Thermal bridging, condensation, and VCL in EWI
What it looks like
Lack of proper VCL installation or insufficient consideration of thermal bridges, leading to condensation issues and mould growth.
Why it happens
External wall insulation (EWI) systems must be designed to minimise thermal bridging and mitigate condensation risk, typically by incorporating a vapour control layer (VCL) on the warm side of the insulation. A condensation risk analysis, often performed to accredited standards, is required to validate the system's performance.
What it costs if it is missed
External wall insulation (EWI) systems must be designed to minimise thermal bridging and mitigate condensation risk, typically by incorporating a vapour control layer (VCL) on the warm side of the insulation. A condensation risk analysis, often performed to accredited standards, is required to validate the system's performance.
How to inspect for it
Check that the design includes a VCL on the warm side of the insulation and that the system has undergone a condensation risk assessment.
How to prevent it
External wall insulation (EWI) systems must be designed to minimise thermal bridging and mitigate condensation risk, typically by incorporating a vapour control layer (VCL) on the warm side of the insulation. A condensation risk analysis, often performed to accredited standards, is required to validate the system's performance.
Basis in regulation
BS 5250
Read the source
Last updated 23 August 2026. This is our editorial summary of publicly available regulation and site practice, not building control approval or a warranty determination.