If you're cleaning mould out of your bathroom every few weeks and it keeps coming back, the bleach isn't the problem — your bathroom is. Mould is a symptom of moisture sitting on a surface for long enough that the spores can colonise it. Eliminate the moisture or the surface, and the mould has nowhere to grow. This guide is what we'd tell you at the home visit if you booked us specifically because of a mould problem.
Some context before the fixes, because it helps to know you're not unusual: when the Scottish Government last surveyed the country's housing, it found visible mould in around 195,000 homes. That's roughly one in twelve. So if you're quietly embarrassed about the black speckling along the silicone, don't be — a lot of Scotland is living with it.
The important caveat, and it's the reason we don't just sell everyone a bigger fan: that survey recorded mould anywhere in the home, not specifically in bathrooms, and mould rarely has a single cause. Cold external walls, heating patterns, insulation and plain old leaks all feed into it alongside ventilation. We look at all of them before quoting.
Scottish Government, Scottish House Condition Survey 2024. Mould observed in 8% of Scottish dwellings, around 195,000 homes. Whole-home measure, not bathroom-specific.Why Mould Grows in Bathrooms
Mould needs three things: spores (always present in indoor air), moisture, and a surface to grow on. You can't eliminate the spores — they're in the atmosphere. The whole game is removing the moisture before the spores can colonise, or making the surface unsuitable for growth.
In bathrooms, moisture comes from showering, bathing, and the steam that condenses on cold surfaces. The "cold surfaces" matter — moisture in the air doesn't cause mould; moisture condensing on a wall, ceiling or grout joint does. That's why mould always shows up at the same spots: top corners (cold ceiling), grout lines (porous surface), silicone seals (porous surface), and external walls (cold).
Standard internet advice — "ventilate while showering, dry the walls down" — works in theory but only if your ventilation actually shifts the air. Most older bathrooms have ventilation that moves enough air to satisfy the regulations and not enough to actually clear the steam.
Fix 1 — Proper Ventilation
The single biggest cause of recurring mould is undersized or broken ventilation. The Scottish Building Standards minimum is 15 litres per second; in practice, that's not enough for most bathrooms. Aim for 20–25 l/s with humidity sensing.
What "proper ventilation" actually means:
- Humidity-sensing fan — switches on automatically when humidity rises above ~65%, runs until it drops back. Doesn't depend on the user remembering to switch on the light.
- Run-on timer — fan continues for 15+ minutes after the trigger ends, clearing residual steam.
- Rigid duct — concertina flexible duct loses 50% of its rated capacity over a 3m run. Rigid 100mm PVC is what should be there.
- External termination — duct ends in a sheltered grille on an outside wall, not in the loft.
- RCD-protected circuit — required by BS 7671. Bonus: the circuit can be set to run independently of the bathroom light.
If your bathroom has a 1990s extractor that runs only when the light is on and only for 60 seconds afterwards, replacing it with a properly-sized humidity-sensing unit is genuinely the single highest-impact thing you can do. Cost: £180–£350 fitted with proper certification. Full ventilation guide here.
Fix 2 — Actual Heat in the Room
Cold rooms are mould-prone because cold surfaces make moisture condense. A bathroom that warms up properly — and stays warm long enough to dry out after a shower — has less mould than a cold one with the same ventilation.
Three changes that meaningfully shift the heat picture:
- Heated towel rail with proper thermostat. Most bathroom rads run 24/7 in winter. Put a TRV on it and program it to run hard during shower windows and shoulder hours.
- Underfloor heating on a programmer. Genuinely warms the room from the floor up, dries out the residual humidity faster than air-only heating. More on UFH here.
- Insulation behind tiled or paneled walls. See Fix 4 below.
Fix 3 — Replace Problem Surfaces
Some surfaces don't grow mould because there's nowhere for it to take hold. Some surfaces grow it because they're porous and moisture sits on them. Replacing the second category with the first is the biggest single change you can make to a bathroom.
Cement grout in the shower zone
Cement-based grout is porous. It absorbs moisture, the moisture sits in the joint, the spores colonise. Within 18 months in a poorly-ventilated bathroom, the grout darkens permanently. The two real fixes:
- Re-grout with epoxy grout. Non-porous, doesn't darken, doesn't need re-sealing. About £180–£250 to retrofit on an existing tiled shower zone.
- Switch to wet wall. No grout joints in the shower zone at all. The single most effective material change you can make. Wet wall service here.
Old silicone seals
Silicone has a finite life — typically 5–7 years before it starts to fail at the wall and bath joint. Once moisture gets behind the silicone, mould grows in the gap and is visually impossible to remove without stripping and re-applying. See Fix 5.
Fix 4 — Insulate Cold Walls
External walls in older properties are the worst culprits — solid concrete in CDC-era flats, single-skin sandstone in tenements, solid stone in village cottages. The internal wall surface stays cold all year, and shower steam condenses on it instantly.
The fix is invisible: when re-fitting the bathroom, fix 50×25mm timber battens to the cold external walls, fit 25mm PIR insulation between them, and panel/tile over the top. The wall surface temperature rises by 5–8°C, condensation stops happening on that surface, mould loses its foothold.
This is the standard wall build-up on every bathroom we fit in cold-walled property — Kildrum, Seafar, Abronhill CDC-era flats; Cumbernauld Village stone cottages; older sandstone tenements in Coatbridge and Motherwell. It adds £180–£300 to a typical fit and pays back in mould-free years.
Fix 5 — Sanitary Silicone, Properly Applied
Most domestic silicone joints fail because they're not sanitary-grade and not applied to the right surface preparation. The right approach:
- Sanitary-grade silicone only (Dow 785 or equivalent). Anti-fungal additive built in. £8–£12 per cartridge vs £3 for the cheap stuff.
- Surfaces fully clean and dry before application. Even a residual film of soap or limescale stops the silicone bonding properly.
- Tooled to a smooth finish — beads with thumb-prints and creases trap moisture in the irregularities.
- Replaced every 5–7 years as a maintenance item, not after it's already gone mouldy.
If you've got mould growing visibly behind your silicone, the only fix is to strip it and reapply — bleach won't penetrate the gap to the moisture trapped inside. We do silicone refresh as a standalone job for around £180–£280 if a customer doesn't need a full bathroom refit.
Related reading: bathroom ventilation rules, condensation causes & fixes, our wet wall bathroom service.
Got a recurring mould problem you'd like solved properly rather than re-bleached? Call 01236 801802 — we'll come out, identify the actual cause, and quote you for the fix.