Seems they're mandatory unless you're deliberately fitting leaky windows! > When windows are replaced, a background ventilator or ‘trickle vent’ should be plac

Seems they're mandatory unless you're deliberately fitting leaky windows! > When windows are replaced, a background ventilator or ‘trickle vent’ should be placed in the new window. This will replace any ventilation you lost, when installing a new window, because your previous window was leaky. If replacing your window did not reduce the amount of useful ventilation, this should be proven, and more ventilation is not needed. [0] https://www.gov.uk/government/publications/home-user-guide-template/existing-home-ventilation-guide https://www.gov.uk/government/publications/home-user-guide-t...