About U-Value Calculator
The U-value calculator determines the thermal transmittance of wall, roof, and floor assemblies per ISO 6946, BR 443, and ASHRAE 90.1. It is used by architects, building physicists, and energy assessors to verify that constructions meet the U-value limits set by building regulations.
Add material layers with their thickness and conductivity, set the internal and external surface resistances, and the tool returns the U-value, total R-value (metric and imperial), and a layer-by-layer resistance breakdown in real time.
How It Works
- Calculate each layer resistance R = d / lambda from thickness and conductivity.
- For bridged layers, combine the insulation and bridge paths via the ISO 6946 combined method.
- Sum the surface resistances Rsi and Rse with all layer resistances to get R_total.
- Invert the total resistance to obtain U = 1 / R_total, and report upper/lower bound R for bridged assemblies.
Worked Example
A wall with Rsi = 0.13, 100 mm mineral wool (lambda = 0.038, R = 2.63), and Rse = 0.04 gives R_total = 2.80 m2K/W and U = 1 / 2.80 = 0.357 W/m2K.
Formulas
- Layer resistance
R = (d / 1000) / lambda- Thermal transmittance
U = 1 / (Rsi + sum(R_layers) + Rse)- Bridged layer (combined method)
1/R_combined = f/R_bridge + (1-f)/R_insulation
Standards & References
- ISO 6946
- BR 443
- ASHRAE 90.1
Frequently Asked Questions
What surface resistances should I use?
ISO 6946 defaults are Rsi = 0.13 and Rse = 0.04 m2K/W for walls, Rsi = 0.10 for roofs (upward heat flow), and Rsi = 0.17 for floors (downward heat flow). The tool pre-fills these per assembly type.
How does the calculator handle thermal bridging?
Set a bridging fraction and bridge conductivity on a layer (for example timber studs in insulation) and the tool combines the two heat-flow paths using the ISO 6946 combined method, also reporting upper and lower bound R-values.
What is the difference between U-value and R-value?
R-value is the thermal resistance of the assembly (m2K/W), and U-value is its reciprocal transmittance (W/m2K). A higher R-value and a lower U-value both indicate better insulation.
Is the U-value calculator free to use?
Yes, it runs entirely in your browser at no cost and stores none of your input data.