Ventilation ductwork has never been the glamorous end of Building Regulations. It gets designed late, squeezed into whatever space is left, and finished with a metre of concertina’d flexible duct behind a hatch. Approved Document F Volume 1, 2026 edition takes a much firmer line — and unusually, it asks for paperwork before the ceiling goes on. It’s a short change, but it alters how jobs get run.
Rigid is now the default
The 2026 guidance is specific about what ducts should be made of:
- Centralised continuous MEV and MVHR systems should use rigid or smooth semi-rigid ducts.
- Intermittent extract fans and decentralised (d-MEV) fans should use rigid ducting, with a maximum duct length of 2.0 m where practical.
- Flexible ductwork is limited to 200 mm in length, and only for final connections to fan unit spigots on centralised MEV or MVHR systems.
- Where a system’s fan control relies on constant pressure within the ductwork, flexible ducting is not permitted anywhere in the system.
Any flexible duct that is used has to be installed so the full internal diameter is maintained and flow resistance is minimised — no sagging, no crushing, no lazy bends.
A hard ceiling on static pressure
AD F 2026 introduces Table 1.8, which caps the designed system static pressure for centralised continuous MEV and MVHR by total flow rate:
| Total air flow rate | c-MEV (multiple extract spigots) | MVHR / MEV (single extract spigot) |
|---|---|---|
| Up to and including 37 l/s | 30 Pa | 60 Pa |
| 38 to 53 l/s | 50 Pa | 110 Pa |
| 54 l/s and above | 80 Pa | 160 Pa |
Paragraph 1.81 sets out how you’re expected to hit those numbers: minimise overall duct length, minimise bends and offsets, and size the ducts appropriately for the flow rate. None of that is new engineering advice — it’s just now written down as guidance you can be held to.
The evidence requirement
This is the one to flag with site teams. Paragraph 1.78: for all systems with a duct length greater than 2.0 m, design calculations and drawings should be provided to the building control body to demonstrate that the fans, the connected ducting and the external air terminal can achieve the required flow rates, and that the static pressure principles have been met. That evidence should come from a suitably competent person.
Two metres is not a lot of duct. In practice, most centralised MEV and MVHR installations — and plenty of d-MEV runs — will clear that threshold, which means a ventilation design package becomes a routine Building Control submission rather than something worked out on site.
The insulation rule that reads backwards
Paragraph 1.83 asks for more duct insulation in heated space than in cold. Intake and exhaust ductwork through conditioned areas — the heated part of the dwelling — needs the equivalent of at least 50 mm of material at ≤0.04 W/(m·K), fully vapour sealed. Through unheated areas such as lofts and sub-floor voids, it drops to 25 mm.
Read as a Part L heat-loss rule that looks like a typo. It isn’t one: it’s condensation control. Intake and exhaust ducts are the two runs between the unit and the outside world, so they carry air at roughly outdoor temperature. Push a duct full of 2°C air through a warm, humid hallway and its surface sits below the room’s dew point — you get condensation on the outside of the duct, and it finds its way through the ceiling. In a cold loft, duct and surroundings are at much the same temperature, so there’s little driving force. The exhaust run is arguably the greater risk of the two: after heat recovery that air has been cooled and is close to saturation.
Mind the terminology. In Part F, “intake” means the outdoor air intake — Section 2 is all about siting intakes away from traffic and downwind of exhaust outlets. It does not mean the supply ducts delivering tempered air to habitable rooms. An MVHR unit has four runs: intake and exhaust (unit to outside, covered by 1.83), and supply and extract (unit to rooms, not covered). Supply air leaves the heat exchanger close to room temperature, so it isn’t a condensation risk indoors.
That leaves a gap worth closing yourself. An MVHR supply or extract duct crossing a cold loft carries near-room-temperature air through an unheated space — a genuine heat-loss and condensation risk that paragraph 1.83, read strictly, doesn’t cover. The installation checklist at paragraph 4.19(g) is looser, asking that “all ductwork is insulated following the guidance in paragraph 1.83”, so expect some Building Control bodies to read it as covering everything. Insulating supply and extract runs in unheated spaces is sensible practice regardless of which reading you take.
The rest of the detail
A few more things worth having on the checklist:
- External air terminals should have a free area of at least 90% of the free area of the duct they serve. A restrictive grille can undo an otherwise good design.
- Connections should be both mechanically secured and adequately sealed; flexible ducts need mechanical fastenings such as rigid connectors or jubilee clips.
- Access for maintenance must be reasonable, and on MVHR units filter access should not require tools or removal of the main front cover.
Why it matters
Industry evidence has said the same thing for years: the gap between designed and delivered ventilation performance is usually an installation gap, not a design one. A well-specified MVHR system with 3 m of crushed flexible duct on the kitchen extract will underperform quietly, and nobody finds out unless it’s measured.
AD F 2026 attacks that from both ends — tighter rules on what gets installed, and a requirement to show your working to Building Control before it’s buried. The amended Approved Documents L and F come into force on 24 March 2027 for most new building work (24 September 2027 for higher-risk building work), with transitional protection to 24 March 2028 for qualifying plots. Systems being designed now for 2027 starts should already be working to these rules.
Worth doing now
Pull the ventilation design forward in the programme. Decide unit locations and duct routes early enough that they can be short and straight, allow space for insulation, and make sure someone owns the design calculations that Building Control will ask for. It’s a lot cheaper than re-routing ductwork through a plasterboarded ceiling.
If you’re working through an FHS-compliant plot and want a second pair of eyes on how the Part L and Part F requirements stack up together, get in touch.
Sources: Approved Document F Volume 1, 2026 edition (PDF), GOV.UK — paragraphs 1.75 to 1.86 and Table 1.8 · CIBSE Journal CPD Module 266: Ventilation design under the Future Homes Standard, July 2026 · The Building Regulations etc. (Amendment) (England) Regulations 2026 (SI 2026/335), legislation.gov.uk