Approved Document F: Ventilation Requirements for Dwellings

Summary

Ventilation is the second most misunderstood building regulation after insulation, and the 2021 revision of Approved Document F significantly increased the requirements — particularly for airtight new-build dwellings where inadequate ventilation causes condensation, mould growth, and poor indoor air quality.

The fundamental principle of Part F is a two-tier system: background ventilation (continuous low-level air exchange through trickle vents and purpose-provided openings) plus rapid ventilation (openable windows or extract fans for wet rooms and cooking areas). For new buildings, the whole-dwelling ventilation rate must be calculated and documented. For existing dwellings, when ventilation is modified or replaced, the replacement must meet current standards.

For tradespeople, Part F is most relevant when: fitting or replacing kitchen extractor fans or cooker hoods, installing bathroom extract fans, fitting replacement windows (trickle vent requirements), and installing whole-house MVHR systems. Understanding the minimum extract rates and background ventilator requirements prevents costly remedial work.

Key Facts

• ADF 2021 (Volume 1: Dwellings) — In force from 15 June 2022. Supersedes ADF 2010. Significant updates: higher whole-dwelling ventilation rates, specific new-build requirements, new commissioning and testing requirements. This remains the current in-force edition. A new Approved Document F Volume 1 was published on 24 March 2026 as part of the Future Homes and Buildings Standards, but it does not come into force until 24 March 2027 (with transitional arrangements for work notified before that date)
• Four ventilation strategies — System 1 (intermittent extract), System 2 (passive stack), System 3 (MEV — continuous extract), System 4 (MVHR — balanced continuous). ADF 2021 itself presents these as natural ventilation, continuous mechanical extract, and MVHR
• Minimum whole-dwelling ventilation (ADF Table 1.3, by number of bedrooms):
  • 1 bedroom: 19 l/s
  • 2 bedrooms: 25 l/s
  • 3 bedrooms: 31 l/s
  • 4 bedrooms: 37 l/s
  • 5 bedrooms: 43 l/s (add 6 l/s per additional bedroom)
  • The rate must meet both Table 1.3 and a minimum of 0.3 l/s per m² of internal floor area (all floors combined)
  • If the dwelling has only one habitable room, use 13 l/s
• Extract fan minimum rates (intermittent — ADF Table 1.1):
  • Kitchen with a cooker hood extracting to the outside: 30 l/s
  • Kitchen with no cooker hood, or a hood that does not extract to the outside: 60 l/s
  • Utility room: 30 l/s
  • Bathroom: 15 l/s
  • Sanitary accommodation (WC/cloakroom): 6 l/s (purge ventilation via an openable window is an accepted alternative)
• Extract fan minimum rates (continuous — ADF Table 1.2, high rate): kitchen 13 l/s, utility 8 l/s, bathroom 8 l/s, sanitary accommodation 6 l/s
• Overrun (run-on) timer — ADF paragraph 1.51: an intermittent extract fan in a room with no openable window must have controls that keep the fan running for at least 15 minutes after the room is vacated. It is good practice elsewhere but is not a blanket ADF requirement
• Background ventilators (trickle vents), ADF Table 1.7 — minimum equivalent area (EA) for natural ventilation: habitable rooms 8000 mm² and kitchen 8000 mm² in dwellings with multiple floors, rising to 10,000 mm² each in single-storey dwellings; bathroom 4000 mm²; utility room and sanitary accommodation — no minimum. Ventilators should be at least 1700mm above floor level. Where continuous MEV is used instead, provide 4000 mm² EA per habitable room and no ventilators in wet rooms
• Commissioning — Mechanical ventilation systems must be commissioned in accordance with an approved procedure, using the completion checklist and commissioning sheet in ADF Appendix C. This applies to MEV and MVHR
• Part F and airtightness — the natural ventilation guidance in ADF 2021 is only suitable for less airtight dwellings, defined as a design air permeability higher than 5 m³/(h·m²) at 50 Pa or an as-built air permeability higher than 3 m³/(h·m²). At or below those figures, continuous mechanical extract or MVHR is required — intermittent fans alone will not comply

Quick Reference Table

Detailed Guidance

System 1 — Intermittent Extract Fans

System 1 is the most common domestic ventilation strategy. It relies on:

• Extract fans in wet rooms (kitchen, bathroom, WC)
• Background ventilators (trickle vents in windows, or purpose-provided wall ventilators) in habitable rooms
• Opening windows for rapid/purge ventilation

Fan specification: For a bathroom, the minimum 15 l/s means the fan's certified flow rate (from the manufacturer's data sheet, measured at a typical installed resistance) must be ≥15 l/s. Do not rely on the nominal rating alone — measure or verify against resistance in the installed duct run.

Run-on timer: ADF paragraph 1.51 requires a 15-minute overrun only where the room has no openable window — the classic internal bathroom or internal WC. In rooms with an openable window a run-on timer is good practice rather than a requirement. Where automatic (e.g. humidity) controls are fitted, ADF paragraph 1.50 requires a manual override so the occupant can switch the fan on or off. Note that humidity sensors should not be used in sanitary accommodation, where odour rather than moisture is the main pollutant.

Kitchen extract: A cooker hood that extracts to the outside must achieve at least 30 l/s (and should span at least the full width of the cooker, mounted 650–750mm above the hob unless the manufacturer specifies otherwise). Where there is no hood, or the hood only recirculates, the kitchen extract must achieve at least 60 l/s. This is why a 100mm (4 inch) fan in a kitchen is marginal — a 150mm (6 inch) fan delivering 60 l/s is preferred. Extract terminals and fans other than cooker hoods should be as high as practicable and no more than 400mm below the ceiling.

Duct length and resistance: Centrifugal fans in extract fans work against the duct resistance. A long duct run (over 3m) with bends will significantly reduce flow rate. For each 90° bend, add an equivalent length of 1.5m to the duct. For metal duct (150mm diameter) with a 5m run and two bends (8m equivalent), the actual flow rate may be 25% lower than rated. Use larger-diameter duct or a more powerful fan to compensate.

Recirculation vs ducted cooker hoods: A recirculation cooker hood does NOT provide Part F ventilation — it filters and recirculates kitchen air without removing moisture. Ducted extraction is required for Part F compliance. For retrofit where ducting is impractical, a System 1 compliant alternative must be provided.

System 4 — MVHR (Mechanical Ventilation with Heat Recovery)

MVHR is increasingly common in new-build housing (especially detached and semi-detached housing where duct runs are shorter) and in Passivhaus or EnerPHit retrofit projects.

How it works: A central unit with two fans draws stale air from wet rooms (kitchen, bathroom, WC) and supplies fresh air to habitable rooms (bedrooms, living rooms). A heat exchanger in the unit transfers 75–93% of the heat from extract air to supply air — avoiding the heat loss penalty of conventional extract ventilation.

Design requirements:

• MVHR must be designed by a competent person; BRE Digest 398 (Continuous mechanical ventilation in dwellings) and manufacturers' design software are the usual references. (Note: BRE Digest 498 is Selecting lighting controls and is sometimes cited here in error)
• Supply air must be filtered (minimum F7 filter class for urban areas)
• The system must be balanced: supply airflow rate = extract airflow rate ±10%
• Commissioning: certified to design flow rates using calibrated airflow measurement equipment

Duct design:

• Rigid ducts should be used wherever possible. Flexible ductwork is for final connections only, a maximum of 1.5m long, meeting BSRIA BG 43/2013, pulled taut so the full internal diameter is maintained
• Minimum 100mm diameter for individual room branches; 150–200mm for main trunk duct
• All ductwork joints must be sealed to prevent air leakage reducing efficiency and contaminating supply air
• External air intake and exhaust must be separated by at least 1m and positioned to avoid short-circuiting (intake to windward side; exhaust to leeward, or use separated duct terminals)

Commissioning certificate: ADF 2021 requires a commissioning certificate for MVHR systems, which must be provided to the building control inspector on completion.

Trickle Vents (Background Ventilators)

Background ventilators allow a low level of continuous air exchange even when windows are closed. The most common form is the trickle vent in a window frame — a small controllable slot ventilator, typically 4000–10,000 mm² equivalent area (EA), in the window head or frame. ADF sizes ventilators by equivalent area, not free area, measured to BS EN 13141-1.

When do trickle vents apply?

• New build (natural ventilation, ADF Table 1.7): habitable rooms and the kitchen 8000 mm² EA each in a dwelling with more than one floor, or 10,000 mm² EA each in a single-storey dwelling; bathroom 4000 mm² EA; utility room and sanitary accommodation have no minimum. There must be at least four ventilators in a one-bedroom dwelling and at least five in any larger dwelling. Ventilators should be at least 1700mm above floor level and, where a fan is in the same room, at least 500mm from it
• New build with continuous MEV: 4000 mm² EA per habitable room, and no background ventilators in wet rooms. With MVHR, background ventilators should not be fitted at all
• Replacement windows where the old windows HAD trickle vents (ADF 3.14): the replacements must include background ventilators that are not smaller than the originals and are controllable
• Replacement windows where the old windows had NO trickle vents (ADF 3.15): this is the widely misunderstood case. Replacing the windows increases airtightness, so ventilation must be no worse than before. The accepted route is to fit background ventilators of 8000 mm² EA in habitable rooms, 8000 mm² in the kitchen and 4000 mm² in bathrooms — or demonstrate an equivalent alternative to the building control body. Saying "the old ones didn't have vents" is not on its own a compliance route
• Existing windows (no replacement): If windows are not being replaced, trickle vents in existing windows need not be upgraded (though it is good practice)

Part F and window replacement: Replacing a window is controlled building work, notifiable either through a competent person scheme (FENSA, CERTASS) or to the building control body. Part L (energy efficiency) and Part F (background ventilation) both apply. A window installer who removes trickle vents on replacement windows without providing equivalent background ventilation is creating a Part F defect.

Part F and Existing Dwellings (Renovation)

In existing dwellings, Part F applies to replacement ventilation systems (new extract fans replacing old ones) and to new extensions that create new habitable or wet rooms. The key rule: any new or replacement ventilation element must meet ADF 2021 standards.

For renovations and extensions:

• New wet room added to an existing dwelling (ADF 3.25) → intermittent extract at the Table 1.1 rate plus a background ventilator of at least 5000 mm² EA; or continuous extract at the Table 1.2 rate; or a single-room heat recovery ventilator
• New bathroom → Part F extract fan required (minimum 15 l/s)
• New kitchen extension → Part F extract fan required (30 l/s with a hood ducted to outside, or 60 l/s otherwise)
• New habitable room added to an existing dwelling (ADF 3.17) → background ventilators of at least 12,000 mm² EA between the new room and the existing room, and 12,000 mm² EA between the new room and outside, where the existing room retains at least 5000 mm² EA
• Energy efficiency works (insulation, draught-proofing, window replacement) → ADF Table 3.1 and Diagram 3.1 categorise the works as A, B or C by the number of major and minor measures. Category B or C means the ventilation provision has likely been reduced below F1(1) and must be upgraded

Note: Part F does not typically require wholesale ventilation upgrades in a like-for-like renovation. The trigger is disturbance or change — replacing like with like (e.g., new extract fan of equivalent or better performance) is compliant.

Frequently Asked Questions

Does a new bathroom in an extension need an extract fan if there's an openable window?

Yes for a bathroom. ADF paragraph 1.47 requires intermittent extract fans in all wet rooms, and a bathroom must achieve 15 l/s. An openable window provides purge ventilation but does not substitute for extract ventilation in a bathroom. A 15-minute run-on timer is only mandatory where the room has no openable window (ADF 1.51), though it is good practice everywhere.

The one exception is sanitary accommodation (a WC or cloakroom without a bath or shower): ADF paragraph 1.49 allows the extract rate to be met using windows by following the purge ventilation guidance instead of fitting a fan.

I've installed a new cooker hood that only recirculates. Is this compliant?

No. ADF 2021 Diagram 1.2 states explicitly that a recirculating cooker hood on its own does not provide a means of ventilation that complies with Part F. Recirculation hoods filter cooking fumes and odour but do not remove moisture or provide air exchange. Where the hood does not extract to the outside, the kitchen extract rate is 60 l/s, not 30 l/s — so a separate ducted extract fan in the kitchen delivering 60 l/s is needed.

What's the difference between Part F and Part L on window replacement?

Approved Document L Volume 1: Dwellings (2021 edition, the current in-force version — the old "Part L1B" naming was retired in 2021) deals with the thermal performance of the replacement window: a whole-window U-value of 1.4 W/m²K or better, or WER band B. Part F deals with background ventilators. Both apply to replacement windows. FENSA or CERTASS self-certification covers both — the installer must confirm compliance with both parts. Removing trickle vents on replacement windows without equivalent provision is a Part F defect, and so is fitting replacement windows with no ventilators where the original windows also had none (see ADF 3.15 above).

My MVHR unit needs servicing. What's involved?

MVHR servicing typically every 12 months:

1. Clean or replace supply air filters (G4/F7 filter pads)
2. Clean heat exchanger core (remove and brush or wash according to manufacturer's guidance)
3. Clean extract grilles and supply diffusers
4. Check and clean condensate drain from heat exchanger
5. Check and clean external air intake and exhaust terminals
6. Verify airflow rates using an airflow measurement hood (commissioned units should be re-verified) Record the service in a commissioning/service log.

Regulations & Standards

• Approved Document F (2021 edition) — Ventilation: Volume 1 Dwellings (in force 15 June 2022). Still the current in-force edition

• Approved Document F Volume 1 (Future Homes and Buildings Standards edition) — published 24 March 2026 alongside the Building Regulations etc. (Amendment) (England) Regulations 2026; in force 24 March 2027, with transitional arrangements for work notified before that date and started within 12 months

• BS EN 13141 — Performance testing of ventilation components/products for residential buildings (Part 1 background ventilators, Part 3 cooker hoods, Part 4 intermittent extract fans, Part 6 MEV, Parts 7 and 8 MVHR)

• BSRIA BG 43/2013 — Flexible ductwork standard referenced by ADF 1.77

• BRE Digest 398 — Continuous mechanical ventilation in dwellings

• BS 5250 — Code of practice for control of condensation in buildings (linked to Part F intent)

• GOV.UK — Approved Document F (2021) — Full text of the 2021 revision

• NHBC Technical Guidance — Ventilation — Practical guidance for housebuilders on ADF 2021 compliance

• BRE Group — Ventilation in Housing — BRE Digest 398 and related research

• Future Homes and Buildings Standards: Building Circular 01/2026 — the 2026 Approved Documents and their 24 March 2027 commencement

• bathroom fan wiring — Extract fan wiring and timer connections

• kitchen appliance circuits — Kitchen extract fan circuits and FCU connections

• airtightness — Airtightness and its interaction with Part F

• part l energy — Part L energy efficiency requirements for windows and replacement work