Ventilation
Extraction fans — get the moisture out
Bathrooms, shower rooms, utility rooms and kitchens all generate large volumes of warm, moisture-laden air. If it isn't removed at source it condenses on the coldest surfaces — window reveals, external corners, behind wardrobes — and within days you have black mould, damaged decoration and, over time, rotting timbers. A correctly sized, correctly sited extract fan is the single cheapest thing you can do to protect a building.
The governing principle
Extract at source — "Awaab's Law"
Paul references what he calls Awaab's Law of extraction: capture moisture and pollutants at the point they are produced, before they mix with room air. Once warm moist air has left the shower cubicle or the hob and diffused into the rest of the room, you have to shift ten times the volume to achieve the same result — and most of it will already have found a cold surface to condense on. The fan therefore belongs directly above (or immediately adjacent to) the moisture source, not on the far wall as an afterthought.
The name is a nod to the real Awaab's Law, introduced through the Social Housing (Regulation) Act 2023 after the death of Awaab Ishak. It requires social landlords in England to investigate damp and mould hazards within 14 days and begin remedial work within 7 days of a report (or 24 hours in emergency cases). While the statutory duty sits with landlords, the underlying message is the same for every property: ventilation is not a decorating issue, it is a health issue.
Three corollaries follow from that principle:
- Replacement air matters. A fan can only push out what fresh air can replace. Undercut the door by 10 mm or fit a transfer grille — a sealed room simply stalls the fan.
- Duct route matters. Every 90° bend and every metre of flexible ducting slashes measured flow. Short, straight, insulated rigid duct to a wall or roof terminal always beats a long concertina run into a soffit.
- Run-on time matters. Moisture keeps evaporating from wet tiles and towels for 10–20 minutes after the shower stops. A fan that switches off with the light does almost nothing.
UK minimum extract rates — Building Regulations Approved Document F
Approved Document F (Ventilation) sets the minimum intermittent extract rates for wet rooms in dwellings. These are floors, not targets — Paul typically specifies a fan with 30–50% headroom to cope with dirty filters, long duct runs and real-world siting compromises.
| Room | Minimum intermittent extract | Typical spec |
|---|---|---|
| Kitchen — cooker hood | 30 l/s adjacent to hob | Ducted extractor hood over the hob |
| Kitchen — elsewhere | 60 l/s | 100–150 mm inline or wall fan |
| Utility room | 30 l/s | 100 mm axial with humidistat + run-on |
| Bathroom / shower room | 15 l/s | 100 mm axial or centrifugal with humidistat |
| WC (separate) | 6 l/s | 100 mm timer fan |
Figures from Approved Document F Volume 1 (dwellings). Continuous mechanical extract systems (MEV / MVHR) use different, lower whole-dwelling rates.
Siting, zones and electrical considerations
Bathroom zones (BS 7671 Section 701)
A fan in Zone 1 (directly above the bath or shower) needs to be at least IPX4 and supplied through a 30 mA RCD/RCBO via a fused connection unit outside the zones. Most ceiling extract fans are SELV 12 V through a remote transformer, which sidesteps the zone restriction entirely.
Humidistat vs pull-cord vs timer
A humidistat fan responds to actual moisture in the air — it comes on when the shower fills the room with steam and runs until the RH has fallen back to a set point (typically 60%). A timer fan switched from the light circuit will always run for exactly its programmed run-on, even when nobody used the shower. Paul specifies humidistat + adjustable run-on as the default.
Ducting — losses are brutal
A 100 mm axial rated 85 m³/h in the catalogue commonly delivers 25–35 m³/h once fitted with 3 m of ribbed flexi and two 90° bends. Use rigid smooth-wall duct, keep runs short, use long-radius bends, and insulate any duct in a cold roof void to stop condensation dripping back through the fan.
Combustion appliances (Part J)
A powerful kitchen extract in the same room as an open-flue gas appliance can pull combustion products back into the room. Where an extract fan is fitted in a kitchen with any open-flued appliance, a spillage test to BS 5440-1 is required. Paul will coordinate with a Gas Safe engineer where needed.
Black mould is a symptom, not a decorating problem
Painting over mould with anti-mould paint without fixing the ventilation buys you a few months at best. Fit an appropriately sized humidistat fan, verify the actual delivered flow with an anemometer, and the mould stops coming back.