Cooler Attic, Cooler House

By Mike McClintock

When it's a comfortable, air-conditioned 70 degrees F in the house despite a heat wave outside, the uncooled attic could be baking at 125 degrees or more. You don't live under the eaves so you don't feel the heat directly. But hot-head attics drive up cooling costs, shorten the life span of air-conditioning equipment and can contribute to mold, rot and other problems. You can increase ventilation in the attic to reduce the baking effect, but the first step should be to provide an insulating barrier between the attic and the living areas below.

Insulation levels

The old rule of thumb is to fill the bays between ceiling joists. If you have 2-by-6 joists, for example, the bays can hold 5 1/2 inches of insulation with a total R-value of about 20. (R-value is the standard measure of insulation effectiveness; the higher the number, the better.) The U.S. Department of Energy standard for attic insulation in most eastern states used to be R-38, but it has been increased to R-49. So even if you have 2-by-10 joists (figuring R-3.5 per inch for fiberglass batts), you'll still have a total R-value of only about 33. To reach R-38 -- or certainly R-49 -- you'll need to add another layer. You can blow in loose fill, spread loose insulation from bags or lay another layer of batts across the tops of the joists. Whatever approach you take, remember that you should not have a vapor barrier between layers. The top layer can't have a foil or paper face that traps moisture that could eventually rot most insulation and framing.

It's also important to keep extra insulation away from recessed light fixtures in the living-space ceiling. Leave a few inches of airspace between insulation and fixtures so heat from the lights can dissipate. Also, keep insulation away from any vents at the edges of the attic, where it might block the airflow. You don't need insulation in the roof overhang in any case, because there is no conditioned living space below it.

Finally, add extra batts around any exposed air-conditioning ducts. In many houses, and particularly in those where central air was added years after construction, ducts run above the joists, where they can absorb attic heat and reduce the efficiency of the cooling system.

Vent inlets and outlets

There are many types of attic vents, and many good places to install them. But one type of vent in just one place can't handle the job of keeping attic air close to the temperature of the air outside.

You could install a huge vent in the roof, but not much hot air would flow out unless there were a way for fresh air to flow in and replace it. You need a combination of vents with about the same square footage of air inlet and air outlet.

A significant mismatch of inlet and outlet area can make the situation even worse, encouraging leaks of conditioned air through cracks and other openings in the attic floor, wasting energy and unnecessarily taxing your cooling system.

You can install turbine vents on the roof that spin as hot air escapes and encourage airflow. Another less commercial-looking option is a basic roof vent with a hood that keeps rain out. Both types require a rooftop installation, where you have to cut back shingles and cut a hole in the plywood roof deck.

The most effective roof installation is a continuous ridge vent. The plywood roof deck is built (or, in an older house, cut back) short of the roof peak so that air can sweep up both undersides of the attic roof and out the vent. You can cover many ridge vents with shingles so that a ridge-vented roof looks about the same as a standard roof.

One of the most common outlet options is a gable-end vent. This louvered vent nestles under the roof ridge on the end wall of the attic. If you have two exposed gable walls, a louver on each one encourages cross ventilation at the highest and hottest area of the attic. (They will need to be screened on the back to keep out insects.)

The best place for an inlet is along the roof overhang because cooler outdoor air will replace hotter attic air rising to the ridge. There are four basic ways to bring air into the attic along the overhang:

• Plug vents are small-screened and louvered circles made of metal. Installing them is easy, one per bay in the spaces between rafters. You cut a hole in the soffit (usually a flat piece of plywood forming the base of the overhang) and snap in a vent.

• You can increase the vent area with the same approach by cutting large openings between bays (or every other bay) and installing rectangular vents about the size of floor registers, although these can look a little clunky in an overhang.

• A better option is to install a continuous strip-grille vent. It's only a few inches wide, but extends the length of the overhang to provide a larger vent area and eliminate unvented dead spots. Use a circular saw to cut out a narrow channel of plywood and nail the strip grille into the recess.

• If your wood soffit is badly deteriorated, consider replacing it with perforated aluminum panels. This venting system usually is used with matching aluminum siding and trim, but you can set the panels and trim the edges with wood.