What "heat loss" means
Heat always moves from warm to cold. On a winter night, the warmth in a room leaks out through every surface that touches the outdoors or an unheated space, and through cold air that sneaks in around windows, outlets and doors. The heat loss is the rate of that leak, in BTU per hour (or watts). A heater that puts out the same amount holds the room at temperature. Anything less, and the room slowly cools.
Rough rules such as "40 BTU per square foot" ignore whether a room has one small window or a wall of glass. A heat loss calculation looks at the actual surfaces, so it can tell a snug interior bedroom from a drafty corner room over the garage.
How to calculate heat loss for a room
The calculator adds up two kinds of loss.
1. Conduction through surfaces. Each wall, window, ceiling and floor loses heat in proportion to its area and the temperature difference across it, and in inverse proportion to its R-value. Windows are usually rated by U-factor instead, which is simply 1 ÷ R.
surface loss (BTU/hr) = area (sq ft) × ΔT (°F) ÷ R-value
window loss (BTU/hr) = area (sq ft) × U-factor × ΔT (°F)
2. Air leakage (infiltration). Outdoor air that leaks in must be warmed. Air holds about 0.018 BTU per cubic foot per °F, so:
air leakage (BTU/hr) = 0.018 × room volume (cu ft) × air changes per hour × ΔT (°F)
ΔT is the design temperature difference: the indoor temperature you want minus the outdoor temperature you design for. Designers use the local 99 percent heating design temperature, meaning only about 1 percent of winter hours are colder. Your building department, an HVAC contractor or ACCA Manual J tables can give you the figure for your county. It is usually well above the record low.
Worked example
A 14 × 12 ft corner bedroom with 8 ft ceilings has 26 ft of exterior wall and 30 sq ft of clear double-pane windows. The walls are 2×4 with R-13 batts (about R-11 for the whole wall once studs are counted), the attic has R-30, the floor sits over a heated basement, and the house is average for air leaks. Design conditions are 70°F inside and 0°F outside, so ΔT is 70°F.
| Part | Math | Heat loss |
|---|---|---|
| Walls | (26 × 8 − 30) = 178 sq ft × 70 ÷ 11 | 1,130 BTU/hr |
| Windows | 30 sq ft × 0.50 × 70 | 1,050 BTU/hr |
| Ceiling | 168 sq ft × 70 ÷ 30 | 390 BTU/hr |
| Air leakage | 0.018 × 1,344 cu ft × 0.7 × 70 | 1,190 BTU/hr |
| Total | 3,760 BTU/hr (1,100 W) |
That works out to 22.4 BTU/hr per square foot of floor, or about 71 W/m², well under the 40 to 50 BTU per square foot that quick rules of thumb suggest for a cold climate. Notice the windows: they are only 14 percent of the wall area but cause 28 percent of the loss.
Typical R-values and U-factors
The menus use whole-assembly figures, not the number printed on the insulation. Wood studs conduct heat far better than insulation, so a 2×4 wall with R-13 batts performs closer to R-11 overall. These are rules of thumb; your actual walls may differ.
| Assembly | Typical value | Metric |
|---|---|---|
| Single-pane window | U-1.0 | U 5.7 W/m²K |
| Double-pane, clear | U-0.50 | U 2.8 W/m²K |
| Double-pane, low-e | U-0.30 | U 1.7 W/m²K |
| Uninsulated wall | R-4 | RSI 0.7 |
| 2×4 wall, R-13 batts | R-11 | RSI 1.9 |
| 2×6 wall, R-19 to R-21 batts | R-16 | RSI 2.8 |
For a newer window, use the U-factor on its NFRC label instead. For attic depths and what it takes to reach a given R-value, see the insulation calculator and the guide to attic insulation R-values.
What makes the biggest difference
- Air leaks are often the largest single item. In the example, moving from "average" to "tight" cuts air leakage from 1,190 to 590 BTU/hr. Sealing gaps at baseboards, outlets, attic hatches and window trim is cheap compared with new windows.
- Windows punch above their size. Swapping the example's clear double-pane units for low-e glass drops the total from 3,760 to 3,340 BTU/hr. Cellular shades and storm windows help too.
- Rooms over garages and crawlspaces run cold. An uninsulated floor over an unheated space adds 2,940 BTU/hr to the same bedroom, nearly doubling the total. The calculator uses the full outdoor ΔT for these floors, which is slightly conservative because a crawlspace or garage is usually a little warmer than outside.
- More attic insulation has diminishing returns once you are past about R-30. Going from R-30 to R-49 saves only 150 BTU/hr here; the walls and air leaks matter more.
Using the result
For a space heater, baseboard or mini-split head in one room, pick a unit whose rated output meets or slightly exceeds the heat loss. Divide watts by 1,000 for kW; a 1,500 W plug-in heater delivers about 5,100 BTU/hr. To see what running it will cost, try the space heater cost calculator. For a quick check without measuring every wall, the heating BTU calculator uses floor area alone.
Safety and code notes
- This is a planning estimate, not an engineering design. In the US, the International Residential Code calls for heating equipment to be sized with ACCA Manual J and Manual S, and Canada and Australia have their own methods. Ask your installer for the room-by-room calculation; see what size furnace you need for more.
- Do not seal up a home so tightly that fuel-burning appliances lose their combustion air. If you have a gas furnace, water heater or fireplace, have a professional check venting after major air sealing, and keep working carbon monoxide alarms.
- Electric heaters need a circuit that can carry them. Fixed baseboard or wall heaters should be installed to local electrical code, often by a licensed electrician.
Frequently asked questions
How do you calculate heat loss in a room?
Add the conduction loss through each surface (area × temperature difference ÷ R-value, or area × U-factor × temperature difference for windows) to the air-leakage loss (0.018 × room volume × air changes per hour × temperature difference). The result is in BTU per hour; divide by 3.412 for watts.
How many BTU does a 12 × 14 room need?
It depends on its walls, windows and climate. A 12 × 14 ft corner bedroom with R-11 walls, R-30 attic, 30 sq ft of double-pane windows and average air leakage loses about 3,760 BTU/hr at 0°F outside. The same room in a 30°F design climate (ΔT 40°F) loses about 2,150 BTU/hr.
What design temperature should I use?
Use your area's 99 percent winter design temperature, which is warmer than the record low. Local building departments, HVAC contractors and Manual J tables list it. Most people keep the indoor side at 68 to 72°F.
Do single-pane windows really lose that much heat?
Yes. In the example room, the 30 sq ft of glass loses 1,050 BTU/hr as clear double-pane, 2,100 BTU/hr as single-pane and 630 BTU/hr as low-e double-pane, while the 178 sq ft of insulated wall loses 1,130 BTU/hr.
