On a warm, sunny summer day, does it seem like your central airconditioning system is working overtime? Have you ever felt the ceiling in a room and found it to be unusually warm? If you can answer “yes” to either of these questions, the problem (and the solutions) may be in your attic.
Adequate attic insulation is only one aspect of keeping your house cool and reducing your air-conditioning costs. By “insulation,” most folks mean thermal insulation that blocks heat from their attics from moving into their homes. This includes fiberglass, rock wool, foam, and/or cellulose insulation on the attic floor.
There are three modes of heat transfer: conduction, convection and radiation. Conduction typically refers to heat flow through solid materials. This is how the handle of a metal skillet gets hot on the stove. Convection is similar to conduction, but it occurs in fluids and gases. This is why you feel cooler in the wind (or in front of a fan) than you do in still air.
Regular thermal insulation in your home’s ceiling blocks conduction and convection heat losses. Most insulation charts that give recommended R-values refer to thermal insulation.
The third mode of heat flow, radiation, is how the sun heats the Earth or why you feel warm in front of a fireplace. Unfortunately, standard thermal insulation is not very effective for blocking this type of heat flow, so on a hot summer afternoon a roof – especially one covered in dark asphalt shingles – gets extremely hot. This heat then radiates downward through the attic floor insulation and into your home.
You can tell if the ceiling is hotter than the walls just by putting the back of your hand against the ceiling during the afternoon. If it feels much warmer, this may be a major reason for high electric bills during warmweather months.
Even with your air conditioner running and air in the room reasonably cool, you still may feel uncomfortable under a warm ceiling. This heat often causes you to set the air conditioner thermostat lower than usual to compensate, which further increases your power bills.
Even though your roof and attic still will be hot, adding more insulation and upgrading inadequate attic ventilation significantly can help reduce heat transfer to the rooms below – resulting in cooler ceilings. For ventilation, a combination of continuous ridge vents at the peak of the roof and inlet soffit vents works best; incoming cool air will move low over the insulation, become less dense as it warms up and flow out the ridge vent.
If your house needs a new roof, consider using light-colored – preferably white – shingles to reduce the roof temperature. Also look at new-style asphalt shingles, available in several colors, that include a special coating to reflect a higher percentage of sunlight than conventional shingles. Finally, check into metal roofs – particularly aluminum ones with heat-reflective (not visibly reflective) paint, which stay even cooler and minimize heat transfer to the ceiling below.