By Zencalcy Editorial TeamLast reviewed About us
Heat loss through a building element is the product of three things: how big it is, how well it resists heat flow, and how cold it is outside. The U-value carries the insulation story — lower is better — and multiplying area by U-value by the temperature difference gives BTU per hour.
BTU per hour lost through a wall, window or room.
Q = A × U × ΔT
U-values in imperial units: single glass 1.04, double low-E 0.32, insulated wall 0.08, 2×4 fiberglass wall 0.06, attic floor R-38 0.026. The whole-house loss sums every element plus an infiltration term.
| Symbol | Meaning |
|---|---|
| A | Element area in square feet. |
| U | Thermal transmittance, BTU/hr·ft²·°F. |
| ΔT | Indoor minus outdoor temperature. |
1,500 × 0.08 × 50 = 6,000 BTU/hr through the envelope — about half a ton of continuous heating at design conditions.
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Thermal transmittance: BTU per hour flowing through one square foot per degree of temperature difference. It is the reciprocal of R-value, so U-0.08 equals R-12.5.
Results use standard construction estimating arithmetic: volumes from measured area and depth, unit weights from published material densities, and code-referenced spans or clearances where the page names a code. Quantities are rounded up to whole saleable units and include the stated waste allowance. These tools are planning aids, not structural design, and do not replace a licensed engineer, a local code check, or a contractor's site measurement.
Reference material used to build and check this calculator:
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Content updated: September 2026