How the U-value is worked out
Each layer resists heat in proportion to its thickness and in inverse proportion to its conductivity. Adding those resistances, plus the resistance of the air films on each face, gives the total resistance, and the U-value is one divided by that total.
The US figures use the ASHRAE winter surface resistances: 0.68 inside and 0.17 outside for a wall, 0.61 and 0.17 for a roof.
Layers with no thickness are ignored, so a three-layer form still works for a two-layer wall. Leave a layer at zero rather than guessing a value for it.
Worked example
A 2x6 wall with 5.5 inches of R-3.7 insulation, half an inch of drywall at R-0.9 per inch and half an inch of sheathing at R-2.5 per inch gives 20.35, 0.45 and 1.25. Adding the 0.68 interior and 0.17 exterior surface resistances gives a total R of 22.9, so the U-value is 0.0437 Btu/h·ft²·°F.
Change the insulation thickness and the U-value moves almost in proportion, which is why insulation thickness dominates a wall's performance.
What the target means
The U-value your code or specification asks for. Zone 5 wood frame walls are around 0.06.
The target is a comparison, not a compliance check. Building control looks at the whole dwelling, including thermal bridging, airtightness and the services, so a wall that meets its target can still sit in a home that does not.
This page reports a steady-state U-value for one element. It does not check interstitial condensation, which depends on vapour resistance and the order of the layers.
Limits of this calculator
It assumes one-dimensional heat flow through continuous layers. It does not account for repeating thermal bridges such as timber studs, wall ties or mortar joints, which lower the real performance.
It does not model the ground, the air gap resistance in a ventilated cavity, or the effect of reflective foil, all of which change the answer.
It does not check condensation risk or the suitability of the build-up, which needs a hygrothermal assessment.
To use the U-value in a whole-home calculation, go to the US heat loss calculator.
Inputs for the US U value calculator
Building element
Wall, roof or floor. The choice sets the surface resistances at each face.
Layer 1 thickness
Thickness of the main insulation in inches, 5.5 for a 2x6 cavity.
Layer 1 R per inch
R-value per inch of the insulation, about 3.7 for glass fibre batts.
Layer 2 thickness
Thickness of the inner lining in inches, 0.5 for drywall.
Layer 2 R per inch
R-value per inch of the lining, about 0.9 for gypsum board.
Layer 3 thickness
Thickness of the outer sheathing in inches, 0.5 for OSB.
Layer 3 R per inch
R-value per inch of the sheathing, about 2.5 for OSB.
Target U-value
The U-value your code or specification asks for. Zone 5 wood frame walls are around 0.06.
CALCULATOR QUESTIONS
US U value calculator questions
What is a good U-value?
Lower is better. A modern wall is about 0.18 W/m²K, a roof about 0.15 and a window about 1.4.
Why do surface resistances matter?
The air films on each face resist heat too. They add about 0.17 m²K/W to a wall, which is worth more than 50 mm of brick.
Does this account for timber studs?
No. Studs, ties and mortar joints are thermal bridges, and the real U-value is worse than the clear-layer figure.
Is this a compliance check?
No. It compares one element with a target. Building control assesses the whole dwelling.