Stopping mould and condensation in your bathroom

Black mould on grout and ceilings is one of the most common problems in London bathrooms, especially in older houses and flats with small windows. It is almost always a ventilation problem, and it is far easier to solve during a renovation than afterwards.

Why bathrooms get mould

Every shower releases a lot of warm, moist air. When that moisture meets a cold surface, such as an outside wall, a window or an uninsulated ceiling, it condenses. If the room stays damp, mould follows.

How much extraction do you need?

Building Regulations guidance in England (Approved Document F) sets a minimum intermittent extract rate of 15 litres per second for a bathroom. In practice, the fan should be:

• Sized for the room, not just the minimum, especially for large walk-in showers.

• Positioned high up and as far from the door as possible, ideally near the shower.

• Ducted the shortest possible route to outside, with rigid ducting where possible.

• Fitted with an overrun timer or humidity sensor so it keeps running after you leave.

Design choices that help

• Insulate cold external walls and ceilings during the renovation.

• Use a heated towel rail, which also keeps the room warmer.

• Choose mould-resistant grout and silicone, and good-quality tile adhesive.

• Leave a gap under the door or fit a vent so replacement air can enter.

If you already have mould

Clean it with a mould remover, then look at the cause. A quiet fan that never runs, a blocked duct or a missing door gap are all common culprits.

Planning a bathroom renovation?

We plan ventilation into every bathroom we build across South West London and Surrey. Contact us to discuss your project.