Reflective foil a good tool to block heat from homes

Dear Jim: I have plenty of fiberglass insulation on the attic floor, but it still seems as though the bedroom ceiling is warm on sunny days and the air conditioner runs like crazy. What can I do to keep it cooler? – Steve P.

Dear Steve: If you put your hand against the ceiling, you would be surprised how warm it actually is. This extra heat will increase your cooling electric bills. This warmth also radiates to your body making you feel uncomfortably warm even at a reasonably cool room temperature. When this happens, people tend to set the thermostat even lower which further increases the electric bills.

What you are experiencing is radiant heat transfer from the hot roof to the ceiling below. A dark roof can reach 150 degrees in the afternoon sun. Standard fiberglass insulation is effective for blocking conductive heat transfer, but not for radiant heat. Radiant heat from the roof penetrates through the insulation to the ceiling below. Even the insulation gets warm.

The best method to block most of the heat is to install reflective foil underneath the roof and install adequate attic ventilation. The foil will block the direct path of the radiant heat to the ceiling below. The attic ventilation will cool the roof and carry the excess hot air away.

In my own home, this combination lowered my bedroom temperature by 10 degrees.

Attic foil is commonly referred to as reflective foil because it looks reflective. It actually works not by reflecting the heat back up to the roof, but by its low emissivity (similar to low-e windows). The foil gets hot, but its shiny, low-e surface does not easily radiate the heat downward.

You can find attic foil in long rolls which are about 4 feet wide. It is similar to heavy-duty kitchen aluminum foil, except it is reinforced with kraft paper or a nylon mesh. The most inexpensive type uses kraft paper with foil on only one side. Although this may sound strange, because the low-e properties are most important, the shiny side should face down.

Staple the foil up under the roof rafters. The neatness of the job is not critical. It is only important that most of the roof surface is blocked from the floor below. Leave a small gap above the insulation near the floor and at the ridge so the attic and roof are well-ventilated. This is particularly effective with a roll-out or rigid continuous roof ridge vent.

Another option is to have the underside of the roof sprayed with a special reflective, low-e paint. This has a similar effect to the foil.

If you are replacing the roof sheathing or building a new home, sheathing is available with a foil backing already applied so additional foil is not needed.

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) 313-3258, 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.