How to use the conduction heat loss calculator
- Pick feet with degrees Fahrenheit and R-values, or meters with degrees Celsius and RSI.
- Enter the indoor and outdoor temperatures. The outdoor one must be lower.
- List each surface with its name, area and either its R-value (RSI) or its U-factor. Add rows for walls, ceiling, floor and windows.
- Read the total heat loss in Btu per hour and watts, and the share lost through each surface.
Formula
Heat loss through a surface Q = U x area x temperature difference
U = 1 / R, where R is the R-value (or RSI in metric)
Total = sum of the losses of all surfaces
1 Btu per hour = 0.29307107 watts
In US units U is in Btu per hour per square foot per degree F and R is its inverse. In metric units U is in watts per square meter per kelvin and the inverse is RSI.
A small house and a room
With 70 F inside and 30 F outside, a 40 degree difference, walls of 800 square feet at R-19 lose 1,684.2 Btu/h, a ceiling of 1,000 square feet at R-38 loses 1,052.6 Btu/h and 100 square feet of windows at a U-factor of 0.35 lose 1,400 Btu/h. The total is 4,136.84 Btu/h, or 1,212.39 W, and the walls are 40.71 percent of it.
A single 100 square foot wall at R-20 loses 200 Btu/h over the same difference. In metric, a room at 21 C with 1 C outside, walls of 75 m2 at RSI 3.35, a ceiling of 90 m2 at RSI 6.7 and 9 m2 of windows at U 2.0 loses 1,076.42 W, or 3,672.89 Btu/h.
Seeing where the heat goes
The share column shows which surfaces matter most. Windows often lose a large part of the heat even though they cover little area, because their U-factor is much higher than a wall. This helps you decide where insulation or better glazing gives the largest change.
For the R-value of a layered wall use the R-value calculator, and for framing effects see the framing factor and effective R-value calculator. The insulation calculator estimates material, and the heating cost from degree days calculator turns losses into a season cost.
What conduction leaves out
A real heating load also includes air leakage, ventilation, heat lost through the ground and slab, and extra loss through studs and corners. Sun and internal heat reduce the need. Because those parts are not here, the total is a conduction estimate and not a heating system size.
Assumptions and limits
- The temperatures are steady, and each surface has one R-value or U-factor for its whole area.
- The R-value or U-factor is the whole assembly value, including air films, as you enter it.
- The outdoor temperature must be lower than the indoor one, so the page only shows heat loss.
- Air leaks, ventilation, ground contact and thermal bridges are not included.
Frequently asked questions
How do I calculate heat loss through a wall?
Divide the area by the R-value and multiply by the temperature difference. 100 square feet at R-20 with a 40 degree difference loses 200 Btu/h.
What is the difference between R-value and U-factor?
U is the inverse of R. A wall with R-20 has a U-factor of 0.05. Windows are usually rated by U-factor.
How do I convert Btu per hour to watts?
Multiply by 0.29307107. The page shows both.
Is this enough to size a furnace?
No. It is only the conduction part of the load. A full heating calculation also needs air leakage, ventilation and other terms.