Heat loss & R-value.
Calculate the heat that flows through a wall, window, or roof given its R-value (or U-value), area, and the temperature difference across it. Imperial and SI units handled.
How this works
Heat flows from warm to cold through any surface. The rate is governed by Fourier's law of conduction, which for a uniform wall reduces to:
Q = (A × ΔT) ÷ R or equivalently Q = U × A × ΔT
where Q is heat flow, A is area, ΔT is temperature difference across the surface, R is the thermal resistance, and U is the thermal conductance (1/R).
R-value vs U-value
Both describe the same thing — they're reciprocals. R-value is what you'll see on insulation packaging (higher R = better insulation). U-value is what you'll see on window specs (lower U = better window). Multiply the two and you get 1.
Imperial vs metric R-values are NOT interchangeable
Imperial R-value (h·ft²·°F/BTU) and metric R-value (m²·K/W) differ by a factor of about 5.68. An R-13 imperial insulation is roughly RSI-2.3 metric. Many imported building products list both; if you see just "R-2.3," that's metric and would be R-13 to a US carpenter. Use the converter below or our unit converter to verify.
Reference R-values (US, imperial)
| Assembly | Typical R-value | Notes |
|---|---|---|
| Single-pane window | R-1 | Practically no insulation. Replace. |
| Double-pane window (clear) | R-2 | Standard new construction. |
| Double-pane low-E argon | R-3 to R-4 | Premium window. |
| Triple-pane window | R-5 to R-7 | High-end / cold climates. |
| Uninsulated wood-frame wall | R-3 to R-4 | Wood + drywall + air gap only. |
| R-13 batt in 2×4 wall | R-13 (R-11 effective) | Studs reduce effective R; "whole wall" rating ~R-11. |
| R-19 batt in 2×6 wall | R-19 (R-17 effective) | Common in modern construction. |
| R-30 ceiling batt | R-30 | Code minimum for most US climates. |
| R-49 attic insulation | R-49 | Recommended for cold climates (DOE zones 5–7). |
| Concrete block (8") | R-1.1 | Negligible insulation without added foam. |
| Fiberglass batt (per inch) | R-3.1 to R-3.7 | Loose fill ~R-2.5 per inch. |
| Closed-cell spray foam (per inch) | R-6 to R-7 | Highest practical R per inch for retrofits. |
Common pitfalls
- Nominal vs effective R-value. An R-13 batt installed in a stud wall has framing penetrations (studs are roughly R-4 per inch). The whole-wall R-value is typically 70–85% of the batt rating. For accurate heat-loss work, use whole-wall R-values from ASHRAE 90.1 or the IECC.
- Radiation and convection aren't included. R-value measures conductive heat transfer only. Real building envelopes lose heat through air leakage (infiltration) and radiation through windows too. For a typical leaky house, infiltration is 25–40% of total heat loss.
- R-values aren't perfectly additive in series. An R-13 wall plus R-5 sheathing is approximately R-18, not exactly. Air films, framing, and installation quality all change the result by 5–15%.
- This calculator gives steady-state loss. Real walls have thermal mass — heat that's stored in framing and drywall and released later. For peak heating load (sizing a furnace), use Manual J or similar. For annual energy cost, use degree-day methods.
- R-value performance varies with temperature. Fiberglass batts lose about 20–30% of their rating at extreme cold (−30 °C / −22 °F) due to convection inside the insulation. Closed-cell foam doesn't have this issue.
Sources
- Heat transfer formulas: ASHRAE Fundamentals Handbook (latest edition), Chapter 25 (Heat, Air, and Moisture Control).
- Standard R-values: ASHRAE 90.1 / IECC for whole-wall assemblies; manufacturer specifications for batt and rigid insulation.
- Unit conversion: 1 m²·K/W = 5.678 h·ft²·°F/BTU (the conversion factor between SI and imperial R-values).
Disclaimer. This is a simplified steady-state heat loss calculator. For HVAC equipment sizing or energy-code compliance, use Manual J / Manual S (residential) or ASHRAE methods (commercial), which account for solar gain, internal loads, infiltration, and thermal mass.