Your Home Climate
Air Quality · The problem library

Why does my bathroom extractor leave the room foggy, and what should I do?

Recently reviewed·Reviewed June 2026, next review December 2026

A bathroom extractor that cannot clear the steam after a shower is almost always one of three problems. The duct run is too long or too restrictive for the fan. The fan itself is undersized. Or the room has no path for replacement air to enter while the fan runs. Replacing the fan without addressing the underlying cause often disappoints.

Editorial confidence
High confidence. Multiple regulator, government or academic sources agree.
Evidence status
Recently reviewed · reviewed June 2026 · next review December 2026
Monitoring
Every six months, plus whenever Approved Document F intermittent extract rates or FETA commissioning notes are updated.
Evidence drawn fromBuilding standards: ADFProfessional bodies: FETA, EST

You may not need to read the rest of this page

You probably already have enough to act if:

  • Steam still hangs in the air ten minutes after the shower
  • There is visible duct work between the fan and the external terminal
  • You have not yet checked the duct route or run-on timer
Open the ventilation hub

This is probably the wrong page for you if:

  • The bathroom is served by a commissioned MVHR boost terminal
  • Visible water is entering from the roof or external wall

Minimum bathroom intermittent extract rate

15 l/s

Source: Building Regulations; Approved Document F · Approved Document F sets 15 l/s as the minimum for intermittent bathroom extract.

Who this advice is for

Applies to

  • Bathrooms with persistent post-shower mist or peeling paint at the ceiling
  • Properties with long flexible duct runs to the extractor terminal
  • Older fans installed before current Part F extract rates were in force

Not intended for

  • Bathrooms with structural water ingress through the roof or external wall
  • Wet rooms already served by a commissioned MVHR boost terminal

What it usually looks like

These are the symptoms readers describe most often. None of them alone is diagnostic, but together they build a picture.

  • Mirror remains foggy long after the shower
  • Wet wallpaper or peeling paint near the ceiling
  • Black mould around the extractor grille
  • Bathroom door wet on the inside

Most common in: New-build flat · Victorian terrace · Edwardian semi-detached · Interwar semi (1920s–1930s)

Before you buy anything

Watch the house respond as you scroll.

These checks are listed in the order we would work through them. The illustration on the left changes with each one, so you can see what each check is actually addressing before deciding whether it is worth doing.

Warm wet air settles on the coldest surface and stays there. The window streams; mould follows.

01

Trace the duct from the fan to outside

Long flexible duct runs with bends can halve a fan's effective extract rate. A straight, smooth duct often makes a small fan perform like a much bigger one.

02

Confirm the fan's quoted extract rate against building regulations

A bathroom typically needs at least 15 litres per second of intermittent extract. Many old fans are well below this.

03

Check whether replacement air can reach the bathroom

A fan with no air path simply hums. Leaving the bathroom door slightly open during operation often solves it.

04

Look at the run-on timer setting

An extractor that switches off the moment the light goes out cannot clear a humid room. Twenty minutes of run-on is a common upgrade.

05

Decide whether continuous low-rate extract is appropriate

A small continuous fan running at low speed prevents build-up rather than chasing it. It is often quieter and more effective overall.

What we recommend, and why

Every recommendation explains why it comes first, the evidence behind it, how confident we are, and when it would not apply.

01

Shorten and straighten the duct before changing the fan

Why this comes first

Long, kinked or flexible ducts can halve a fan's real-world extract rate. A short, rigid run often makes an existing fan perform like a much bigger one and costs less than replacement.

Evidence

FETA commissioning guidance and Part F both treat duct routing as the primary determinant of installed extract performance.

FETA · ADF

Confidence

High confidence. Multiple independent sources agree on the direction and the order.

Exceptions
  • Installations where the duct route cannot reasonably be changed
Next step
See the ventilation hub
02

Make sure air can reach the bathroom while the fan runs

Why this comes first

A fan with no make-up air simply hums. Leaving the door slightly open during operation, or fitting a door undercut, often resolves performance without any new equipment.

Evidence

Part F treats door undercuts and make-up air as part of the designed extract path; FETA application notes confirm the effect on installed extract rate.

ADF · FETA

Confidence

High confidence.

Exceptions
  • Bathrooms intentionally pressurised by a supply ventilation system
Next step
Plan it on the Home Planner

Real homes that faced this

Publicly documented UK projects that ran into the same problem and reported what happened.

We did not run these projects. Each links back to the original publisher so you can read the figures in context and judge for yourself what carries over to your home.

Evidence behind this page

Every recommendation on this page is traceable to its source. Click a publication to read the original.

Products that may help

Only consider these once the checks above have been ruled out. A product fitted into the wrong cause is rarely satisfying.

House Summary

The cheapest answer to bathroom extractor not coping with steam is usually the one that addresses the cause rather than the symptom. The list above is in the order we would work through it, because the checks at the top tend to rule out the most expensive mistakes further down.

Related decisions
Next Best Step

Ventilation; what your home actually needs

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