies flat against the sheathing.

3. Mount the vent. This is actually a three-step process. The first step is to put flashing around the opening to prevent water from getting in. Use building paper to flash all sides of the rough opening, making sure that upper pieces overlap those beneath. (By the way, the piece of building paper running across the top of the opening will come down over the metal cap flashing, discussed below.) Staple the building paper in place. The new paper should overlap the old by at least 2 inches. Run a bead of caulk under each strip before you insert it, and try to work it under the siding on all sides if you can. If any paper should overlap the opening, just fold it into the opening.

Next, slide a metal cap flashing up under both the siding and the new paper at the top of the opening. Now mount the vent. Caulk liberally under the vent's mounting flange, and then push the vent into place, working the top of the flange up under the cap flashing. This will take a little maneuvering. Check that the vent is plumb, then nail or screw the other three sides of the nailing flange to secure the vent in position. If yours is a shingle wall, replace shingles as necessary. If your vent lacks trim, you'll need to add some that matches the trim on the rest of your house.

TOOLS AND MATERIALS
Hammer
Framing and trim lumber
Screwdriver set
Gable vents
Tape measure
Shims
Drill and bits
16-penny nails
Level
2 1/2-inch screws
Combination square
Staples
Drill bit extension shaft
Acrylic/latex caulk
Chalk line
Reciprocating saw and bimetal blade
Plumb bob
Circular saw
Stapler
Caulk gun
Utility knife