How to Calculate the Heat Loss for a Roof

The heat loss of a structure is the amount of heat energy, usually measured in BTUs, that escapes through the surface every hour. Knowing these values for the different parts of your home, particularly the roof, is important when reroofing or purchasing a furnace. You can calculate your roof's heat loss using a simple equation requiring only basic data about the makeup of your roof and the local temperatures.

Ladder

Instructions

- 1
  Climb onto the roof using a ladder and safety equipment and measure the length and width of every rectangular section. Multiply each pair of numbers and add the products together to calculate the area.
- 2
  Subtract the lowest annual outdoor temperature from the internal temperature of your house. This number is known as your home's temperature difference or temperature lift.
- 3
  Find the R-values of each layer of material used in the construction of your roof. A list can be found at the Archtoolbox website (see Resource for link). Add these together and divide 1 by the total to find their combined reciprocal, known as the roof's U-value or conductance.
- 4
  Multiply the conductance, temperature difference and area of the roof together. The product of the calculation is the hourly heat loss of the roof, expressed in BTUs.

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