ℹ About This Calculator
The U-value (thermal transmittance) measures how readily heat passes through a building assembly — a wall, roof, floor or window — and it drives the conduction portion of a building's heating and cooling load. This calculator computes the conduction heat loss or gain from the assembly's U-value, its area, and the temperature difference across it, using the fundamental relationship Q = U·A·ΔT. It also returns the equivalent R-value (thermal resistance) and the heat flux per square foot, in US units, so you can evaluate insulation, compare assemblies, and feed envelope loads into an ASHRAE or Manual J calculation.
U-value and R-value are reciprocals: U = 1/R. A lower U-value (higher R-value) means better insulation and less heat transfer. The conduction heat loss through an assembly equals its U-value times its area times the temperature difference between inside and outside. This conduction load, summed over all the walls, roof, glazing and floor, is a major component of a building's total heating and cooling load, alongside solar gain, infiltration, ventilation and internal gains. Energy codes such as the IECC and ASHRAE 90.1 set maximum U-values (minimum R-values) by climate zone.
Frequently Asked Questions
What is a U-value?
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A U-value (thermal transmittance) is the rate of heat transfer through a building assembly per unit area per degree of temperature difference, in BTU/hr·ft²·°F. It describes how well a wall, roof, floor or window conducts heat — a lower U-value means less heat passes through, so it is better insulated. The U-value is the reciprocal of the assembly's R-value (U = 1/R) and is used directly in the conduction heat-loss equation Q = U·A·ΔT.
How do I convert U-value to R-value?
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They are reciprocals: R = 1/U and U = 1/R. So a U-value of 0.06 BTU/hr·ft²·°F equals an R-value of 16.7 hr·ft²·°F/BTU, and an R-30 assembly has a U-value of 1/30 = 0.033. R-value is the more familiar term for insulation ratings, while U-value is used in heat-loss calculations and energy codes because it can be multiplied directly by area and temperature difference.
How do I calculate heat loss through a wall?
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Use Q = U·A·ΔT. Multiply the wall's U-value (BTU/hr·ft²·°F) by its area (ft²) and by the temperature difference between inside and outside (°F). For example, a 200 ft² wall at U-0.06 with a 50°F temperature difference loses 0.06 × 200 × 50 = 600 BTU/hr. Sum the heat loss of all the walls, roof, windows and floor to get the building's total conduction heat loss.
What is a good U-value for a wall?
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It depends on the climate zone and code, but modern energy codes typically require exterior walls around U-0.045 to U-0.084 (R-12 to R-22 continuous or cavity plus continuous), roofs much lower (U-0.02 to U-0.03, R-30 to R-50), and windows around U-0.30 to U-0.50. Colder climates require lower U-values. Check the IECC or ASHRAE 90.1 tables for the specific maximum U-values in your climate zone.
What is heat flux?
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Heat flux is the heat loss per unit area, equal to U·ΔT in BTU/hr·ft², independent of the total area. It tells you how intensely heat is moving through the assembly per square foot. Multiplying the heat flux by the area gives the total heat loss. Comparing heat flux across assemblies at the same temperature difference is a quick way to see which parts of the envelope lose the most heat per square foot.
Does the U-value calculation include solar gain or infiltration?
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No — Q = U·A·ΔT captures only the conduction heat transfer driven by the temperature difference. A complete building load also includes solar gain through glass, air infiltration and ventilation, and internal gains from people, lights and equipment. The conduction loss from this calculator is one important component that is summed with those other loads in a full ASHRAE or Manual J heating and cooling load calculation.
Is this U-value calculator accurate?
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It applies the exact conduction relationship Q = U·A·ΔT used throughout building science and energy codes, so it is reliable for evaluating assemblies and computing envelope conduction loads. For a whole-building load, combine the conduction results for every assembly with solar, infiltration, ventilation and internal gains, and use design temperatures from ASHRAE for your location rather than extremes.
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