How Much Is It to Heat a House Per Square Foot?

Thermal Resistance of the Home's Walls

• Determine the R-value of the walls within the home. This value is the thermal resistance of the wall to heat loss. A home loses heat energy to the outside environment over time. You must account for this lost heat when determining heating costs. R-values for many common materials are known and published. Contractors and building supply shops also have these values readily available. Add together the value of each material in the wall, including drywall and wood, to determine an overall R-value. Also consider the thermal resistance of air on both sides of the wall. As an example, a home with standard 2X4 wood construction with 3/4 inch insulating sheeting, 3.5 inches of loose fiberglass insulation, and 1/2 inch drywall has an R-value of 16.31.

Degree Days

• Degree days represent the difference between a standard temperature, 65 degrees Fahrenheit, and the outside temperature. Degree days have been recorded for many regions to provide a standard measure for heating and cooling calculations. This difference helps quantify the severity of a winter in different regions of the world. This difference in temperature creates a thermal gradient that causes heat to escape from a home. Insulation slows this heat transfer.

Calculating BTU

• Calculate the BTU requirements of the home prior to determining the heating costs. The BTU output and fuel requirements of the heater within a home allow for a close approximation of this cost. The BTU heating function is: (24 hours x square feet x degree days) / (R value). Using the R-value in a home with an external wall area of 1,000 square feet and 4,000 degree days: (24 hours x 1,000 square feet x 4,000) / (16.31) = 5,885,960 BTUs of lost heat energy. This is heat energy that must be created using fuel in the heater.

Calculating Heating Costs

• Multiply the fuel usage required to generate the BTUs needed by the cost of the fuel to determine the heating cost per season. Divide this value by the efficiency of the heater. The heater efficiency is under the "Energy Star" heading on the heater's attached label. This calculation is often helpful in determining whether an electric or gas heater is more cost efficient, as per the electrical costs in the area. The cost of heating, using natural gas as an example, is calculated as: (Heat energy used x Cost / BTU) / (Heater efficiency). Using a 90 percent efficient heater at an approximate price of $10 per million BTU yields: (5.885960 million BTU x $10 / million BTU) / ( 90%) = $65.4 in heating costs for the season.

Deviation

• Use degree days and thermal resistance to make a close approximation of the cost to heat a house. Some factors can skew the results. For example, heat leaks within the home can easily cause an underestimate of heating requirements. The most common cause for a heat leak is air infiltration. The air in a house completely exchanges, on average, once per hour. Leaks from door seals and windows can lead to excessive air infiltration, which raises the cost of heating a home.