The Proper Way To Calculate Home Heat Loss

If you are trying to determine heat loss in a home, you should have a basic understanding of heat measurement and units as well as knowledge about insulation and ways heat is transferred. The amount of heat loss may be important when you are considering new insulation, airtight windows or other home improvements designed to make your home more energy-efficient.

You can do the actual calculations for heat transfer yourself, or you can get some help from automatic calculators.

What Units Measure Heat Loss?

Heat loss is usually measured in old British and United States common units. To understand the jargon and be able to compare calculations, you’ll need to use the same units. The most important is BTUs, or British Thermal Units.

One BTU is the amount of heat needed to raise 1 pound of water by 1 degree Fahrenheit. It is the unit that measures heat or cooling energy and the amount of heat loss.

The other term to know is R-factor. This number indicates the amount of thermal resistance a substance has, and it is commonly used to measure the effectiveness of insulation. However, other building materials such as wallboard, shingles and siding also have R-values.

What Contributes to Heat Loss?

There are a number of ways a home loses heat energy. This is important when trying to determine how much heat in winter or cool air in summer a home loses and how energy-efficient a home is.

The primary ways a home loses heat is through:

  • Air leakage around doors and windows (35 percent)
  • Doors and windows (20 percent)
  • Slab or basement (15 to 18 percent)
  • Floors (15 to 18 percent)
  • Walls (12 to 15 percent)
  • Ceilings (10 percent)


To calculate home heat loss, you will come up with a number that calculates the loss of energy expressed in BTUs per hour.

The formula is:

Q over t

Q = (the area of a wall, ceiling, etc.) x (the inside temperature - the outside temperature)

t = the thermal resistance of the wall, which is calculated as (square feet of wall) x (the temperature in Fahrenheit) / BTUs per hour

Do a separate calculation for each wall, ceiling and floor, and make adjustments for doors and windows in the walls.

To calculate heat loss at different temperatures, loss per degree day and annual heat loss, use the formulas at:

Home Heat Loss Calculators

If the calculations are intimidating, another way to determine home heat loss is to use an online heat loss calculator to do the number manipulation for you. While you will still need to measure square footage and inside and outside temperatures, the calculators tell you exactly what numbers you need to add and what to measure.