How to Fit a Bathroom Extractor Fan (and Why You Need One)

Bathrooms produce more moisture than any other room in the house. A single shower releases 1.5–2 litres of water vapour into the air. Without adequate ventilation, that moisture condenses on walls, ceiling, and windows — leading to mould, peeling paint, and timber decay. A properly sized extractor fan is the most effective solution, and fitting one is a manageable DIY project.

Do You Need an Extractor Fan?

UK Building Regulations (Approved Document F) require mechanical ventilation in bathrooms without opening windows. Even if you have a window, an extractor fan is more effective — you’re unlikely to open a window during a winter shower, but a fan runs automatically.

Signs you need better ventilation:

• Condensation on windows and mirrors lasting more than 30 minutes after showering
• Mould on the ceiling, grout, or window frames
• A musty or damp smell
• Peeling paint on the ceiling
• The room feels humid and clammy

Our condensation guide explains the full picture of moisture management in the home.

Types of Extractor Fan

Timer vs Humidity Sensor

• Timer overrun — fan switches on with the light and runs for a set time (typically 15–20 minutes) after the light is turned off. Most common, simplest, effective. The overrun period is adjustable on most models
• Humidity sensor (humidistat) — fan switches on automatically when humidity rises above a set threshold. More sophisticated, runs only when needed. Excellent for ensuring the fan runs even when the light switch isn’t used (e.g., during daylight)
• PIR motion sensor — fan activates when someone enters the room. Less common but useful in combination with a timer

A timer overrun fan is the most practical choice for most bathrooms. The Screwfix extractor fan range includes models with every trigger type.

Sizing: How Powerful Does It Need to Be?

Fan capacity is measured in litres per second (L/s) or cubic metres per hour (m³/hr). Building Regulations require a minimum extraction rate of 15 L/s for bathrooms. To calculate what you need:

1. Calculate the room volume: length × width × height (in metres) = m³
2. A bathroom should achieve at least 15 air changes per hour for effective moisture removal
3. Required flow rate = room volume × 15 ÷ 3,600 (to convert to L/s)

For a typical bathroom (2.5m × 2m × 2.4m = 12m³), you need at least 50 L/s, or about 180 m³/hr. In practice, a standard bathroom fan rated at 15–25 L/s handles most domestic bathrooms adequately — the building regulation minimum of 15 L/s is the target.

Installation: Through an External Wall

The simplest installation — the fan mounts in the wall and exhausts directly outside:

1. Choose the location — as high as possible on the wall (hot, moist air rises) and ideally on the wall opposite the door to create a cross-flow of fresh air
2. Mark the hole position — use the fan’s template or measure the required hole size (typically 100mm or 150mm diameter)
3. Drill a pilot hole from inside to mark the position on the outside wall
4. Core drill from outside — use a 100mm or 150mm diamond core drill. On cavity walls, you’ll need to drill through both skins. Wear safety glasses and a dust mask. If you don’t own a core drill, most tool hire shops rent them
5. Insert the duct sleeve — a plastic tube that lines the hole and gives the fan a smooth surface to seal against
6. Mount the fan — fix it to the wall using the screws provided, connecting to the duct sleeve. Seal around the duct on the outside with silicone or mastic
7. Fit the external grille — this keeps rain, insects, and birds out. Most fans include one. Ensure it has a back-draught shutter to prevent cold air flowing in when the fan is off
8. Wire the fan — see wiring section below

Installation: Through the Ceiling (Ducted)

For bathrooms that don’t have an external wall (en-suites, internal rooms), the fan mounts in the ceiling and ducts through the loft to an external wall or soffit vent:

1. Cut a hole in the ceiling to match the fan size
2. Mount the fan housing in the ceiling, securing to the joists or a mounting bracket
3. Run 100mm flexible or rigid duct from the fan through the loft to the nearest external wall or soffit
4. Keep the duct run as short and straight as possible — every bend and extra metre reduces airflow
5. Insulate the duct in the loft — uninsulated duct in a cold loft causes condensation inside the duct, which drips back into the bathroom
6. Fit an external grille or soffit vent at the termination point

Wiring

Important: Electrical work in bathrooms is notifiable under Part P of the Building Regulations. If you’re not a qualified electrician, you have two options:

• Use a fan that plugs into an existing fan isolation switch (the simplest DIY option — no new wiring)
• Hire a qualified electrician to run the wiring and issue a Part P certificate. For a straightforward fan installation with existing wiring, this typically costs £50–£100 on top of the fan price

Most extractor fans require a permanent live supply (for the timer function) and a switched live from the light switch (to know when the light is on). This is standard wiring that most bathrooms already have if an existing fan was fitted. The Electrical Safety First bathroom guide explains the regulations.

Common Mistakes

• Fan too small — an undersized fan runs constantly without clearing the moisture effectively. Size correctly for the room volume
• Venting into the loft — never vent into the loft space. The moisture will condense on roof timbers, causing rot and mould. Always duct to the outside
• Duct too long or too many bends — each metre of duct and each bend reduces airflow significantly. Keep the duct run as short and direct as possible
• No back-draught shutter — without one, cold air blows in through the fan when it’s off, creating a draught and wasting heat
• No timer overrun — if the fan only runs while the light is on, it stops the moment you leave the room. The bathroom is still full of moisture. A 15–20 minute overrun lets the fan clear the air properly