Heat Loss Calculator

How heat leaves a building

A heated building loses heat in two main ways: conduction through its walls, windows, roof and floor, and air leakage, where warm air escapes and cold air takes its place. Conduction through each surface is area × temperature difference ÷ R-value, or equivalently area × U-factor × temperature difference, where U = 1 ÷ R. Air leakage is volume × air changes per hour × temperature difference × 0.018, the heat needed to warm one cubic foot of air by one degree Fahrenheit. Add them all and you have the heat output a furnace or heat pump must supply to hold the indoor temperature on the coldest design day.

A worked example

Take a single-storey house with 1,100 ft² of net wall at R-13, 180 ft² of double-glazed windows and doors at U-0.30, a 1,200 ft² ceiling with R-38 attic insulation, and 9,600 ft³ of heated volume leaking at 0.5 air changes per hour. With 70 °F inside and a 10 °F design temperature, the difference is 60 °F. The walls lose 1,100 × 60 ÷ 13 = 5,077 BTU/h, the windows 180 × 0.30 × 60 = 3,240, the ceiling 1,200 × 60 ÷ 38 = 1,895 and air leakage 0.018 × 9,600 × 0.5 × 60 = 5,184. The total is 15,396 BTU/h, about 4.5 kW.

Where the heat really goes

The example shows two things that surprise most homeowners. First, the windows are only 7% of the envelope area in the calculation but lose 21% of the heat, because even good double glazing has an R-value of about 3, against 13 for an insulated wall. With old single glazing at U-1.1, the total jumps to 24,036 BTU/h. Second, air leakage is a third of the load. Doubling leakage to 1.0 air change per hour, typical of an older, draughty house, raises the total to 20,580 BTU/h. Air sealing is often cheaper than any other improvement for the heat it saves, which is why energy auditors use a blower door test before recommending insulation.

Design temperatures and limits

The outdoor design temperature is not the coldest temperature ever recorded but a statistical value, often the temperature exceeded 99% of the hours in winter, published for each location by ASHRAE. Using an extreme record low oversizes the heating system. This calculator also leaves out floors, basements and slabs, which lose heat to the ground at different rates, and heat lost from ducts in unheated spaces, which can add 10 to 30%. For equipment selection, a full Manual J or equivalent calculation is the professional standard; this tool shows the size and the main contributors.

How to use it

1. Enter the indoor and outdoor design temperatures in °F.
2. Enter the net wall area and its R-value, and the window and door area and its U-factor.
3. Enter the ceiling area and its R-value, the heated volume and the air changes per hour.
4. Read the design heat loss and the share from each part, and target the largest one first.