BR 443: Conventions for U-Value Calculations

BR 443 is the BRE document that sets out the conventions for calculating U-values for Building Regulations compliance in the UK. It provides standardised thermal conductivity values and calculation methods.

What is BR 443?

Published by the Building Research Establishment (BRE), BR 443 provides:

• Design thermal conductivity values for common materials
• Surface resistance values
• Air cavity thermal resistances
• Conventions for dealing with thermal bridges
• Correction factors for various conditions

Design vs Declared Values

BR 443 distinguishes between two types of thermal conductivity values:

Declared Values (λD)

These are the values declared by manufacturers based on laboratory testing to EN standards. They represent 90% fractile at 90% confidence level.

Design Values (λdesign)

For Building Regulations calculations, design values must be used. These are typically higher than declared values to account for:

• Moisture content in service conditions
• Ageing effects
• Installation tolerances

Standard Surface Resistances

BR 443 specifies the following surface resistances:

Air Cavity Resistances

Standard thermal resistances for air cavities:

Common Material Values

Some commonly used BR 443 design thermal conductivity values:

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