Roof Systems

In our area, attic spaces can heat up to 130°F or more on a hot summer day. Eventually, that heat transfers through the ceiling insulation into living spaces below, increasing the cooling load. To counter this problem, we not only suggest twice as much roof ventilation as most codes recommend, but we also use a radiant barrier, such as Insul Foil (Advanced Foil Systems, 820 S. Rockerfeller Suite A, Ontario, CA 91761; 909/390-5125) to keep the attics cool. You can also use Kool-Ply (Louisiana-Pacific, 111 S.W. Fifth Ave., Portland, OR 97204; 800/299-0028), a labor-saving roof sheathing with foil laminated directly to it. The radiant barrier blocks the transfer of heat from the hot roof to the attic. Apply the radiant barrier to the underside of the roof, not on the attic floor, and make sure to cut open the radiant barrier along the ridge and below the low-profile vents so that attic air can exhaust (Figure 3). Figure 3. Foil radiant barriers can reduce unwanted heat gain by up to 40%. The foil barrier should be installed on the underside of the sheathing and should be cut at the ridge vent to allow heated air in the attic to escape.

Roof Ventilation Details

Figure 4. The author recommends doubling the net ventilation area in both attics and vaulted roofs. He prefers passive venting to power vents, which may draw conditioned air out of the living space. Where this can’t be done with continuous ridge venting, such as on hipped roofs, I supplement the ridge venting with low-profile vents on the roof itself. These should be located on the side of the roof opposite the direction of prevailing summer breezes. Don’t substitute power attic vents for the passive venting techniques just described. They not only consume power, but often draw conditioned air from the living space by depressurizing the attic.

Duct Sealing

Mechanical air distribution systems, including ducts and air handlers, must be installed with minimum air leaks. Otherwise, a vacuum can occur in the living space, creating an imbalance that will draw in moist outside air. Consider this scenario: The air handler is pulling air from the house through a short return air duct or well-sealed chase. But because not all the supply-duct joints and seams are equally well sealed, not all of that air gets delivered back into the living areas of the house. Rather, it gets lost to the attic or someplace other than the building’s occupied zone, creating a negative air pressure, or vacuum, in the conditioned zone. As a result, moisture-laden outside air from the attic — or worse yet, moist and chemically laden air (from insecticides and rat poisons) from the crawlspace — is drawn into the house through cracks and poorly sealed pipe penetrations. To eliminate the imbalance that creates this negative air pressure, we seal all duct joints and transitions with fibrous mastic rather than duct tape (Figure 5).

Figure 5. Properly sealing ductwork with mastic will prevent hvac systems from creating negative pressure in conditioned living space, which could draw unwanted moisture from the outside. We use Versa Grip (Hardcast, P.O. Box 1239, Wylie, TX 75098; 800/527-7092) or RCD #7 Mastic (RCD Corp., P.O. Box 1020, Eustis, FL 32727; 800/854-7494). Sealing ducts this way is important both to maintain indoor air quality and to minimize moisture and mildew. I even suggest setting up the hvac system to provide slightly positive pressure in the house. This can be done in a controlled way via a small outside-air intake duct to the return-air chamber of the air handler. It will provide outside makeup air and create a positive pressure difference between the inside and outside. Positively pressurizing the interior also keeps unwanted moisture from infiltrating the living environment. In addition, this outside air is dehumidified by the air handler before it reaches the home’s interior.

Educate the Homeowner

Using a little common sense, homeowners can keep humidity levels down inside the house, which will decrease the chance of molds and mildew growth during the summer and minimize condensation on windowsills during the winter. Builders can help by explaining why clothes dryers and kitchen exhaust fans must be vented directly to the outside, and by providing bathroom fans with timer switches so the owners won’t have to remember to turn them off. Homeowners will "buy into" these preventive techniques if you explain how they will prolong not just the hidden structural elements, but the interior paint job, windows, drywall, and other finishes as well. Peter L. Pfeiffer, AIA, is a principal in Barley & Pfeiffer, an Austin, Texas, architectural firm specializing in sustainable architecture, planning, and energy consulting.