How to Figure House Heat Loss

Subtract the normal interior temperature of your house from the lowest annual exterior temperature in your area. This number is called your home's temperature difference.

Measure the area of every flat surface on the outside of your house, including every wall, door, window and angle of the roof. Subtract the area of any windows or doors from the walls in which they are located for more accurate results.

Find the R-value of the primary material for each surface. A material's R-value is the degree to which it resists heat loss; the higher the value, the better it insulates. Lists of R-values can be found on numerous websites. Divide 1 by the R-value to find the surface's U-value, or level of heat conductivity.

Multiply the temperature difference, area and U-value of each surface together to find that surface's individual heat loss, measured in BTUs per hour. Add the product of every calculation together to find the hourly heat loss for your entire house.