Background ventilation omitted or blocked at second fix
What it looks like
Condensation on glazing and reveals in the first winter, and mould in corners of bedrooms and bathrooms.
Why it happens
Trickle vents specified but not fitted or not opened through the frame, extract terminals fitted with restrictive grilles, or ducting crushed in the void.
What it costs if it is missed
Failure to meet the ventilation strategy, customer care condensation claims, and remedial work to finished windows and ceilings.
How to inspect for it
At pre-handover, check every habitable room for background ventilation, confirm vents are open through to outside, and measure extract rates at each terminal.
How to prevent it
Check the ventilation schedule against the window order at the point of ordering, and commission extract rates plot by plot rather than by sample.
Basis in regulation
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.