What is Part F? A Joiners Guide

10 February 2026

Part F is the section of the Building Regulations that addresses ventilation, ensuring adequate air quality, preventing damp and mould, and removing moisture and pollutants in dwellings. Ventilation entails providing a flow of fresh air into a room or building and removing stale air. Below, we’ll discuss the important considerations when manufacturing timber windows and doors.

You can view the full regulations here.

What Part F means for windows & ventilation in homes

Background (continuous) ventilation, “trickle vents” or equivalents

When you install or replace windows or doors, Part F requires that the background ventilation provision must not be worse than before the work. In many cases, this means fitting “background ventilators” (often trickle vents) in windows, shown here.

Key points:

• If the existing window had background ventilation (e.g., vents), the replacement window must include equivalent or better vents than previously fitted. https://www.bwf.org.uk/bwf-blog/trickle-vents-in-replacement-windows-and-doorsets/
• If the existing window did not have vents, the updated regulations (post-2022) generally require that new windows have background ventilation, unless an alternative compliant ventilation system (e.g. mechanical ventilation) is provided.
• “Night-vent” (i.e. window locked slightly ajar) does not count as background ventilation under Part F.
• Background ventilators (fixed devices allowing fresh air in) must be controllable, either manually or through automation, and are usually installed about 1700mm above floor level.

 

Equivalent Area (EA) requirements

Part F sets minimum ventilation capacity in terms of “Equivalent Area” (EA) of ventilation per room, not per window. The “Equivalent Area” is a measure of the amount of air that can pass through the vent under certain pressure conditions.

As of the 2022 update to Part F (applies to work started after 15 June 2022)

• Habitable rooms (e.g. living rooms, bedrooms, and kitchens): minimum 8,000 mm² EA per room (in multi-floor dwellings). Source
• Bathrooms (with or without a toilet): minimum 4,000 mm² EA. Source
• For single-storey dwellings, some guidance suggests a higher EA requirement for habitable rooms/kitchens: e.g. 10,000 mm².

Additional rules:

• If a kitchen and living room are combined (open-plan), at least three ventilators (each of the EA required for a habitable room) should be provided in that space.
• If the property uses mechanical ventilation with extraction (e.g. continuous mechanical extract ventilation, or a mechanical system with heat recovery) instead of just passive ventilation, different EA requirements may apply (or background vents might not be required in some wet rooms).

 

When do these rules apply (new builds vs replacement / refurbishment)

• For a new build, Part F applies to ventilation design: background ventilators (trickle vents) are required for rooms with external walls, unless mechanical ventilation is used.
• For existing dwellings undergoing refurbishment (e.g. replacement windows), from 15 June 2022, replacement windows must have background ventilation (unless the property uses an alternative compliant ventilation system).
• If the original window had vents, the new ones must provide at least the same ventilation capacity (or better). Source

Important caveats & special situations

• Ventilation must not worsen indoor air quality. If the home is near a busy road or pollution source, vents should ideally be located in areas that minimise ingress of polluted air (e.g. on less-exposed façades). Source
• For bathrooms, kitchens, utility rooms, etc. There is also a requirement for extract ventilation (to quickly remove moisture, cooking fumes, odours, etc.), and windows/trickle vents alone may not suffice in many cases.
• Background ventilation is about continuous/occasional airflow even when windows are closed. For “purge ventilation” (when you want to clear the air quickly, e.g. after cooking, showering), windows or external doors should open sufficiently. Guidance suggests that for natural purge ventilation, the openable area should be at least 1/20 of the floor area of the room.
• There are some exemptions: e.g. for buildings of certain historic/conservation status (e.g. listed buildings, conservation areas, or very old/“breathable” construction), it may be impractical or inappropriate to meet modern ventilation standards.

What this means for homeowners (in England) when replacing windows or building/renovating

• If you’re replacing windows, expect that the new windows will need background ventilators (trickle vents), even if the old ones didn’t have them (unless your home has an alternative ventilation system).
• Ensure the total ventilation capacity (EA) for the room matches the minimums: 8,000 mm² for habitable/kitchen, 4,000 mm² for bathroom (or 10,000 mm² in some single-storey cases).
• Make sure any ventilation is controllable (manual or auto), properly installed (e.g. vent location, seal, weather-proofing), and that there is still adequate provision for moisture/fume extraction (especially in kitchens/bathrooms).
• Keep evidence of what the previous ventilation provision was, in case of inspection or future sale (e.g. photos of old windows/vents, specification from installer).
• If your home is historic or listed, check whether there are special rules or exemptions; you may need to consult the local authority / conservation officer before making changes.

Looking for Trickle Vents?

Find a wide range of high quality trickle vents on our website.