How to Install an Extractor Fan Through a Wall

Installing an extractor fan through a wall avoids the need for a duct run through ceiling voids and is often the simplest solution for a bathroom or kitchen on an external wall. The core principle is straightforward: bore a hole through the wall, fit the fan, connect it to power, and seal against draughts and weather. The complications are mostly regulatory, this work touches on both electrical and ventilation compliance.

A competent DIYer can handle the drilling and fan fitting. The electrical connection is the part that may be notifiable, and whether it is depends on where in the room the work sits and whether a new circuit is involved, as set out below. Notifiable work must either be carried out by a registered competent person who self-certifies it, or be notified to Building Control before work starts.¹

Tools and Materials

Total for a basic installation, hiring the SDS drill and core bit for a day: approximately £80–£185, depending on fan specification and wall thickness. That is the table’s line items added up: £43–£138 for the fan and materials (with 1 m of duct and up to 5 m of cable) plus £35–£45 of hire.

At the budget end, the Manrose MG100T 100 mm timer fan is £27.38 at Screwfix.² Vent-Axia’s quieter 100 mm Silent fan is £74.99 with a timer at Toolstation, and £109.98 in its humidistat version, so a premium humidity-sensing fan takes the total above this range.³

Planning Before You Start

Zone rules (bathrooms): BS 7671 (the IET Wiring Regulations, 18th Edition, now BS 7671:2018+A4:2026) divides bathrooms into zones based on proximity to water.⁴ Zone 1 is the space directly above the bath or shower tray up to 2.25 m from the floor, and Zone 2 extends 0.6 m beyond it.⁵ A fan installed in Zone 1 or Zone 2 must be rated at least IPX4 (splash-proof); most modern bathroom extractor fans meet this requirement.⁶

Ventilation rates (Building Regs Part F): Extract ventilation to the outside is required in every kitchen, utility room and bathroom, and an openable window does not remove that requirement.⁷ A bathroom needs at least 15 l/s, a utility room 30 l/s, and a kitchen 30 l/s where a cooker hood extracts to the outside or 60 l/s where it does not.⁷ Separate sanitary accommodation needs 6 l/s, and that is the one case where the rate can instead be met by a window meeting the purge ventilation guidance.⁷ Ratings vary from fan to fan: Manrose’s MG100T, a basic 100 mm timer fan, is rated at 85 m³/hr, about 24 l/s, which clears the bathroom minimum but not the 30 l/s a utility room needs.² Check the product specification.

Run-on timer or humidity sensor: In a room with no openable window, Part F requires an intermittent extract fan to keep running for at least 15 minutes after the room is vacated.⁷ A timer-controlled or humidity-sensing fan satisfies this, though any automatic control should still have a manual override.⁷

Step 1: Locate the Position

Position the fan as high as possible on the wall (warm moist air rises) and as far from the air inlet (e.g. door gap) as practical to maximise air movement across the room. Part F puts a number on “as high as possible”: an extract terminal or fan, cooker hoods aside, should be as high as is practicable in the room and no more than 400 mm below the ceiling.⁷ Avoid positioning directly above a shower where steam concentration is highest unless the unit is specifically rated for that.

Check both sides of the wall:

• Inside: avoid existing wiring, pipes, and stud positions
• Outside: avoid being under an overhang that will restrict the external grille, and check for any pipes or cables in the wall using a pipe and cable detector

Mark the centre point and confirm the core drill will miss any obstructions.

Step 2: Core Drill the Wall

Fit the core drill bit to the SDS hammer drill. Core drilling requires firm, steady pressure, brace yourself and let the drill do the work. For a cavity wall (two leaves of masonry with a 50–100 mm cavity), the total core depth will be 250–350 mm. Drill from the inside initially to establish the hole, then complete from outside if the wall is thick.

Angle the hole very slightly downward towards the exterior, typically 5° is sufficient, so any water that enters the duct exits outward rather than back into the room.

Remove the core (it usually comes out in sections), clear any rubble, and brush out the hole.

Step 3: Fit the Duct and External Grille

Push the duct through the hole. For a cavity wall with a rigid duct, use a single continuous piece of 100 mm rigid PVC duct that spans the full wall thickness, seating it in the hole. If a short section of flexible duct is needed to connect the fan to a rigid duct (for minor alignment), keep it as straight and short as possible. Approved Document F says rigid duct should be used wherever possible, and flexible duct only for final connections, no more than 1.5 m long and pulled taut.⁷

Fix the external louvred grille to the outside of the wall using rawlplugs and screws. Apply a bead of exterior-grade silicone sealant around the perimeter to weatherproof the joint between grille and masonry. A non-return louvre (spring-loaded blades that close when the fan is off) prevents cold draughts and insect entry.

Step 4: Mount and Connect the Fan

Fix the fan body to the wall per the manufacturer’s instructions, typically a faceplate that clips or screws over the duct entry. The fan should sit flush against the wall with the grille covering the hole interior.

Wiring the fan (Part P): In a bathroom, the fan is usually wired either through a fused connection unit (FCU) or from the lighting circuit via a switched live, often with a fan isolator switch, and the specific arrangement depends on the fan type (run-on timer, humidity sensor) and whether you want it to operate with the light or independently.

Whether this wiring is notifiable depends on the work, not simply on the room. In England only three things are notifiable under Part P: a new circuit, replacing the consumer unit, and work inside a special location, which in a bathroom means from floor level up to 2.25 m and within 0.6 m of the bath or shower tray (or within 1.2 m of the shower head centre where there is no tray).¹ Notifiable work must be carried out by a registered competent person (NICEIC, NAPIT or another scheme authorised by government) who self-certifies it, or be notified to Building Control before work begins.⁸ Spurring a fan off an existing lighting circuit high on the wall, outside that zone, is not notifiable, but it still has to comply with Part P and BS 7671, and most people hand it to an electrician anyway.¹ An electrician charges approximately £80–£150 for the wiring element of a straightforward fan installation.

Step 5: Seal and Test

Once the fan is wired and operational, seal any remaining gap between the duct and wall internally using decorator’s caulk or expanding foam (trim flush when set). Switch on and verify operation: the fan should be audible running and flow can be checked by holding a tissue near the grille. Part F also expects the installer to check that the fan works, responds to its controls, and has any run-on timer set to at least 15 minutes.⁷

Cost Summary

Building Regulations Compliance

The installation must comply with:

• Part F (Ventilation), adequate extract rate and run-on control
• Part P (Electrical Safety), notifiable only for a new circuit, a consumer unit replacement, or work inside the bathroom special location

A registered electrician will self-certify notifiable work under their competent person scheme, and the occupier should receive a Building Regulations compliance certificate within 30 days, which is useful when selling the property.¹