Wall U-Value & R-Value Builder
Add construction layers from the material library to build up your wall assembly. Get total R-value, U-value, and compliance check against UK Building Regs Part L and Passivhaus targets. Nothing uploaded.
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Wall Assembly
| Layer | Thickness | R (m²K/W) |
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Learn more: wall U-value and R-value fundamentals
Understanding R-value and U-value - the inverse relationship
R-value measures thermal resistance - the higher the R-value, the better the insulation performs. U-value measures the rate of heat transfer through a material - lower is better. They are mathematical inverses of each other: U = 1 / (total R-value). A high-resistance wall has a low U-value, and vice versa. Both matter for building physics calculations and building code compliance.
How to calculate wall U-value from layers
Each layer contributes R-value calculated as: R = thickness (in metres) divided by thermal conductivity (lambda, in W/mK). You sum the R-values of all layers, including internal surface resistance (0.13 m²K/W) and external surface resistance (0.04 m²K/W) as per ISO 6946 standards. Once you have the total R-value, U-value is simply 1 divided by total R. The calculator automates this calculation from the material library.
UK Building Regs Part L and Passivhaus targets
UK Building Regulations Part L requires new external walls to achieve a U-value of 0.18 W/m²K or better. Passivhaus standard requires 0.15 W/m²K or better. An uninsulated cavity wall typically achieves 1.5-2.0 W/m²K - about 10 times worse than new-build standard. Adding insulation, reducing air gaps, and choosing materials with lower thermal conductivity all improve the U-value (lower is better).
FAQ
What is a good U-value for a wall?
UK Building Regs Part L requires 0.18 W/m²K or lower for new-build external walls. Passivhaus requires 0.15 W/m²K or lower. For comparison, an uninsulated cavity wall is typically 1.5-2.0 W/m²K - 10 times worse than the new-build standard. Retrofit recommendations are higher due to practical constraints.
Does the order of layers matter in the calculation?
No - R-values are additive regardless of layer order. However, layer order matters for moisture management, condensation risk, and structural performance in real buildings. Consult a building physicist for layer sequencing and vapour barrier placement specific to your climate and wall type.
Should I include air gaps between layers?
Yes - air gaps have thermal resistance. A 25mm air gap with reflective surfaces can contribute significant R-value (0.15-0.20). Closed air gaps are more effective than ventilated gaps. The calculator's material library includes standard air gap resistances for common cavity widths.