Dear Jim: Even though my house is adequately insulated, my air conditioner runs a lot. On sunny days, the bedroom ceiling seems hot, so heat must be coming from the hot roof. How can I reduce this afternoon heatflow?— Shana G.

Dear Shana: Adequate insulation is only one aspect of keeping your house cool and reducing your air-conditioning costs. By “insulation,” most people mean thermal insulation which blocks heat conduction. This includes fiberglass, rock wool and/or cellulose insulation on the attic floor and in the walls.

It is easy to tell if the ceiling is hotter than the walls just by putting the back of your hand against it. If it really is much hotter, this may be one of the significant sources of your high cooling electric bills during summer. Thermal insulation is not particularly effective at blocking the radiant heat from a hot roof.

Even with the air conditioner running and the room air reasonably cool, you may still feel uncomfortably warm under a warm ceiling. This often causes you to set the air conditioner thermostat even lower and this further increases your electric bills.

You are correct that the heat is likely coming from a hot roof. A dark-colored shingle roof can easily reach 150 degrees in the hot summer sun. Light-colored shingles reduce the roof temperature. Metal roofs, particularly aluminum ones, stay even cooler and minimize the heat transfer down to the ceiling below. This is why some qualify for energy tax credits.

Other than installing an expensive metal roof, stapling reflective aluminum foil under the attic rafters and adding adequate attic ventilation can help significantly. When I installed the foil and more attic vents in my own home, I could immediately feel the difference in my bedroom temperature.

Although it is often called “reflective” attic foil because it is shiny, it is actually the low-emissivity properties of the underside of the foil which makes it most effective. Its low emissivity reduces the amount of heat it radiates downward from the foil and roof to the room ceiling below.

It is not critical how neatly the foil is stapled under the roof rafters. Most two-sided foil is reinforced with nylon mesh. Low-cost single-sided foil over kraft paper is the least expensive type. When using single-sided foil, the shiny side should face downward. Another option is low-emissivity silver paint which is sprayed on the underside of the roof sheathing.

Install attic vents, preferably a ridge vent and inlet soffit vents. This air flow, between the hot roof and the top surface of the foil, carries the heat away and keeps both cooler. You will be surprised at how hot the exiting air actually gets.

The following companies offer attic foil: Fi-Foil, (800) 448-3401,

www.fifoil.com

; Solec, (609) 883-7700,

www.solec.org;TVM

Building Products, (888) 699-1645,

www.tvmi.com

; and ridge vents: Cor-A-Vent, (800) 837-8368,

www.cor-a-vent.com

; and Lomanco, (800) 643-5596,

www.lomanco.com

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