Heat Loss Calculator

Background.

An insulation label that says R-13 tells you how well a wall resists heat. It does not tell you how much heat leaves through it, and that rate is what sizes a furnace, compares one insulation upgrade with another, or explains why one room is always cold. The rate depends on three things: the size of the surface, how strongly it resists heat flow, and how much colder it is outside than in. Multiply the area by the temperature difference, divide by the R-value, and you have the heat flow in BTU per hour.

Written as Q = U × A × ΔT, with U = 1/R, this is the conduction equation of the ASHRAE Handbook — Fundamentals and of the Department of Energy's building-science course notes. Every term acts in proportion. Double the area or the temperature difference and the loss doubles; double the R-value and it halves. That last rule has a practical edge. Taking a surface from R-1 to R-2 halves its loss, and so does taking an attic from R-30 to R-60, but per square foot the first upgrade saves thirty times as many BTUs as the second.

Each run covers one assembly: a stretch of wall, a roof, a floor, a window or a door. A house is the sum of its assemblies plus the warm air that leaks out through gaps and is replaced by cold air, which a conduction figure does not include. The outdoor temperature to use is a design temperature, the cold night the heating must cope with, not a seasonal average; for houses that is normally the 99% heating design temperature from ASHRAE's climate tables for the nearest weather station. Enter an outdoor temperature above the indoor one and the result turns negative, meaning heat is flowing in rather than out.

What is heat loss calculator?

Heat loss here is the steady rate at which heat conducts through one building assembly when the air on each side is held at a fixed temperature, measured in BTU per hour or watts. In US units R-value is in ft²·°F·h/BTU, which reads naturally: an R-13 wall needs 13 °F of temperature difference to push one BTU per hour through each square foot. U-value is the reciprocal, the BTU per hour that flow through a square foot for each degree of difference. The R-value that belongs in the calculation is the whole assembly's: every layer from drywall to siding, plus the thin films of still air on each face, added together, and reduced where framing bridges the insulation. Because the result is a rate, it describes one moment at design conditions, not a season's fuel use.

How to use this calculator.

1. 01Measure one assembly in square feet. For a wall, take length times height and subtract the windows and doors, which get their own runs.
2. 02Enter the R-value of the whole assembly. The R-13 in the field's hint is the rating of batts in a 2×4 stud cavity; studs lower the effective figure and sheathing, drywall and air films raise it, so use an assembly table where you can.
3. 03Enter the indoor design temperature; 70 °F is the usual figure for heating.
4. 04Enter the outdoor design temperature for your location, typically the 99% heating dry-bulb from ASHRAE climate data, not the average January temperature.
5. 05Read the heat loss in BTU per hour and watts, then repeat for each assembly and add the results for the room or house.

The formula.

The temperature difference comes first: ΔT = indoor temperature − outdoor temperature, in °F. The R-value is inverted to give U = 1/R, in BTU per hour per square foot per °F. Heat loss is then Q = U × A × ΔT in BTU per hour, and dividing by 3.412142, the number of BTU per hour in a watt from the NIST SP 811 conversion tables, gives the same rate in watts. The product U × A, often written UA, is the assembly's loss per degree of temperature difference and is the handiest number to carry around. Two combining rules follow from the physics. Layers stacked in series, such as drywall, insulation and sheathing, add their R-values. Areas side by side, such as a wall and the window in it, add their UA values; averaging R-values by area understates the loss whenever the areas differ much in quality. The equation assumes steady conditions. It ignores the heat stored in heavy materials, sunshine on the surface, air leakage and the effect of wind beyond what the outside air film already allows for, which is why it is a design-day conduction figure and not a prediction of what a thermostat will see.

A worked example.

Frequently asked questions.

References& sources.

1. [1]ASHRAE. "ASHRAE Handbook — Fundamentals." American Society of Heating, Refrigerating and Air-Conditioning Engineers.
2. [2]U.S. Department of Energy, Building Science Education. "Heat Loss Calculations and Principles" (Course No. M05-003).
3. [3]U.S. Department of Energy, Building Science Education. "Conduction and U-value vs. R-value."
4. [4]National Institute of Standards and Technology (2008). "Guide for the Use of the International System of Units (SI), NIST Special Publication 811." U.S. Department of Commerce.