What is a U-value and why does it matter?

A U-value (thermal transmittance) measures how effective a building element is as an insulator. It is expressed in W/m²K — the lower the U-value, the better the insulation. UK Building Regulations Part L sets maximum U-values for new buildings: 0.30 W/m²K for walls, 0.16 for roofs, and 0.25 for floors.

What U-values do I need to meet UK Building Regulations?

Under Approved Document L (Conservation of fuel and power), the limiting U-values for new domestic buildings are: external walls 0.30 W/m²K, pitched roof with insulation at ceiling level 0.16 W/m²K, flat roof 0.18 W/m²K, and ground floor 0.25 W/m²K. These are maximum values — lower is better.

What are Rsi and Rse surface resistances?

Rsi (0.13 m²K/W) is the internal surface thermal resistance and Rse (0.04 m²K/W) is the external surface resistance. These account for the thin layers of still air at the surface of walls, roofs and floors. They are standard values used in all U-value calculations to BS EN ISO 6946.

How does a cavity air gap affect U-value?

An unventilated air cavity of 25mm or more has a thermal resistance of approximately 0.18 m²K/W. On its own this provides limited insulation. Filling the cavity with insulation (mineral wool, blown bead or foam) dramatically improves thermal performance.

What is the best insulation material for the lowest U-value?

PIR (polyisocyanurate) boards such as Celotex or Kingspan have the lowest thermal conductivity at around 0.022 W/mK, meaning you need less thickness to achieve a given U-value. Phenolic foam (0.020 W/mK) is even better. Mineral wool (0.035 W/mK) is cheaper but requires greater thickness for the same U-value. Use our insulation thickness calculator (/insulation/insulation-thickness-calculator/) to compare materials.

U-Value Calculator — Calculate Wall, Roof & Floor Thermal Transmittance UK

U-Value Calculator \u2014 Calculate Thermal Transmittance for Walls, Roofs & Floors

Calculate the U-value (thermal transmittance) of any building element by adding construction layers. Compare against UK Building Regulations Part L targets for walls, roofs and floors.

Building Element Type

Select the element type to compare against Building Regs targets

Construction Layers (outside to inside)

Layer 1 — Material

Thickness

Layer 2 — Material

Thickness

How We Calculate This

This calculator determines the U-value (thermal transmittance) of a building element using the standard method from BS EN ISO 6946. The U-value tells you how much heat passes through 1 m² of the element for every 1 degree Kelvin temperature difference.

The formula

U-value = 1 / (Rsi + R1 + R2 + ... + Rn + Rse)

Where each layer's thermal resistance R = thickness (m) / thermal conductivity (W/mK). Rsi (0.13) is the internal surface resistance and Rse (0.04) is the external surface resistance.

UK Building Regulations Part L targets

External walls: 0.30 W/m²K maximum
Pitched roof (insulation at ceiling): 0.16 W/m²K maximum
Flat roof: 0.18 W/m²K maximum
Ground floor: 0.25 W/m²K maximum

Thermal conductivity values used

Brick: 0.77 W/mK
Concrete block: 1.13 W/mK
Lightweight block: 0.19 W/mK
Mineral wool: 0.035 W/mK
PIR board (Celotex/Kingspan): 0.022 W/mK
EPS: 0.034 W/mK
Plasterboard: 0.16 W/mK
Timber: 0.13 W/mK
Cavity air gap: R = 0.18 m²K/W (fixed thermal resistance)

These values are based on manufacturer data and BS EN 12524 / BS EN ISO 10456 standards. Actual performance may vary depending on moisture content and installation quality.

Frequently Asked Questions

Last updated: February 2026

All calculations are estimates. Verify with your supplier.

U-Value Calculator \u2014 Calculate Thermal Transmittance for Walls, Roofs & Floors

Construction Layers (outside to inside)

How We Calculate This

The formula

UK Building Regulations Part L targets

Thermal conductivity values used

Frequently Asked Questions

Related Calculators

Insulation Thickness Calculator

Heat Loss Calculator

Cavity Wall Insulation Calculator

Loft Insulation Calculator

EPC Rating Estimator