Summary
- The 2021 edition of Approved Document F (in force June 2022) tightened ventilation for both new dwellings and work on existing ones — replacement windows now normally need trickle vents equivalent to or better than what they replace.
- Wet rooms need intermittent extract (kitchen 30/60 l/s, bathroom 15 l/s) or continuous mechanical extract; whole-dwelling background ventilation rates depend on airtightness and bedroom count.
- Part F is inseparable from Part L: every airtightness improvement reduces incidental ventilation, so your evidence log should show the two were designed together, with commissioning results at handover.
What the Building Safety Act requires for Part F on non-HRB domestic jobs
Requirement F1 requires adequate means of ventilation for people in the building. For dwellings the 2021 edition sets out three system types: natural ventilation with background ventilators and intermittent extract (System 1), continuous mechanical extract (MEV, System 3), and mechanical ventilation with heat recovery (MVHR, System 4). Your Stage 3 job is to pick one and size it, not to leave 'fans by electrician' in the spec.
Work on existing dwellings is where small practices get caught: replacing windows is notifiable building work, and the new windows must not worsen ventilation — in practice, trickle vents sized per Table 5.2a (typically 8,000 mm² equivalent area to habitable rooms, 4,000 mm² to wet rooms where the existing windows had vents, and specified minimums even where they did not).
As Principal Designer (BSA), record the ventilation strategy, the vent sizing, and — critically — the commissioning: since 2022, measured airflow rates for mechanical systems should be recorded and given to Building Control and the owner.
Key design risks and evidence needed (RIBA Stages 2–4)
- Stage 2: choose the system type against the airtightness ambition (a 'build tight' Part L strategy with System 1 ventilation rarely stacks up); flag open-flued appliances that constrain extract choices (see Part J).
- Stage 3: size background ventilators room by room; coordinate extract routes, duct runs and external terminal positions with elevations and structure.
- Stage 4: specify fan performance at installed duty (l/s at system resistance, not free-flow catalogue figures), trickle vent equivalent areas on the window schedule, and require commissioning sheets recording measured flows.
Standards and references you should be citing
- Approved Document F Volume 1 (2021 edition) — Tables 1.1–1.4 for whole-dwelling and extract rates; Table 5.2a for replacement window ventilators.
- BS 5250:2021 — condensation risk management; the design companion that justifies your ventilation strategy in refurbishments.
- BS EN 13141 series — laboratory testing of ventilation components (how fan and ventilator performance is declared).
- Domestic Ventilation Compliance Guide — installation and commissioning benchmarks for Systems 1, 3 and 4.
Extract ventilation rates for dwellings
| Room | Intermittent extract (System 1) | Continuous extract (System 3, high rate) |
|---|---|---|
| Kitchen (cooker hood to outside) | 30 l/s | 13 l/s minimum |
| Kitchen (no hood / not to outside) | 60 l/s | 13 l/s minimum |
| Utility room | 30 l/s | 8 l/s |
| Bathroom | 15 l/s | 8 l/s |
| Sanitary accommodation (WC only) | 6 l/s | 6 l/s |
Common pitfalls that catch out sole practitioners
- Replacement windows ordered without trickle vents because 'the client didn't want them' — record the Part F advice given; the FENSA/Building Control declaration will surface it.
- Extract fans specified by catalogue flow rate, then installed on long flexible duct runs that halve the delivered rate — commissioning sheets expose this.
- MVHR in an extension bolted onto a leaky existing house, delivering neither heat recovery nor compliance.
- No interaction check with open-flued combustion appliances — extract can depressurise a room and pull flue gases back in (Part J spillage testing).
Frequently asked questions
Do replacement windows legally need trickle vents?
Under the 2021 edition of Approved Document F, replacement windows should provide background ventilation at least equivalent to the windows they replace, and where there were no vents, the guidance still expects minimum equivalent areas in habitable and wet rooms unless another ventilation strategy is demonstrated. Record the sizing decision on the window schedule.
Does an extension need its own ventilation design?
Yes — each new habitable room needs background ventilation (typically 8,000 mm² equivalent area under System 1) and each new wet room needs extract at the rates in AD F. If the extension makes the dwelling significantly more airtight, check the whole-dwelling rate rather than treating rooms in isolation.
What commissioning evidence should I keep?
Measured airflow rates for all mechanical ventilation (installed duty, per room), recorded on commissioning sheets and passed to Building Control and the owner. As PD, hold copies in the project record — it is the proof the designed system was actually delivered.
Track this natively instead of in Word templates
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