Background Ventilator Calculator — Trickle Vent Requirements

A trickle ventilator (also called a background ventilator) is a small, closable ventilation opening typically fitted in window frames or walls. They provide continuous background ventilation to dilute indoor pollutants and control moisture when windows are closed. Under Approved Document F, trickle vents are mandatory in all new dwellings and replacement windows in existing dwellings (since June 2022). They are part of System 1 (background ventilators plus intermittent extract fans).

Approved Document F 2021 Table 1.7 specifies minimum equivalent areas per room: 8,000 mm² for habitable rooms and the kitchen (the kitchen is treated the same as a habitable room; 10,000 mm² in single-storey dwellings), 4,000 mm² for a bathroom, and no minimum for a utility room or WC. These can be achieved with one or more trickle vents per room. Standard vent sizes range from 2,500 mm² to 12,000 mm² — a 5,000 mm² vent in a bedroom would need two to meet the 8,000 mm² requirement. The dwelling must also have at least 5 background ventilators (4 if one-bedroom).

Yes. Since June 2022, Approved Document F requires that replacement windows include trickle ventilators if the existing windows had them OR if the existing windows did not have them. The only exception is where adequate alternative background ventilation exists (such as an MVHR system). This applies in England; Scotland, Wales, and Northern Ireland have their own regulations with similar requirements.

Trickle vents should be positioned at least 1.7m above floor level to avoid direct draughts on occupants. In windows, they are typically fitted in the window frame head (top). They should be evenly distributed around the dwelling to promote cross-ventilation. In rooms with an extract fan (kitchens, bathrooms), the trickle vent provides the make-up air path. Trickle vents should not be located where they could short-circuit with extract fans.

Equivalent area (EA) measures the actual airflow performance of a ventilator, accounting for the resistance of the grille, mesh, and any internal baffles. It is always less than the "free area" (the physical opening size). Equivalent area is measured per BS EN 13141-1 and is the correct measure specified in Approved Document F. A ventilator with a 6,000 mm² free area might have an equivalent area of only 5,000 mm². Always check the manufacturer's stated equivalent area, not the free area.