Heat Loss Calculator
Find how much heat a house or room loses through its walls, windows, doors, ceiling, and floors, in BTU/h, watts, and kilowatts. Enter the size and construction of each surface and the indoor and outdoor temperatures. Add the air changes per hour to include the heat lost to air leakage.
- Choose US or Metric. You can still change the unit of any single field.
- Add a surface for each wall, window, door, ceiling, and floor that faces the outdoors, a vented attic, or a vented crawl space. Choose its type under Surface, then enter its size, or switch Measure by to Area if you know the area from a plan. Use Quantity for windows or doors of the same size.
- Measure each wall end to end, including its windows and doors. The calculator takes the windows and doors out of the wall area for you.
- Choose the Construction closest to yours. If you know the R-value of the whole wall, ceiling, or floor, choose Enter R-value. For a window or door with an NFRC label, choose Enter U-factor and enter the U-factor on the label.
- Check the Indoor temperature. It starts at 70 °F (21 °C). Enter the Outdoor design temperature for your area.
- Optionally, enter the Air changes per hour, the Floor area, and the Ceiling height to add the heat lost to air leakage.
Dimension fields accept whole numbers, decimals, fractions, and mixed
numbers, such as 12, 10.5, 3/4, or
6 1/2. Temperatures can be negative, such as
-10.
How to calculate heat loss
Heat flows out through each surface in proportion to its U-factor, its area, and the difference between the indoor and outdoor temperatures:
Heat loss = U-factor × area × (indoor temperature − outdoor temperature)
In US units, a U-factor in Btu/(h·ft²·°F) times ft² times °F gives BTU/h. In metric units, a U-factor in W/(m²·K) times m² times °C gives watts. The U-factor is the inverse of the R-value of the whole surface:
U-factor = 1 ÷ R-value
Air that leaks out is replaced by cold outdoor air, which has to be heated too. In US units:
Air leakage heat loss = 0.018 × air volume (ft³) × air changes per hour × temperature difference (°F)
The calculator adds the heat loss of every surface and the air leakage to get the total. The windows and doors are taken out of the walls in proportion to each wall's area, so you don't have to say which wall each window is in.
These are the formulas in Performing Heat Loss Calculations, a factsheet from the Washington Energy Extension Service (August 1988), which adds up the windows, doors, walls, ceilings, floors, and infiltration of a home to get its total hourly heat loss. 1 BTU/h is about 0.293 W.
This is different from a rule of thumb of so many BTU/h per square foot of floor area, which the BTU Calculator and the Furnace Size Calculator use. Adding up each surface shows where the heat goes, and how much a better window or more insulation would change it.
U-factors for common constructions
The construction choices use these U-factors. They include the framing, drywall, siding, and the air films on each side, not only the insulation. US values are in Btu/(h·ft²·°F), and metric values in W/(m²·K):
| Construction | US | Metric |
|---|---|---|
| Wall | ||
| 2×4, no insulation | U-0.356 | U 2.02 |
| 2×4, R-11 | U-0.11 | U 0.62 |
| 2×4, R-13 | U-0.102 | U 0.58 |
| 2×4, R-15 | U-0.095 | U 0.54 |
| 2×6, R-19 | U-0.074 | U 0.42 |
| 2×6, R-21 | U-0.069 | U 0.39 |
| Window | ||
| Single pane, metal frame | U-1.28 | U 7.27 |
| Single pane, wood or vinyl frame | U-0.99 | U 5.62 |
| Double pane, metal frame | U-0.79 | U 4.49 |
| Double pane, metal frame with thermal break | U-0.66 | U 3.75 |
| Double pane, wood or vinyl frame | U-0.58 | U 3.29 |
| Double pane low-E, insulated frame | U-0.3 | U 1.7 |
| Door | ||
| Wood door, 1 3/4 in thick | U-0.5 | U 2.84 |
| Wood door, thinner | U-0.6 | U 3.41 |
| Insulated steel door | U-0.5 | U 2.84 |
| Steel door, not insulated | U-0.7 | U 3.97 |
| Ceiling or roof | ||
| Attic floor, no insulation | U-0.3 | U 1.7 |
| Attic floor, R-19 | U-0.049 | U 0.28 |
| Attic floor, R-30 | U-0.032 | U 0.18 |
| Attic floor, R-38 | U-0.026 | U 0.15 |
| Attic floor, R-49 | U-0.02 | U 0.11 |
| Attic floor, R-60 | U-0.017 | U 0.1 |
| Floor | ||
| Over crawl space, no insulation | U-0.097 | U 0.55 |
| Over crawl space, R-13 | U-0.046 | U 0.26 |
| Over crawl space, R-19 | U-0.037 | U 0.21 |
| Over crawl space, R-30 | U-0.028 | U 0.16 |
| Over open air, no insulation | U-0.238 | U 1.35 |
| Over open air, R-19 | U-0.049 | U 0.28 |
| Over open air, R-30 | U-0.034 | U 0.19 |
- Walls, ceilings, floors, and doors come from Joint Appendix JA4 of the California Energy Commission's 2022 Reference Appendices (CEC-400-2022-010-AP). Walls are wood studs 16 in on center with no insulation on the outside. The table assumes a stucco exterior, and other siding changes the U-factor only a little.
- Attic floor rows are for insulation on the ceiling under a vented attic, and include the attic and roof above it. For a sloped ceiling with insulation between the rafters, enter its R-value or U-factor instead.
- Floors over a crawl space include the vented crawl space below, so you can use the outdoor temperature for them too.
- Windows come from Table 110.6-A of the California Energy Commission's 2022 Building Energy Efficiency Standards (CEC-400-2022-010-CMF). These are default values for windows without a rating and are higher than most rated windows. The low-E window is the high end of the 0.23 to 0.30 range in the Efficient Windows Collaborative's double low-E glazing table, run by the National Fenestration Rating Council. If your window has an NFRC label, enter its U-factor instead. Enter a patio door or a door that's mostly glass as a window.
- The same constructions are used in Metric. Wood-framed 2×4 and 2×6 walls (38 × 89 mm and 38 × 140 mm) are built the same way in Canada. For other construction, enter your own RSI value or U-factor.
If you enter an R-value, the calculator uses it as the R-value of the whole surface. The R-value on the insulation label leaves out the studs or joists, which let more heat through, so it gives less heat loss. A 2×4 wall with R-13 insulation is U-0.102 in the table, about the same as R-9.8 for the whole wall, while 1 ÷ 13 is U-0.077.
Indoor and outdoor temperatures
The indoor temperature starts at 70 °F, the heating design temperature in the EPA's ENERGY STAR National HVAC Design Report. In Metric, it starts at 21 °C, which is 70 °F (21.1 °C) rounded to a whole degree.
For the outdoor temperature, use your area's 99% heating design temperature: the temperature it stays above for 99% of the hours in a year. ENERGY STAR recommends the 99% heating design temperature of the nearest weather station in ACCA Manual J. In the US, its Design Temperature Limit Reference Guide lists limits by state and county. Elsewhere, use the design temperature from your local building or heating guidance. Don't use the record low. It gives much more heat loss than a home needs to handle on a normal cold day.
Air leakage
The 0.018 in the formula is the heat it takes to warm 1 ft³ of air by 1 °F: air weighs about 0.075 lb/ft³ and its specific heat is 0.24 Btu/(lb·°F), and 0.075 × 0.24 = 0.018. ACCA's article The Physics of Heat and Air Flow explains the same numbers behind the 1.08 used for airflow in ft³/min, and 1.08 ÷ 60 = 0.018. In metric, it's about 1,207 J/(m³·K). These are values for air near sea level. At high altitudes, the air is thinner, and the calculator gives a little more air leakage heat loss than there is.
The Washington Energy Extension Service factsheet puts most US homes between 0.3 and 1.5 air changes per hour, with newer homes at 0.3 to 0.8 and older homes at 0.8 to 1.5. Those values are from 1988, and tight newer homes can be lower. Use the natural air changes per hour, not the ACH50 from a blower door test, which is measured with the house under pressure and is much higher.
Examples
A one-story house
A house has 120 ft of exterior wall 8 ft high with 2×4, R-13 walls, eight 3 ft × 5 ft double-pane windows, two 3 ft × 6 ft 8 in wood doors, a 40 ft × 25 ft attic floor with R-30, and the same floor over a crawl space with R-19. At 70 °F indoors and 10 °F outdoors, a 60 °F difference:
- Walls: 960 − 160 = 800 ft² × U-0.102 × 60 = 4,896 BTU/h
- Windows: 120 ft² × U-0.58 × 60 = 4,176 BTU/h
- Doors: 40 ft² × U-0.5 × 60 = 1,200 BTU/h
- Ceiling: 1,000 ft² × U-0.032 × 60 = 1,920 BTU/h
- Floor: 1,000 ft² × U-0.037 × 60 = 2,220 BTU/h
- Air leakage at 0.5 air changes per hour: 0.018 × 8,000 ft³ × 0.5 × 60 = 4,320 BTU/h
The total is 18,732 BTU/h (5,490 W). Without the air leakage, it's 14,412 BTU/h.
R-value instead of the construction
If you enter R-13 for the same walls, they lose 800 × 60 ÷ 13 = 3,692 BTU/h instead of 4,896 BTU/h, because the label R-value leaves out the studs. Use the construction or the whole-wall R-value when you can.
Metric
Four walls 10 m × 2.5 m at RSI 2.5, six 1.2 m × 1.5 m windows at U 1.4, a 0.9 m × 2.1 m door at U 1.8, a 100 m² ceiling at RSI 6, and a 100 m² floor at RSI 2.5, with 21 °C indoors, −5 °C outdoors, and 0.5 air changes per hour in 250 m³ of air, lose 3,953 W (13,487 BTU/h).
Walls with different constructions
With 600 ft² of 2×4, R-13 wall, 400 ft² of 2×6, R-21 wall, and 100 ft² of windows, the calculator takes 10% off each wall and gives 8,275 BTU/h at a 60 °F difference. If all the windows are in the 2×4 wall, the actual heat loss is 8,196 BTU/h, about 1% less. When all the walls have the same construction, it doesn't matter which wall the windows are in.
FAQ
The U-factor is 1 ÷ the R-value. The R-value says how well a surface resists heat flow, and the U-factor says how much heat gets through, so a lower U-factor means less heat loss. An R-20 wall is U-0.05.
Your area's 99% heating design temperature, not the record low. It's a cold temperature that only about 1% of the hours in a year are colder than. In the US, ENERGY STAR's Design Temperature Limit Reference Guide lists the heating limits by state and county.
Only as a rough check. The result is a whole-house estimate from typical U-factors. A heating contractor sizes equipment with a room by room Manual J calculation that also counts duct losses and local design conditions.
Choose the construction closest to yours instead. The insulation's R-value leaves out the studs or joists, which let more heat through. For a 2×4 wall with R-13, the construction gives 4,896 BTU/h for 800 ft² at a 60 °F difference, while R-13 alone gives 3,692 BTU/h.
No. ACH50 is measured with the house under pressure and is many times the natural air changes per hour. Enter a natural value, such as 0.3 for a tight newer home to 1.5 for an older, drafty one.
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