Ventilation is the single most under-specified part of a bathroom refit. Get it wrong and the consequences are steady rather than dramatic — grout mould along the splash, condensation pooling on the windowsill, panels lifting after three years. Get it right and the room stays dry, the finish lasts, and the air quality is genuinely better than the original. This guide covers what Scottish Building Standards require, what's actually worth fitting, and the question to ask before signing the quote.
What the Regulations Say
The Building (Scotland) Regulations 2004, supported by the Domestic Technical Handbook, set out ventilation requirements in Section 3.14. The relevant points for a bathroom refit:
- Any room used for bathing or showering must have ventilation to outside.
- An openable window meets the requirement for rooms with one — minimum opening area of 1/30th of the floor area.
- Where there's no opening window (inboard bathrooms, internal en-suites, basement WCs), mechanical extract is mandatory.
- Intermittent extract must achieve a minimum of 15 l/s; continuous extract must achieve a minimum of 8 l/s.
- The system must be capable of running independently of the lighting, or have an over-run of at least 15 minutes after the light switches off.
For a refit (rather than a new build), these requirements apply when the bathroom is "altered substantially" — which a strip-out and full refit comfortably is. We design every fit to meet the current standard, even where the existing extractor was technically grandfathered in.
How Fans Are Sized
The headline number on a fan box is its free-air flow rate — what it shifts in still air with no ducting. Real-world performance is always lower because ducting, bends and grilles all add resistance. A typical CDC-era flat bathroom (1.9 m × 1.7 m × 2.4 m ceiling = ~7.7 m³) needs around 20 l/s to achieve good air change rates in real-world conditions, even though the regulation minimum is 15 l/s.
Rule of thumb: spec a fan rated about 30% higher than your minimum target. A fan badged at 25 l/s free-air will typically deliver 18–20 l/s through a short run of properly-fitted 100mm ducting. That's the right number for most Cumbernauld bathrooms.
Intermittent vs Continuous Extract
Intermittent
The classic bathroom fan. Switches on (with light, with humidity sensor, or with a separate switch), runs hard, switches off. Higher peak flow, lower average flow. Good for rooms used in concentrated bursts (most domestic bathrooms). Cheaper to buy, slightly noisier in use, simpler to install.
Continuous
Runs constantly at a low background rate, ramps up to a "boost" mode when humidity rises. More expensive, virtually silent in trickle mode, and gives better long-term air quality. Required if you're hitting whole-house Mechanical Extract Ventilation (MEV) standards rather than room-by-room compliance.
For most domestic refits in Cumbernauld and the surrounding G postcodes, an intermittent humidity-sensing fan is the right answer. Continuous makes sense in newer airtight homes and in households with chronic damp issues — but for a 1970s flat refurb, intermittent is the practical choice.
Humidity Sensors & Timers
The single best feature on any bathroom extractor is a humidity sensor. It detects when relative humidity rises above a set threshold (usually around 65%) and runs the fan automatically until the humidity drops back. Two practical benefits:
- The fan runs after you've left the room — exactly when you need it most. The peak humidity event is post-shower, when you've already turned the light off.
- The fan doesn't run pointlessly when nobody's showering — it's not slaved to the light, so you don't get airflow during a 30-second WC visit.
A timer-only fan (runs for 15 minutes after the light turns off) is the regulatory minimum. A humidity-sensing fan is what you actually want, and the price difference is small — typically £25–£50.
Ducting — The Part Everyone Gets Wrong
A high-rated fan with bad ducting performs worse than a budget fan with good ducting. Three common failure modes we see during strip-outs:
- Concertina flexible ducting on a long run — the corrugated interior creates massive resistance. Performance can drop by 50% on a 3-metre run.
- Multiple 90° bends — every sharp bend adds the equivalent of a metre of extra ducting in resistance terms. Two right-angles in a short run can halve the effective extract rate.
- Ducting terminating in the loft, not outside — surprisingly common in 1970s installs. The fan does extract from the bathroom, but it dumps the moist air into the loft space where it condenses on the cold rafters. This causes loft mould, not bathroom dry.
The fix on a refit: rigid 100mm PVC duct, smooth bore, minimum bends, terminating in an external grille on a sheltered elevation. We replace any concertina flexible ducting we find as standard during strip-out.
What to Ask Your Fitter
- What's the rated extract rate of the fan you're fitting? Should be 20+ l/s for a domestic bathroom.
- Is it humidity-sensing? Should be yes.
- Does the duct terminate outside or in the loft? Should be outside — no exceptions.
- Is it rigid or flexible duct? Should be rigid where the run is over 1 metre.
- Is the fan circuit RCD-protected and certified? Should be yes — covered by the BS 7671 certificate at completion.
Related reading: bathroom electrics & IP zones, Scottish building regs, waterproofing 101, and our wet wall bathroom service.
Got a bathroom with persistent condensation or mould issues? Call us on 01236 801802 — most of the time the fix is in the ventilation, and it's cheaper than you'd think.