Ventilation
Background ventilator (trickle vent) equivalent area (Approved Document F)
- where
- natural ventilation: total equivalent area (mm²)
- equivalent area per habitable room or kitchen: 8000 mm², or 10,000 mm² for a single-storey dwelling
- habitable rooms
- kitchens
- bathrooms
- continuous extract: total equivalent area (mm²)
- continuous extract: number of ventilators
- bedrooms
Explain this equation
Works out the background ventilator (trickle vent) equivalent area needed when replacing windows or upgrading ventilation, following Approved Document F.
With natural ventilation every habitable room and kitchen needs a set equivalent area of background ventilation, larger in single-storey homes, and each bathroom 4000 mm². With continuous mechanical extract, only habitable rooms need background vents, at 4000 mm² each. The calculator also gives the minimum number of vents for each system.
Simple per-room allowances added up.
Sense check: a two-storey home with 4 habitable rooms, a kitchen and a bathroom needs about 44,000 mm² with natural ventilation, but only 16,000 mm² with continuous extract.
Watch out: equivalent area is not the slot size. Use the figure marked on the vent, and do not reduce existing ventilation when replacing windows.
Totals the minimum background ventilator equivalent area for a dwelling with natural ventilation and intermittent extract (ADF Table 1.7), and for continuous mechanical extract (ADF para 1.64).
Enter your values to see the result.
Good to know
Natural ventilation: 8000 mm² per habitable room and kitchen in a dwelling on more than one storey (10,000 mm² if single-storey), 4000 mm² per bathroom, none required for utility rooms and WCs; at least four ventilators for a one-bedroom dwelling and five otherwise; an open-plan kitchen/living space needs at least three. Continuous extract: 4000 mm² per habitable room, none in wet rooms, total number bedrooms + 2. When replacing windows in an existing home, ADF para 3.15 asks for 8000 mm² (habitable, kitchen) and 4000 mm² (bathroom) where there were none, and never less than before.
Common mistake
Using the free area or slot size printed on a vent. ADF uses equivalent area, which is lower; check the manufacturer’s equivalent area figure.
Limits
The natural ventilation guidance only applies to “less airtight” dwellings (design q50 above 5, or as-built above 3 m³/(h·m²)). Not for MVHR systems.
Sources
- HM Government, Approved Document F Volume 1: Dwellings, 2021 edition (para 1.24 and Table 1.3 whole dwelling ventilation rates; Table 1.7 background ventilators; para 1.64 continuous extract; paras 1.45–1.46 airtightness; Section 3 existing dwellings)
- HM Government, Approved Document F Volume 1: Dwellings, 2026 edition (in force 24 March 2027; Table 1.3 and Table 1.7 values unchanged)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow BS EN ISO 6946, BS 5250, PAS 2035, Building Regulations Part L and manufacturers’ data. Whole-house retrofit needs a qualified retrofit assessor and coordinator.