What This Requirement Covers
Approved Document F (Ventilation) requires that adequate means of ventilation are provided in all buildings to maintain indoor air quality and control condensation. The 2021 edition of Approved Document F (effective June 2022) updated the ventilation standards alongside the changes to Part L, recognising that more airtight buildings require more controlled ventilation.
Key Requirements
Ventilation Systems
Approved Document F describes four ventilation system types for dwellings
- System 1 (Intermittent extract fans with background ventilators): Extract fans in kitchens, bathrooms, and utility rooms, with trickle ventilators in habitable rooms. The most common system in new housing.
- System 2 (Passive stack ventilation): Uses ducts from wet rooms to roof-level terminals, driven by natural buoyancy. No fans required. Background ventilators in habitable rooms.
- System 3 (Continuous mechanical extract): A central extract fan runs continuously at a low rate, with boost rates for kitchens and bathrooms. Background ventilators in habitable rooms.
- System 4 (Mechanical ventilation with heat recovery - MVHR): A central unit extracts stale air from wet rooms and supplies fresh air to habitable rooms, recovering heat from the exhaust air. No background ventilators required.
Minimum Extract Rates
- Kitchen: 13 litres/second (intermittent) or 30 litres/second (cooker hood)
- Bathroom: 8 litres/second (intermittent)
- Utility room: 8 litres/second (intermittent)
- WC (separate): 6 litres/second (intermittent)
- Continuous whole-dwelling rate: 0.3 litres/second per m² of internal floor area (minimum 13 litres/second for System 3)
Background Ventilation (Trickle Ventilators)
- Required for Systems 1, 2, and 3
- Minimum equivalent area of 5000 mm² per habitable room
- Minimum equivalent area of 2500 mm² per wet room (kitchen, bathroom, utility, WC)
- Trickle vents must be controllable by the occupant (openable and closable)
- Not required for System 4 (MVHR), as the system provides all ventilation mechanically
Purge Ventilation
- Every habitable room must have a means of purge ventilation (rapid ventilation to remove pollutants)
- Typically provided by an openable window with an opening area of at least 1/20th of the floor area of the room
- Purge ventilation may also be provided by an external door
MVHR (System 4) Specific Requirements
- Heat recovery efficiency must be at least 73% (as measured to BS EN 13141-7)
- Specific fan power must not exceed 0.5 W/(l/s) for balanced systems
- Ductwork must be installed to minimise pressure drops, with rigid duct preferred over flexible duct
- The unit must be accessible for maintenance (filter changes every 3-6 months)
Practical Compliance Tips
- Choose the ventilation system at the design stage; retrofitting MVHR in particular is difficult and expensive
- MVHR is the preferred system for buildings with low air permeability (below 3 m³/(h.m²)) as it recovers heat while providing controlled ventilation
- For System 1, specify quiet extract fans to encourage occupants to use them (maximum 30 dBA in bedrooms)
- Ensure trickle ventilators are installed in all windows as specified; missing vents will fail Building Control inspection
- Commission the ventilation system and provide flow rate measurements for Building Control sign-off
- Provide homeowner information on how to operate and maintain the ventilation system
- Do not block or seal trickle ventilators; they are essential for indoor air quality