Blocking Heat Keeps Rooms Cool
May 24th, 2012 by James Dulley
This schematic shows the hot air flow from an attic through a rigid vent. Notice it is covered with shingles for a nice appearance.

This schematic shows the hot air flow from an attic through a rigid vent. Notice it is covered with shingles for a nice appearance.

Q: Although I think my house is adequately insulated, my air conditioner runs a lot. On sunny days, the bedroom ceiling seems hot, so I assume heat is coming from the hot roof. How can I reduce this heat flow?

A: Adequate attic insulation is only one aspect of keeping your house cool and reducing your air-conditioning costs.

By “insulation,” most people mean thermal insulation that blocks heat conduction. This includes fiberglass, rock wool, foam and/or cellulose insulation on the attic floor and in the walls.

The three modes of heat transfer are conduction, convection and radiation. Conduction refers to heat flow typically through solid materials. This is how the handle of a metal skillet gets hot on the stove.

Convection is similar to conduction, but occurs in fluids and gases. This is why you feel colder in the wind than in still air.

Regular thermal insulation in walls and the ceiling blocks conduction and convection heat losses. Most recommended insulation charts, which mention R-values, refer to thermal insulation.

Radiation is how the sun heats the Earth or you feel warm in front of an open fireplace. Unfortunately, standard thermal insulation is not very effective for blocking this type of heat flow. On a hot summer afternoon, a roof—especially a dark asphalt shingle one—gets extremely hot. The heat then radiates downward through the attic floor insulation and into your house.

You can tell if the ceiling is hotter than the walls just by putting the back of your hand against it in the afternoon. If it feels much warmer, this may be a major reason for high electric bills.

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

If your house needs a new roof soon, replace it with light-colored—preferably white—shingles to reduce the roof temperature. Metal roofs, particularly aluminum ones with heat-reflective (not visibly reflective) paint, stay even cooler and minimize heat transfer to the ceiling below.

Other than replacing the roof, adding more insulation and adequate attic ventilation can help significantly. When I installed more attic vents in my own home, I could immediately feel the difference in my second-floor bedroom.

Putting in extra insulation also will cool ceilings that meet attic space because it blocks heat transfer. Attic vents, continuous ridge or inlet soffit work best. This allows cool air to move low over the insulation—become less dense as it warms up—and then flow out the ridge vent.

Your attic and roof will still be hot, but extra insulation and ventilation will help cool the living space underneath your attic.