Heat Loss Through Walls, Windows and Roofs

Understanding your result

The headline is the total conduction loss through the elements you entered, with the temperatures in the sub-line. The bars and stats show each element’s loss, and the table gives each one’s R-value and U-factor, its loss and its share of the total, then the temperature difference, the total, the heat loss per degree (UA) and the average R-value across all the elements.

The notes name the element that loses the most per square foot, which is where improvements pay best, and flag an attic below R-38. The average R-value row is a quick way to compare two versions of a house: replacing double-pane with low-e windows, or adding attic insulation, moves it more than the area suggests.

The UA is the input the degree-day fuel estimate turns into a season’s fuel. For sizing equipment, the furnace size calculator and the heat pump size calculator use whole-house loads per square foot that include air leakage; this calculator is the element-level view behind them. If the ceiling is the weak point, our sister site Roofings Calculator works out how much attic insulation to add.

How we calculate this

This is steady-state conduction, the basis of the heat-loss method in the ASHRAE Handbook, Fundamentals, and in ACCA Manual J. R-values are in US units (hr·sq ft·°F per BTU); to use metric RSI values, multiply them by 5.678 first. Typical R-values for common assemblies and the climate-zone loads that include air leakage are on the BTU per square foot table, and the unit conversions, including W/K and RSI, on the unit conversions page.

The assumptions behind the numbers

Assumptions last reviewed October 8, 2026.

The calculator leaves out air leakage, ground losses, thermal bridging through framing, solar gain through windows, and duct losses, which together often make up half or more of a house’s heat loss. It is therefore lower than a whole-house load and should not be used alone to size a furnace. The guide to BTU per square foot by climate zone explains what the whole-house figures include, and the guide on sizing an air conditioner covers what Manual J adds.

Two worked examples

Where to find your inputs

Areas. Measure walls (exterior perimeter × height, minus windows and doors), windows and doors (rough openings), and the ceiling (usually the floor area of the top storey).

R-values. For insulation, printed on batts or estimated from depth (about R-3.5 per inch for fibreglass or cellulose); for windows, the NFRC label’s U-factor (R = 1 ÷ U); for walls, R-11 to R-13 for insulated 2 × 4 walls, R-19 to R-21 for 2 × 6.

Temperatures. The indoor setting and, for sizing, your winter design temperature.

Common mistakes

• Counting window area as wall. Subtract windows and doors from the wall area.
• Using the insulation’s R-value for the whole wall. Framing lowers it; use a whole-wall figure for precision.
• Confusing U and R. U = 1 ÷ R; a U-0.30 window is R-3.3.
• Sizing a furnace from conduction alone. Air leakage and ground losses are missing.
• Ignoring the attic. It is often the cheapest place to cut heat loss.
• Mixing metric RSI with US R. Multiply RSI by 5.678 for US R-value.

Questions people ask

How do I calculate heat loss through a wall?

Multiply the area by the temperature difference and divide by the R-value. A 200 sq ft wall insulated to R-13, with 70 °F inside and 20 °F outside, loses 200 × 50 ÷ 13 = 770 BTU per hour. The same formula works for windows, ceilings, doors and floors over unheated space; the U-factor printed on windows is 1 ÷ R, so you can also multiply area by U by the temperature difference.

How much heat is lost through windows?

Far more per square foot than through walls. A single-pane window is about R-1, a standard double-pane about R-2, a low-e double about R-3 and a triple about R-5, against R-13 to R-21 for an insulated wall. In the calculator's default house, 200 sq ft of double-pane windows lose 6,000 BTU per hour at a 60 °F difference, more than 1,200 sq ft of R-13 wall.

What R-value should my attic have?

The International Energy Conservation Code recommends R-30 in the warmest US climate zones and R-49 to R-60 in most of the country, which is 10 to 20 inches of blown insulation depending on the material. Many older houses have R-11 to R-19. Because the attic is usually the largest single surface, bringing it up to the recommendation is one of the cheapest heat-loss reductions.

What is UA?

The heat loss per degree of temperature difference, the sum of each element's area divided by its R-value, in BTU per hour per °F (or W/K in metric). It is the property of the house that, multiplied by a temperature difference, gives the heat loss at that moment, and multiplied by 24 hours and the heating degree days, gives the season's heat. The degree-day fuel calculator uses it.

Why is this lower than my furnace size?

Because it counts only conduction through the elements you enter. A full heat loss also includes air leakage (often a third or more of the total), heat lost to the ground through slabs and basement walls, the extra loss through framing that bypasses the insulation, and duct losses. And most furnaces are oversized. A Manual J includes all of these; this calculator is the part you can check by hand.

Is the R-value on the insulation the R-value of the wall?

Not quite. In a wood-framed wall the studs, about 15 to 25% of the area, have a much lower R-value than the insulation between them, so an R-13 batt in a 2 × 4 wall gives an effective whole-wall R-value nearer R-11, and the drywall, sheathing and air films add a little back. For a quick estimate use the insulation's R-value; for a careful one, use the whole-wall figure.