Radiant Heat Transfer

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

A: 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. Radiant heat transfer is unique in that it increases at a very fast rate as the temperature increases. For example, a roof at 150 degrees can radiate more than 10 times as much heat to the ceiling below as a roof at 120 degrees.

Standard fiberglass insulation is effective for blocking conductive heat transfer, such as the heat from inside the home, to the cold attic during the winter. It is not very effective for blocking the radiant heat from a hot roof. Such heat penetrates through the insulation to the ceiling below. even the insulation itself gets warm and can actually hold the heat in the ceiling once it gets warm.

The best method to block most of the heat is by installing reflective foil underneath the roof and installing adequate attic ventilation. The foil blocks the direct path for the radiant heat to the ceiling below. The attic ventilation will cool the roof and carry the excess heat away by natural air flow ( the hot air is less dense and rises).

Attic foil is commonly referred to as reflective foil because it looks like aluminum foil. It actually works not by reflecting the heat back up to the roof, but by its low-emissivity ( similar to low-e-windows) properties on its bottom surface. The foil gets hot, but it’s shiny low-e surface does not easily radiate the heat down. this why room radiators for heating are a dull dark color instead of shiny, so they radiate heat more effectively.

You can find attic foil in long rolls that are about four feet wide at many building supply outlets. It is similar to regular kitchen aluminum foil, except it is reinforced with kraft paper or a nylon mesh or grid so the staples don’t pull through. 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 every part 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.

Installing a ridge vent is most effective. The hot air naturally rises to the peak of the roof where it is exhausted. Ridge vents are available in long rolls or as rigid sections that are nailed over a slot along the roof ridge beam. It is also important to install adequate inlet vent areas along the soffit or underside of the roof overhang.