Dear Jim: I opened the attic access panel in my closet to get a box and the heat was staggering. We have plenty of attic insulation, so is this increasing our cooling costs? If so, what can we do to reduce the heat? – Michael N.
Dear Michael: Yes, that hot attic can increase your electric bills for air-conditioning. Most people are not aware how hot their attic gets in the afternoon sun. That heat does not just stay in the attic. It comes through the insulation to make the ceiling hot and your entire house warmer.
The 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 heats 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 which can easily pass through the attic floor insulation.
This radiant heat not only heats up rooms, which 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 method to reduce this radiant heat is by installing an aluminum foil barrier under the roof rafters and increasing 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 from suppliers. It comes in long rolls, often four feet wide. You can find insulation contractors to install it for you, but the cost of the foil alone, not including installation, may be marked up several times.
The attic foil is stapled up under the attic rafters, not placed on the attic floor. The goal is to block the radiant heat before it gets into the attic area. Also, if it is on the floor, it is susceptible to damage and heavy dust buildup.
The least expensive type of foil is laminated to brown kraft paper for strength. If you install this, face the shiny side downward. It is the low-emissivity properties of the aluminum which block the heat more than the reflectivity from the top.
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 soffit inlet vents to bringing in cool outdoor air over the attic floor insulation.
These companies offer attic foil: AtticFoil, www.atticfoil.com; Fi-Foil, www.fifoil.com; TVM Building Products, www.tvmi.com; and ridge vent: Cor-A-Vent, www.cor-a-vent.com; and Lomanco, www.lomanco.com.