Heat Loss

Calculating Conductive Heat Loss Through a Surface

A surface’s heat loss depends on its area, how well it resists heat flow (its R-value), and how big a temperature difference it separates: Heat Loss = Area × Temperature Difference ÷ R-value. Enter a surface’s area, its R-value, and the indoor and outdoor temperatures, and this calculator finds how many BTU per hour that surface loses.

A lower R-value (less insulation) or a bigger temperature swing between inside and outside both drive heat loss up. This is the same underlying relationship that makes a poorly insulated wall feel cold to the touch on a winter day, and why adding insulation is one of the most effective ways to cut a home’s heating and cooling costs.

Key Factors to Consider

• This calculates one surface at a time — a home’s total heat loss is the sum across every wall, window, roof, and floor. Windows typically have much lower R-values than walls (often R-2 to R-5 for a window versus R-13 or higher for an insulated wall), so they’re frequently a disproportionately large contributor to total heat loss per square foot of surface area.
• This formula captures conductive heat loss only, not air infiltration. Air leaking through gaps, cracks, and unsealed penetrations is a separate, often substantial heat-loss pathway this calculation doesn’t include — a home can have well-insulated (low heat-loss) surfaces by this formula and still lose significant heat through air leaks the formula doesn’t measure.
• For sizing heating equipment, a “design temperature” is typically used instead of an average or record-cold temperature. Engineers commonly use a regional winter design temperature (a realistically cold, but not record-setting, value specific to your area) rather than either an average day’s temperature or the coldest day ever recorded, for a sizing estimate that’s conservative without being wildly oversized.
• A full heating or cooling load calculation accounts for more than one surface’s conduction. A proper whole-building load calculation (like the industry-standard Manual J) also factors in air infiltration, internal heat gains from people and appliances, and solar heat gain through windows — this calculator estimates one piece of that larger picture, not a complete load calculation on its own.

The Formula

$$Q = \frac{A \times \Delta T}{R}$$

Where $Q$ is heat loss (BTU per hour), $A$ is surface area (square feet), $\Delta T$ is the temperature difference between indoors and outdoors (°F), and $R$ is the surface’s R-value (ft²·°F·hr/BTU) — the standard imperial unit R-values are labeled in.

Worked Example

A 500 sq ft wall with an R-13 insulation rating, with the indoor temperature at 70°F and outdoor at 30°F:

1. Temperature difference: $70 - 30 = 40$°F.
2. Heat loss: $500 \times 40 \div 13 \approx 1,538$ BTU per hour.

Doubling the R-value (to R-26, roughly double the insulation) would cut that heat loss in half — the same wall, the same temperature difference, but half as much energy escaping every hour.

Common Mistakes

• Entering a U-factor as if it were an R-value. Windows are often rated with a U-factor rather than an R-value, and the two move in opposite directions — a lower U-factor means better insulation, while a lower R-value means worse insulation. Since R-value = 1 ÷ U-factor, plugging a U-factor straight into the R-value field produces a wildly wrong result.
• Using today’s outdoor temperature or a seasonal average when sizing heating equipment. As noted above, equipment sizing should use a regional winter design temperature, not just whatever the thermometer reads on a mild day — undersizing equipment based on an average temperature leaves a home unable to keep up on the coldest nights.
• Treating one surface’s heat loss as the home’s entire heating requirement. This calculator finds the loss through a single wall, window, or roof section — a home’s total heat loss (and therefore the furnace or heat pump size needed) is the sum across every exterior surface, not just the one entered here.
• Assuming this figure already includes drafts and air leakage. As covered above, this is conductive loss only — a home with excellent wall and window R-values can still feel drafty and lose substantial heat through unsealed gaps that this formula doesn’t capture at all.

Useful to Know

R-value and the metric RSI-value aren’t interchangeable by just moving a decimal point. Outside the U.S., insulation is commonly rated in RSI (m²·K/W) rather than R-value (ft²·°F·hr/ BTU) — converting between them means multiplying R-value by about 0.1761 (or dividing RSI by the same factor), so an American R-13 wall is roughly RSI-2.3, not “13” in metric terms. Comparing an insulation spec sheet from one system directly against a number from the other without converting first will make otherwise-equivalent insulation look very different.