Building Regs Part F UK 2026 — Ventilation Rules Every Trade Should Know

What Is Part F?

Part F of the Building Regulations for England covers ventilation. The accompanying guidance — Approved Document F — sets the requirement for adequate means of ventilation for people in the building. In plain terms, it exists to make sure there is enough fresh air coming in and enough stale, moisture-laden air going out to protect health and prevent condensation and mould.

Part F does not work in isolation. It is the direct partner of Part L, which governs the conservation of fuel and power — insulation and airtightness. The two were uplifted together in the 2021 edition (in force from June 2022), and the relationship between them is the single most important thing for a tradesperson to understand. The trade principle is "build tight, ventilate right": as you reduce uncontrolled air leakage to save energy under Part L, you must add controlled, purpose-provided ventilation under Part F to replace the fresh air the building used to get through gaps and draughts.

Get the airtightness without the ventilation and you create the perfect conditions for condensation, damp and black mould — the kind of problem that triggers callbacks, complaints and, increasingly, legal action over unhealthy housing. Part F is what stops that happening.

The Ventilation Strategies Part F Covers

Approved Document F describes four jobs that a dwelling's ventilation must do, and several systems that can deliver them. You don't need every system in every home — but you do need to provide each function.

Background ventilation (trickle vents)

Small, controllable openings — typically trickle vents built into window frames — that provide a continuous, low rate of fresh air without the occupant having to do anything. These are the quiet workhorse of dwelling ventilation and the source of most window-replacement compliance failures (more on that below).

Intermittent extract ventilation

Extract fans in the "wet rooms" — kitchens, bathrooms, utility rooms and WCs — that run when the room is in use to remove moisture and odours at source. Each room type has a minimum extract rate set out in the guidance.

Continuous mechanical extract (MEV / dMEV)

Instead of fans that switch on and off, a continuously running system extracts a trickle of air all the time, boosting when a room is in use. This can be a centralised unit (MEV) or a decentralised fan in each wet room (dMEV). Continuous systems are quieter and more effective at controlling humidity in airtight homes.

Whole-house mechanical ventilation with heat recovery (MVHR)

A balanced supply-and-extract system that recovers heat from the outgoing stale air and uses it to warm the incoming fresh air. MVHR is increasingly common in new-build and deep-retrofit projects because it pairs well with very airtight, well-insulated fabric — exactly the kind of building Part L now demands.

Purge ventilation

Rapid ventilation to clear pollutants or moisture quickly — for example after spillage, decorating or heavy cooking. In most homes this is provided by openable windows. Part F sets out the openable area required relative to floor area.

The Big One for Trades: Replacement Windows

This is the compliance point that catches more trades out than any other, so it's worth being crystal clear. When you carry out work on an existing home, Part F says you must not make the building's ventilation provision worse than it was before. This is the "no worse than" rule.

For window installers, this has a direct and unavoidable consequence: replacement windows generally need to include trickle vents. Older windows leaked air around poorly fitting frames and through worn seals — that uncontrolled leakage was effectively providing background ventilation, even if nobody designed it that way. When you fit modern, tightly sealed double or triple glazing, you remove that air path. If the new window has no trickle vent, you have made the ventilation worse, which is a Part F breach.

The safe and compliant position is to fit trickle vents (background ventilation) to replacement windows to maintain or improve the existing provision. If the original window already had a trickle vent, the replacement must have one of at least the same equivalent area. Skipping this is one of the most common reasons a window job fails to comply — and it's also one of the most common causes of condensation appearing on brand-new windows, which then comes straight back to you as a complaint.

The same logic applies to insulation and airtightness work. If you draught-proof, insulate or otherwise tighten up a home without addressing ventilation, you risk pushing the property into condensation and mould. Whenever you make a home tighter, check that purpose-provided ventilation is keeping pace.

Minimum Extract Rates (Guidance Figures)

For intermittent extract fans in wet rooms, Approved Document F sets minimum extract rates. Use these as approximate guidance figures when specifying and commissioning fans — always check the current edition for the exact values that apply to your job.

• Kitchen: around 30 l/s if the fan is over the hob, or around 60 l/s if it's elsewhere in the room
• Bathroom: around 15 l/s
• Utility room: around 30 l/s
• WC (toilet): around 6 l/s

These rates matter because a fan that's undersized, badly ducted or restricted by long, kinked or flexible ducting often fails to achieve its rated airflow in practice. A 15 l/s bathroom fan on the box can deliver far less once it's pushing air through six metres of flexible duct and a clogged external grille — and an underperforming fan in an airtight home is a mould problem waiting to happen.

How Compliance Is Demonstrated

Part F work is shown to comply in one of two main ways: through building control, or through a Competent Person Scheme where the installer self-certifies. Which route applies depends on the trade and the work.

• Replacement windows: usually self-certified under FENSA or CERTASS. The installer certifies that the work — including the trickle-vent / background ventilation provision — meets the regulations, and you receive a compliance certificate.
• Extract fans and mechanical systems: electricians and ventilation installers commission the system, measure the actual air flow rate achieved, and record it. For notifiable electrical work, a Competent Person Scheme (such as a registered electrical scheme) can self-certify.
• Larger or non-self-certifiable work: goes through building control, who will want to see evidence that the ventilation provision meets Part F.

The 2021 update placed real emphasis on commissioning and recording. For mechanical systems you are expected to measure the air flow rates actually delivered and complete a commissioning sheet. That sheet is your proof the system performs as designed — keep a copy on file. A fan rated at 15 l/s is worthless if nobody checks it's actually moving 15 l/s once installed, and the commissioning record is what protects you if a condensation dispute arises later.

Keeping clear records of which jobs you certified, what you commissioned and what you handed over is just good business. The same habit that keeps you compliant — knowing exactly which jobs you've done and where they came from — also helps you see which marketing actually brings in paid work. Trades who track that link tend to grow faster, which is exactly what the Trade2Base tools are built to help with.

Quick Reference: Part F by Job Type

Why It Matters

Beyond ticking a regulatory box, getting Part F right protects you commercially. The most expensive outcomes in this area are not fines — they're callbacks. A homeowner who finds black mould on their new windows or streaming condensation after a loft conversion will call the last trade who touched the property, and they will expect it fixed for free. Designing in the right ventilation from the start de-risks every window, insulation and airtightness job you take on.

There is a health and reputational dimension too. Condensation and mould are linked to respiratory problems, and tolerance for damp, unhealthy housing has fallen sharply. A trade known for "build tight, ventilate right" delivers homes that stay dry, comfortable and healthy — and that reputation wins referrals. To get it right, treat ventilation as part of the spec from the quote stage, not an afterthought: specify the right fans and vents, fit them properly, commission them, and record what you did.

A Note on Devolution

Everything above describes the England regime — the Building Regulations and Approved Document F as they apply in England. Wales operates a broadly similar system with its own version of the guidance. Scotland has an entirely separate framework: the Building Standards system and its Technical Handbooks, which cover ventilation under their own sections rather than a "Part F". Northern Ireland has its own technical booklets as well. If you work across borders, always check the rules for the nation where the property is — the broad principles are similar, but the documents, figures and certification routes differ.