What is purge ventilation and when is it needed?

Purge ventilation is the ability to rapidly ventilate a room to dilute and remove high concentrations of pollutants — for example, after painting, applying chemicals, or burning food. Approved Document F requires all habitable rooms to have purge ventilation capability, typically provided by openable windows. It is not intended for continuous use; rather, it provides the ability to quickly flush out a room when needed.

What openable window area does Part F require?

Approved Document F Table 5.2a requires a minimum openable window area equal to 1/20th of the room floor area for all rooms. For example, a bedroom with 15m² floor area needs at least 0.75m² of openable window area. This equates to approximately 4 air changes per hour, which is considered adequate for purge ventilation. External doors may contribute to this area.

Do all window types count equally for purge ventilation?

No. The effective openable area depends on the window type and opening mechanism. A side-hung casement window opening to 90 degrees provides 100% of its frame area as openable area. A top-hung window opening to 30 degrees provides only about 50%. A sliding sash window provides about 50% because only one half slides open. When checking compliance, use the equivalent area rather than the full window frame size.

Can external doors count towards purge ventilation?

Yes. Approved Document F allows external doors (such as French doors, patio doors, or back doors) to contribute to the purge ventilation openable area. A standard 900mm x 2100mm door provides 1.89m² of openable area when fully open. However, building control officers may prefer that the purge requirement is met by windows alone, as doors may be locked or obstructed.

Does purge ventilation apply to kitchens and bathrooms?

Yes. Approved Document F requires purge ventilation in all rooms, including kitchens, bathrooms, and WCs. These rooms still need 1/20th of floor area as openable window area. Note that purge ventilation is separate from the mechanical extract ventilation requirement — a bathroom needs both an extract fan (8 l/s) AND openable windows equal to 1/20th of floor area. Internal rooms without external walls may use an alternative route through an adjacent room.

Purge Ventilation Calculator — Openable Window Area

Calculate the minimum openable window area for purge ventilation per Approved Document F Table 5.2a. Checks compliance for all habitable room types.

Room Type

All rooms need 1/20th floor area

Room Length

Room Width

Window Type

Opening type affects equivalent area

Window Width

Openable pane width (not whole frame)

Window Height

Openable pane height

Number of Windows

Number of identical openable windows

How We Calculate This

This calculator determines whether a room meets the purge ventilation requirements of Approved Document F (2021 edition) Table 5.2a by comparing the available openable window area against the minimum requirement.

The requirement

Minimum openable area = Floor area / 20

This applies to all rooms (habitable rooms, kitchens, bathrooms, WCs, and utility rooms). The openable area should provide approximately 4 ACH, which is adequate to rapidly dilute and remove pollutants.

Window type adjustment

Different window types provide different equivalent openable areas due to their opening geometry. The calculator applies appropriate factors for each type:

Side-hung casement: 100% of frame opening
Top-hung / awning: ~50% of frame opening
Sliding sash: ~50% of total window area
Tilt and turn: ~85% in turn mode
Centre pivot: ~75% of frame opening

Estimated purge rate

The estimated ventilation rate during purge is calculated using a simplified orifice flow equation assuming light wind conditions (1 Pa pressure difference). Actual rates will vary with wind speed and direction, temperature difference, and room geometry.