Approved Document F (Ventilation): What Homeowners Need to Know

Part F covers four ventilation strategies for dwellings (Systems 1–4):

• System 1 — Background ventilation + intermittent extract fans: the most common approach in existing homes. Trickle vents in windows provide background ventilation; extract fans remove moisture from kitchens and bathrooms.
• System 2 — Passive stack ventilation: uses natural air movement through ducts to ventilate without powered fans.
• System 3 — Continuous mechanical extract: a continuously running low-power extract fan removes moist air from wet rooms.
• System 4 — MVHR (Mechanical Ventilation with Heat Recovery): a balanced whole-house system that supplies fresh air and extracts stale air, recovering heat from the outgoing air. Required in very airtight new buildings under the Future Homes Standard.

Under Part F, background ventilators (trickle vents) must be provided in habitable rooms. The minimum equivalent area (EA) requirements are:

• Living rooms and bedrooms: 8,000 mm² EA per room
• Kitchens (with openable window): 4,000 mm² EA
• Bathrooms/WCs: 4,000 mm² EA

If you are replacing windows in an existing home, the new windows must be fitted with trickle vents of at least this size if the original windows had them — or if the building control officer judges that ventilation would be inadequate without them. This requirement has caught many homeowners out when installing new double glazing.

Intermittent extract fans (System 1) must meet minimum airflow rates:

• Kitchen with cooker hood: 30 litres/second (l/s) adjacent to the hob, or 60 l/s elsewhere in the kitchen
• Utility room: 30 l/s
• Bathroom (with or without WC): 15 l/s
• Sanitary accommodation (WC only): 6 l/s

Fans should have a 15-minute overrun timer (continuing to run after the light is switched off) and ideally a humidistat to respond to moisture levels automatically.

The Future Homes Standard (FHS), expected to come into full effect from 2025–2026, will require new homes to be far more airtight than current standards. This makes mechanical ventilation — particularly MVHR — essential in new-build homes, because natural ventilation through gaps in the fabric will no longer be sufficient.

Key changes relevant to homeowners include:

• New extensions and loft conversions will need to demonstrate adequate ventilation provision as part of building control approval.
• MVHR systems will be increasingly common in new homes and high-quality refurbishments. They recover up to 90% of heat from extracted air.
• Air permeability testing (blower door test) is becoming more common for extensions under the FHS regime to verify airtightness.

Building control will check ventilation provision at key stages, particularly:

• Window schedules submitted with Full Plans to confirm trickle vent provision
• Ductwork routes and fan specifications for extract ventilation
• MVHR system design (duct layout, unit specification) if used

For MVHR systems, a commissioning record showing the system is delivering the designed airflow rates is typically required before a completion certificate is issued.