How To Outsmart The Heat

I want to make my house more energy efficient. First, I think I should understand how a house loses or gains heat. For example, since heat flows upward, is it important to insulate the basement and foundation?—Steve N.

Many do-it-yourself guides offer advice on home improvements to save energy. But each house is unique, and improvements that may make economic sense for one may not be the best for another.

An understanding of how a house loses heat in winter and gains heat in summer can help when making energy decisions. The first step is learning the basics of heat transfer—how heat moves from one object to another.

A basic misunderstanding is that heat flows upward. Although hot air does rise because it is less dense than cool air, heat itself, as a form of electromagnetic energy, is unaffected by gravity and flows in all directions. For example, if you put a heating element in the center of a metal block, the bottom will get just as hot as the top and sides.

Heat moves through your home in three ways. The first type, heat conduction, would apply to the metal block example. Conduction is how heat moves through solid material. In your house, this is how most of the heat flows through insulation in the walls and ceiling.

The temperature difference between air on each side of a wall determines how fast heat will flow through it. For a given wall insulation R-value, if it is 68 degrees indoors and 28 degrees outdoors (a 40-degree temperature difference), about twice as much heat will be lost through the wall than when it is 48 degrees outdoors (a 20-degree difference).

Convection heat transfer is similar to conduction, but occurs in fluids and gases. When it is windy outdoors, cold air increases the heat loss from the wall more than if the air, at the same temperature, was still. Similar to conduction, when the temperature difference is doubled, the heat transfer is also doubled.

Radiation is the third type of heat transfer, and it is the most difficult to understand. This is how the sun heats the Earth through millions of miles of empty space and how the top of a steak gets seared in a broiler. Radiant heat transfer is generally more of an issue during the summer, but cannot be ignored during winter.

Radiant heat transfer increases exponentially as the temperature difference between two areas grows. This isn’t a major issue when it’s relatively mild outside, but during the summer, when a black roof can reach 160 degrees and the inside of your house is 80 degrees (an 80-degree temperature difference), the heat transfer can become a very large number (80 x 80 x 80 x 80). This radiant heat is not blocked by standard insulation, but reflective foil under the roof rafters will do the trick.

Applying this knowledge can help in evaluating improvement options. For example, if there is a particularly cold wall in your house that faces oncoming winds from the northwest, there is likely both conductive and convective heat loss.

Making sure the wall is well-insulated is the first step to reduce both modes. Evergreen trees or a privacy fence outside would help block the wind and further reduce convective heat loss. And since heat can flow downward, add insulation to the band joist immediately above the foundation to reduce conductive heat loss. Also, use foam caulk to seal along the top of the foundation.

Don’t forget the typical heat loss areas of windows, doors, and fireplaces. Windows and doors lose heat by conduction, convection, and air infiltration through leaks.