When Can Flexible Ducting Be Used? Part F 2026

Spoiler alert: Flexible ducting has just taken a massive hit. Under Approved Document F's 2026 edition (Volume 1: Residential), flexible ducting can only be used as a final connection to a fan spigot, and only up to 200mm long. Everywhere else in a dwelling's mechanical ventilation system, it's got to be rigid or semi-rigid ducting. Every contractor, developer, and FM team specifying ventilation should know what this update entails before it takes effect in 2027.

What are the new Part F rules on flexible ducting?

Approved Document F, Volume 1 (2026 edition) restricts flexible ductwork to a maximum of 200mm in length, used only for the final connection to a fan unit spigot on centralised continuous mechanical extract ventilation (MEV) or mechanical ventilation with heat recovery (MVHR) systems. That's just a short stub at the fan — not the standard length of duct that many contractors are still running through ceilings, or loft space.

Even within that 200mm allowance, the flexible section has to be installed with the full internal diameter maintained, with flow resistance kept to a minimum. No kinks, no compression, no slack.

How does this compare to the 2021 rules?

The 2021 edition allowed flexible ductwork up to 1.5 metres, provided it was used only for final connections and met BSRIA's BG 43/2013 standard. The 2026 edition cuts that down to 200mm — a roughly 85% reduction in maximum permitted length.

The 2021 edition also just said rigid ducts "should be used wherever possible," without naming semi-rigid as a defined alternative. The 2026 edition is more precise: it explicitly names rigid or smooth semi-rigid ducting as the compliant choice for centralised MEV and MVHR systems — putting semi-rigid on equal footing with rigid metal duct for the first time.

Other restrictions on flexible ducting in Part F 2026

There are several other changes from Part F 2021, and they apply across the whole system, not just the fan connection:

• Intermittent extract fans and decentralised continuous MEV: must use rigid ducting, kept to 2.0m or less where practical. Anything longer needs design calculations submitted to building control.
• Pressure-controlled systems: where the fan's automatic control relies on constant pressure within the ductwork — can't use flexible ducting anywhere in the system, not even the 200mm spigot connection.
• Static pressure limits: now specified in a table, from 30Pa up to 160Pa depending on air flow rate and configuration. A run full of flexible duct makes these limits far harder to hit.

Why has Part F cracked down on flexible ducting?

The 2026 edition doesn't spell out its reasoning line by line, but the direction of travel points at a few well-known problems with flexible duct in residential ventilation. Here's our speculation:

• Airflow and pressure drop: Flexible duct's ridged inner wall creates far more resistance than a smooth bore, and any sagging or compression makes it worse, which is exactly what the new static pressure limits in Table 1.8 are designed to catch.
• Air hygiene: Ridged, porous, flexible ducting is harder to keep clean and more prone to condensation and mould growth than a smooth-walled product. This is a big concern given Part F's whole-dwelling ventilation and mould-prevention focus.
• Fire safety: Most thin flexible ducting performs far worse under fire-safety classifications than a semi-rigid or rigid product built to Class B-s1, d0 or B-s2, d0 (EN 13501-1) — increasingly non-negotiable post-Grenfell.

Elephant Duct doesn't suffer these problems. Our line of semi-rigid ducting is fire-rated up to Class B-s1, d0 and Class 1 (D.M. 26/06/84), carries an antimicrobial hygienic coating, and holds its shape without sagging, which we believe is precisely why the 2026 edition now names semi-rigid as a compliant alternative to rigid metal duct.

Is semi-rigid ducting a compliant alternative to flexible ducting?

Yes, under paragraph 1.76 of the 2026 edition, "rigid or smooth semi-rigid ducts" are required for centralised continuous MEV and MVHR systems — these are the exact systems where flexible duct has just been cut to a maximum of 200mm. Semi-rigid ducting bends around joists, studwork, and tight loft runs the way rigid metal can't, while keeping the smooth bore and shape retention that flexible duct lacks.

What does the flexible ducting restriction mean for the industry?

• Contractors and installers: check any spec that still calls for flexible duct beyond a fan spigot; it could fail commissioning against the new static pressure limits.
• Residential property developers: specify semi-rigid or rigid ducting at design stage to avoid a costly reroute post-completion, especially on projects landing after the March/September 2027 effective dates.
• Building owners and FM companies: existing flexible duct runs aren't automatically non-compliant, but any new ventilation work, major refurbishment, or fan replacement triggers the current rules.

Get in touch

Specifying or retrofitting a ventilation system that needs to meet Part F 2026? Talk to us about where semi-rigid ducting fits your project.

Approved Document F is periodically revised — always verify the current wording and effective dates against the live gov.uk publication before specifying or signing off installation work.