Trickle Vent Calculator

Yes. Under Part F (2021), trickle vents are required in new and replacement windows unless an alternative whole-house ventilation strategy (such as MVHR) is installed. They provide background ventilation to reduce condensation and maintain air quality. FENSA installers must fit them.

ADF 2021 Vol 1 Table 1.7 specifies minimum background ventilator equivalent areas per room: habitable rooms (living, bedroom, dining) AND the kitchen need 8000 mm² EA each — the kitchen is treated the same as a habitable room. In single-storey dwellings (bungalows and single-level flats) that figure rises to 10,000 mm² for both habitable rooms and the kitchen. A bathroom (with or without a WC) needs 4000 mm² EA. A utility room and a standalone WC (sanitary accommodation) have no statutory minimum. The required area can be split across multiple vents in the room.

As well as meeting the per-room equivalent areas in Table 1.7, ADF 2021 Vol 1 paragraph 1.57 sets a whole-dwelling minimum: the total number of background ventilators across the habitable rooms and kitchen should be at least five — or at least four for a one-bedroom dwelling. Because this calculator works one room at a time, check the dwelling total separately so you do not under-provide across the property.

Modern trickle vents are designed to minimise draught discomfort. They can be closed by the occupant in extreme weather. The small openings create gentle air movement rather than strong draughts. Positioning them at the top of the window helps as warm air rises and mixes with incoming cool air.

Standard slot vents (typically 2500 mm² EA) fit into the frame or glazing rebate. Night vents (4000 mm² EA) provide extra ventilation for summer cooling. Over-frame canopy vents (5000 mm² EA) sit above the frame. Acoustic vents provide sound attenuation while maintaining equivalent area.

If the original windows had no trickle vents and there is no condensation or air quality issue, Building Control may accept like-for-like replacement without vents. However, FENSA guidance strongly recommends fitting them on all replacement windows. In practice, most installers now fit them as standard.