How to Calculate Heat Loss on a Commercial Building

Updated March 16, 2017

A great deal of money can be lost every year due to the costs of heating a poorly insulated building. Heat loss is measured in British Thermal Units or BTUs. A BTU is the amount of heat energy that is required to increase the temperature of one pound of water one degree. If your building is losing a large amount of BTUs every winter, it might be a good idea to invest in insulation, weatherstripping or replacement windows.

Measure the length, width and height of each room in the building you wish to determine heat loss in. You will need the surface area of the floor, ceiling, and walls of each room in the building. Record these measurements on a piece of paper as you determine them.

Calculate the heat loss of the roof using the following equation: Ceiling area X multiplier for building material used = heat lost through roof. Different building materials have different multipliers. The multiplier for glass and corrugated asbestos is 5.7, sheet asbestos is 6.5, 25mm of wood is 5 and 100mm of concrete is 4. Record your findings on the paper.

Calculate the heat lost through the walls using the following equation: Wall area X multiplier for building materials used = heat lost through walls. Different materials have different multipliers. These multipliers are 1.1 for 140mm of insulation block and 1.4 for corrugated double cladding with 25mm of fibreglass over polythene vapour barrier. You can refer to a table with additional multipliers for different materials at Record your results.

Add the total amount of heat lost through the walls. Record your results.

Calculate the heat lost through the floor using the following equation: Floor area X multiplier for building material used = heat lost through floor. Different floors have different multipliers. The multiplier for an earth floor is 1.9 and the multiplier for a concrete floor is 0.7. Record your results.

Repeat this process for each room of the building and add your results into a single sum. This is approximately how many BTUs your building is losing per hour without accounting for windows, doors and ventilation.


To get a more accurate reading on your building's heat loss, calculate the heat lost through the windows of each room by using the same equation used on other surfaces. Don't forget to subtract the area of the window from the area of the wall it is on.

Things You'll Need

  • Measuring tape
  • Calculator
Cite this Article A tool to create a citation to reference this article Cite this Article

About the Author

Mike Thurau has written for his college newspaper since 2009. He has written editorials and freelance articles for the "BG News" and Cleveland's "Plain Dealer." He is currently enrolled in Bowling Green State University studying political economy.