How it’s calculated
Steady-state conduction through a flat assembly is proportional to area and temperature difference, and inversely proportional to its thermal resistance R (for a single layer R = thickness ÷ conductivity). The U-value is 1/R. Assumes steady temperatures, one-dimensional heat flow and no air leakage, solar gain or thermal bridging through studs and framing.
Example (US units): 100 ft² of wall at R-13 with 70 °F inside and 30 °F outside: Q = 100 × 40 ÷ 13 = 308 BTU/h (90 W).
SI check (OpenStax University Physics, Example 1.10): a foam icebox with A = 0.950 m², 2.50 cm walls and k = 0.035 W/m·°C has R = 0.025 ÷ 0.035 = 0.714 m²·K/W; with ΔT = 35 °C, Q = 0.950 × 35 ÷ 0.714 ≈ 46.6 W.
Add up each surface (walls, windows, roof, floor) and add air-leakage (infiltration) losses for a whole-building estimate; heating equipment should be sized with a full load calculation.
Frequently asked questions
What is the difference between R-value and U-value?
R measures resistance to heat flow (higher is better); U measures conductance (lower is better). For a whole assembly U = 1 / R_total.
Are US and SI R-values the same?
No. US R-values are in ft²·°F·h/Btu; SI (RSI) values are in m²·K/W. Multiply a US R-value by 0.1761 to get RSI.
Can I add R-values of layers?
Yes, for layers in series (e.g. drywall + insulation + sheathing + air films). Parallel paths such as studs need an area-weighted calculation.
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Sources
- University Physics Vol. 2, 1.6 Mechanisms of Heat Transfer — OpenStax
- Energy Saver Guide (R-value and insulation section) — U.S. Department of Energy
- NIST Guide to the SI (SP 811), Appendix B conversion factors — U.S. National Institute of Standards and Technology
Formulas are taken from the free public references above. Results are provided “as is” for informational and educational purposes only. See our disclaimer.
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