Trickle Vents and Ventilation Behind Secondary Glazing
Every airtightness improvement moves the ventilation problem somewhere else. Getting this right matters more in a period house than most installers acknowledge, because the old window was doing ventilation work nobody had specified.
What Part F actually requires
Approved Document F sets background ventilation provision for dwellings. Where a window that provided background ventilation is replaced, equivalent provision must be maintained. Secondary glazing added inboard of a retained window is a different situation to a replacement, but if you seal the last air path into a bedroom you have created a problem regardless of what the regulation says.
Acoustic trickle vents
A standard trickle vent is an acoustic hole straight through your new glazing line. An acoustic vent uses a lined, folded baffle path so air can pass while sound has to travel a longer, absorbent route. It holds most of the acoustic gain at a small cost in airflow resistance. On any bedroom where we seal the window we discuss this at survey.
- Standard vent: cheap, undoes acoustic work
- Acoustic vent: baffled path, holds most of the gain
- Positioned head-height to avoid draught on occupants
Rooms where you should think harder
Bathrooms and kitchens generate the moisture. Bedrooms accumulate it overnight from occupants. If either currently relies on a leaky window as its only air path, sealing that window without adding ventilation moves the condensation to the next coldest surface — usually a wall behind a wardrobe.
Frequently asked questions
Not automatically. It depends on the room, the existing provision and whether the primary window retains a vent. We assess it at survey rather than fitting them by default.
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