Condensation risk analysis for AVCLs
What it looks like
Failure to provide or correctly specify an AVCL based on a robust risk analysis can lead to concealed interstitial condensation, causing structural degradation or mould growth.
Why it happens
A high resistance air and vapour control layer (AVCL) is typically required unless a condensation risk analysis demonstrates it is not necessary, or its function is provided by another compliant solution. Such analyses, often conducted to standards like BS EN ISO 13788, must use appropriate boundary conditions, such as specified internal and external temperatures and internal humidity levels.
What it costs if it is missed
A high resistance air and vapour control layer (AVCL) is typically required unless a condensation risk analysis demonstrates it is not necessary, or its function is provided by another compliant solution. Such analyses, often conducted to standards like BS EN ISO 13788, must use appropriate boundary conditions, such as specified internal and external temperatures and internal humidity levels.
How to inspect for it
Confirm that a condensation risk analysis has been undertaken and that the specified AVCL or alternative solution aligns with its findings and complies with relevant standards.
How to prevent it
A high resistance air and vapour control layer (AVCL) is typically required unless a condensation risk analysis demonstrates it is not necessary, or its function is provided by another compliant solution. Such analyses, often conducted to standards like BS EN ISO 13788, must use appropriate boundary conditions, such as specified internal and external temperatures and internal humidity levels.
Basis in regulation
BS 5250
Read the sourceBS EN ISO 13788
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.