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, but the electrical connection in a bathroom must be carried out by a Part P qualified electrician or notified to Building Control.
Tools and Materials
| Item | Typical cost (2026) | Notes |
|---|---|---|
| Extractor fan (100 mm, humidity sensor type) | £25–£80 | Timer or humidity-controlled recommended |
| Core drill bit (100 mm or 125 mm diameter) | £20–£50 (buy) / £15/day hire | Diamond core for masonry; SDS chuck required |
| SDS hammer drill | £50–£150 (buy) / £20–£30/day hire | For the core drill; a standard drill is not adequate |
| Duct — 100 mm rigid PVC or flexi duct | £4–£10/m | Rigid is preferred; flex only for short straight runs |
| Wall vent grille (external) | £5–£20 | Must be louvred/fly-screened; non-return preferred |
| Silicone sealant (exterior grade) | £4–£8 | Sealing around external grille |
| 1.0 mm² or 1.5 mm² twin and earth cable | £1–£2/m | For new wiring; confirm requirement with electrician |
| Drill bits, rawlplugs, screws | £5–£10 | For mounting |
Total materials for a basic installation: approximately £60–£160, depending on fan specification and wall thickness.
Planning Before You Start
Zone rules (bathrooms): Under BS 7671 (IET Wiring Regulations, 18th Edition), bathrooms are divided into zones based on proximity to water. Zone 1 (inside the bath/shower enclosure) and Zone 2 (within 60 cm of the edge) require appropriately rated IP equipment. Extractor fans in bathrooms must have an IPX4 rating minimum (splash-proof) — most modern bathroom extractor fans meet this requirement.
Ventilation rates (Building Regs Part F): For a bathroom with no openable window, mechanical ventilation must achieve at least 15 litres/second extract rate. For a kitchen, the required rate is 30 l/s adjacent to a hob or 60 l/s elsewhere. Most 100 mm fans achieve 85–120 m³/hr, equivalent to approximately 23–33 l/s — adequate for a bathroom. Check the product specification.
Run-on timer or humidity sensor: Part F also requires that the fan runs for a minimum period after the room is last used (typically 15 minutes). A timer-controlled or humidity-sensing fan satisfies this without manual override.
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. 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 — flex duct over 600 mm significantly reduces flow rate.
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 must be wired via a fused connection unit (FCU) or connected to the lighting circuit via an appropriate switched live — 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.
This wiring work must be carried out by a registered competent person (NICEIC, ELECSA, or similar) or notified to Building Control before work begins. 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.
Cost Summary
| Route | Approximate total cost |
|---|---|
| Full DIY (materials + electrical labour only) | £140–£280 |
| Tradesperson — fan + fitting + wiring | £180–£350 |
| Complex installation (structural wall, long duct run) | £300–£600+ |
Building Regulations Compliance
The installation must comply with:
- Part F (Ventilation) — adequate extract rate and run-on control
- Part P (Electrical Safety) — notifiable if new circuit or work in a bathroom
- In some local authority areas, a new penetration through an external wall of a dwelling may require a minor works notification — check with your local Building Control if uncertain
A reputable electrician will self-certify the work under their competent person scheme and provide a certificate, which is useful when selling the property.
