Basement Ventilation Calculator

Yes, if the basement has no openable windows — which is the case for most below-ground rooms. Approved Document F requires habitable rooms to have adequate ventilation for occupant health and moisture control. Mechanical extract ventilation (MEV) or mechanical ventilation with heat recovery (MVHR) is typically required. Rooms with lightwells and openable windows may qualify for natural ventilation.

Approved Document F 2021 (England) sets intermittent extract rates for WET rooms only: a kitchen needs 30 L/s where a cooker hood ducts to outside adjacent to the hob, or 60 L/s with no hood or a recirculating hood (Table 1.1); a utility room needs 30 L/s; a bathroom needs 15 L/s (or 8 L/s continuous, Table 1.2). Part F does NOT set a per-person extract rate for habitable rooms (bedrooms, living rooms) — their provision comes from the whole-dwelling supply rate distributed by volume, with an overall floor of 0.3 L/s per m² of internal floor area (para 1.24). The widely quoted ‘8 L/s per person’ is a Scottish/sedentary figure, not Part F. This tool sizes habitable-room fresh air to the greater of the CIBSE Guide A rate (10 L/s per person) and the ADF 0.3 L/s per m² floor.

MVHR (mechanical ventilation with heat recovery) is generally the best choice for basements because it provides both supply and extract air while recovering up to 90% of the heat from exhausted air. This is particularly beneficial in basements where the walls and floor are against cold ground. MEV is simpler and cheaper but only extracts — supply air relies on infiltration or separate vents.

There is no national Building Regulation that sets a lightwell area as a fixed percentage of floor area — lightwell sizing is governed by your local planning authority's design guidance and varies between councils. What IS set nationally is the escape window: a basement bedroom needs an emergency escape window per Approved Document B Vol 1 — at least 0.33 m² unobstructed openable area, minimum 450 mm high × 450 mm wide, with the sill no more than 1100 mm above floor — unless an alternative protected escape route is provided. A lightwell is usually what makes such a window workable below ground. Lightwells also give daylight (Part L) and can allow natural ventilation (Part F). Confirm lightwell dimensions with your local planning authority before construction.

Duct size depends on airflow rate: 100mm diameter for up to 30 L/s, 125mm for 30–60 L/s, and 150mm above 60 L/s. These keep air velocity below about 5 m/s — the upper end of the 3–5 m/s domestic design band — to limit noise (a 100mm round duct carries roughly 39 L/s, 125mm about 61 L/s and 150mm about 88 L/s at 5 m/s). For quiet branches to bedrooms aim nearer 3–4 m/s and size up. Runs should be as short and straight as possible — each 90-degree bend adds roughly 1–1.5 m equivalent length. Insulate ducts running through cold spaces to prevent condensation.