Curbing radiant heat means cool savings

Q 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 heat flow?

A: 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 you hand against it. If it really is much hotter, this may be one of the significant sources of your high 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-color shingle roof easily can reach 150 degrees in the hot summer sun. Light-color 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 immediately could 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 that make 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.

Ridge vents:

-- Cor-A-Vent, 800-837-8368, www.cor-a-vent.com.

-- Lomanco, 800-643-5596, www.lomanco.com.

Q: I am planning to install some replacement windows and doors in my house. I have heard about turtle glass and that many people are using it now. What is turtle glass, and is it very energy efficient?

A: Turtle glass is usually used in coastal areas and is actually mandated in many areas. The glass reduces the light transmission from indoors to outdoors at night. It does not affect energy efficiency.

Sea turtles crawl up on the shore and lay their eggs. They use the light from the moon to navigate back to the water. Bright lights can confuse them, and they go the wrong way. If they do not get back to the water, they die.

Send inquiries to James Dulley, The Post and Courier, 6906 Royalgreen Dr., Cincinnati, OH 45244 or visit www.dulley.com.