Beat the heat, reduce costs with aluminum-foil radiant barriers
Q. I opened the attic access panel in my closet to get a box, and the heat was intense. There is attic insulation, so is this heat increasing our cooling costs? If so, how can I reduce the heat?
A. Yes, that hot attic can increase your electric bills for air-conditioning. Most people are not aware how hot their attics get in the afternoon sun, up to 150 degrees. That heat does not just stay up there. It comes through the insulation to make the ceiling hot and your entire house warmer.
This form of heat that comes from a hot roof and attic is called radiant heat. This is a different form of heat transfer than normal conductive heat, which thermal insulation is used to block. Radiant heat is the method the sun uses to heat the Earth and why you feel warm in the sun on a cold day.
As your attic starts to warm up in the morning, the thermal (fiberglass, rock wool, etc.) insulation on the attic floor blocks the heat from the warm attic air. As the underside of the roof gets hotter in the sun, it gives off more radiant heat that can easily pass through the attic-floor insulation.
This radiant heat not only heats up rooms and makes your air conditioner run longer, but it makes you feel hotter. Often, you set the air-conditioner thermostat lower to feel comfortable, and this further increases your summertime electric bills.
The best methods to reduce this radiant heat are to install an aluminum-foil radiant barrier under the roof rafters and increase attic ventilation. The foil blocks the radiant heat from the underside of the hot roof, and the ventilation allows the hot air to escape.
Attic radiant-barrier foil is available at most home centers, or you can order it online. It comes in long rolls, often 4 feet wide. It is best to staple the foil up under the attic rafters to block the radiant heat before it gets into the attic area. If the foil is rolled out on the floor, it is susceptible to damage and heavy dust buildup.
You can find insulation contractors to install the barrier for you, but the cost of the foil from them, not including installation, may be marked up several times. It is an easy do-it-yourself project because the neatness of the installation doesn’t impact its effectiveness. Just use a hand stapler, and tack it up every several feet.
The least expensive type of attic foil is laminated to brown kraft paper for strength. If you install this, face the shiny side downward. The low-emissivity properties of the aluminum bottom surface block the heat more than the reflectivity from its top surface. For slightly more cost, double-sided foil reinforced with mesh is easier to handle.
The best type of attic ventilation to install is a continuous ridge vent. This exhausts the hottest air at the peak of the roof. Also install inlet vents under the roof soffits to bring in cool outdoor air over the attic floor insulation. For a do-it-yourselfer, it may be easier to install a row of round roof vents.
The following companies offer attic foil: AtticFoil (800-595-8772, www.atticfoil.
com), Fi-Foil (888-263-7031, www.fifoil. com), Innovative Insulation (888-420-7289, www.radiantbarrier.com) and ridge vents: Cor-A-Vent (800-837-8368, www.cor-avent.com) and Lomanco (800-643-5596, www.lomanco.com).