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U-value of a wall: add up the layers

Add up to four layers, such as drywall, insulation, sheathing, concrete or brick, to calculate the total R-value, the U-factor and the heat loss of a wall.

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Results

U-factor of the wall

0.0409

U-0.048 or better (R-21 overall): a well-insulated wall
Total R-value, surfaces included
24.5
R-value of the layers alone
23.5
Heat loss through this wall
163 BTU/h

How heat crosses a wall

Heat flows through a wall from the warm side to the cold side. Each layer resists that flow according to its thermal resistance, R = thickness ÷ thermal conductivity (λ). The resistances of layers in series simply add up, along with two thin films of still air on the inside and outside surfaces. The U-value, or U-factor, is the inverse of the total: the lower it is, the less heat escapes.

The calculation

  • R of each layer = thickness in meters ÷ λ in W/(m·K)
  • Still air gap: about R 0.18 m²·K/W, whatever its thickness above about 3/4 inch
  • Surface resistances: inside 0.13, outside 0.04 m²·K/W
  • R total = sum of the layers + 0.17
  • U = 1 ÷ R total, in W/(m²·K); divide by 5.678 for BTU/(h·ft²·°F)
  • Heat loss = U × area × temperature difference

Conductivities used here: drywall 0.25, concrete 2.3, brick 0.8, softwood 0.13, OSB 0.13, mineral wool 0.035, EPS 0.038, XPS 0.034, polyiso 0.022 and cellulose 0.040 W/(m·K).

Example

A wall section of 1/2-inch drywall, 5.5 inches of mineral wool and 1/2-inch OSB, measured between the studs:

  • Drywall: 0.0127 ÷ 0.25 = 0.05
  • Mineral wool: 0.1397 ÷ 0.035 = 3.99
  • OSB: 0.0127 ÷ 0.13 = 0.10
  • R total with surfaces: 4.31 m²·K/W, or about R-24.5
  • U = 0.232 W/(m²·K), or U-0.041
  • Heat loss through 100 sq ft with a 40°F difference: about 163 BTU/h

Framing and thermal bridges

The calculation describes a path through the insulation. Studs, plates and headers cover 15 to 25% of a typical wood-frame wall and have far lower R-values, so the whole-wall U-factor is higher. Continuous insulation outside the studs, such as foam boards or mineral wool panels, cuts this thermal bridging and is required by many energy codes in colder climates.

Using the result

  • Compare designs: adding 1 inch of polyiso on the outside of the example wall raises the R-value by about 6.6 (US), more than adding another inch of cavity insulation.
  • Check code minimums: the IECC lists both prescriptive R-values (such as R-20 + R-5 continuous) and maximum assembly U-factors by climate zone.
  • Estimate the payback of insulation with the heat loss and your heating cost.

Frequently asked questions

What is a U-value?

The rate of heat flow through one square meter of a building element for each degree of temperature difference, in W/m²·K. In the US it is called the U-factor and given in BTU/(h·ft²·°F). Lower is better.

How is the U-value related to the R-value?

U = 1 ÷ R total, where R total is the sum of the R-values of all layers plus the inside and outside surface resistances (about 0.13 and 0.04 m²·K/W for a wall).

Why is my wall's real U-value higher than calculated?

Studs, joists, ties and other framing conduct heat much better than insulation and create thermal bridges. A wood stud wall typically performs 10 to 20% worse than its cavity insulation alone suggests; steel studs much worse.

What U-value should a wall reach?

Building codes vary by climate zone. In the IECC, wood-frame walls in cold zones need about R-20 cavity insulation plus continuous insulation, or an assembly U-factor around 0.045 to 0.060 BTU/(h·ft²·°F).

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Results are estimates provided for information purposes. They do not replace professional advice.

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