Ventilation is rarely the reason anyone renovates their bathroom, yet it decides whether a beautiful new bathroom stays beautiful. Poor ventilation leads to steamed-up mirrors, damp towels, black mould in the grout and peeling paint on the ceiling. In the worst cases moisture finds its way into timber and plaster where you cannot see it.
The easiest and cheapest time to get ventilation right is before a single tile goes on the wall. Once the room is finished, adding or rerouting ducting means opening up ceilings and walls.
Why London Bathrooms Are Prone to Condensation
Many London bathrooms are internal rooms with no window, particularly in flats and mansion blocks. Others are in period houses where the bathroom sits at the back of the property with a small window that is rarely opened in winter. Add a powerful shower, a bath and towels drying on a rail, and a large amount of moisture is released into a small space.
Condensation forms when warm, moist air meets a cold surface. External walls, single glazing, mirrors and uninsulated ceilings below a loft are the usual culprits. The solution is a combination of removing moist air quickly and reducing cold surfaces where it can settle.
What Building Regulations Require
Under Approved Document F of the Building Regulations, a new or refurbished bathroom should have mechanical extract ventilation. As a guide, an intermittent extractor fan in a bathroom should extract at least 15 litres per second, and continuous systems have their own lower minimum rates. The room also needs a way for replacement air to enter, which is usually achieved by leaving a gap under the door.
A window alone is generally not considered enough, because in practice it is rarely opened while the room is in use. Our designers and installation team make sure every bathroom we create meets the current requirements.
Choosing the Right Extractor Fan
Not all fans are equal, and the cheapest units are often noisy enough that people avoid switching them on. Things worth considering include:
- Extraction rate. The figure on the box is measured in ideal conditions. Long or bendy duct runs reduce real-world performance, so a little extra capacity is sensible.
- Noise. Quieter fans get used. Inline fans mounted in the loft or ceiling void, away from the room, are often much quieter.
- Controls. An overrun timer keeps the fan running after the light goes off, and a humidity sensor switches it on automatically when moisture rises.
- Continuous ventilation. Low-speed continuous fans that boost when needed are well suited to internal bathrooms and flats.
Position matters as much as the fan itself. The extract point should be as far from the door as possible, ideally above or near the shower, so that air is drawn right across the room. Any fan in the wet zones around the bath or shower must also be rated and installed to the correct electrical standard.
Ducting: The Part Nobody Sees
We regularly find fans in older properties that are ducted into the loft and simply stop there, pumping warm, wet air into the roof space. Ducting should run to the outside, by the shortest practical route, using rigid ducting where possible and insulated where it passes through cold spaces so that condensation does not form inside the duct and run back down.
In mansion blocks and converted buildings, the route to the outside may need agreement from the freeholder or managing agent. This is one of the reasons our surveyor notes every restriction before design work starts, and it is something we cover in our bathroom design and installation process.
Design Choices That Reduce Condensation
Ventilation does most of the work, but other design decisions help:
- A heated towel rail of the right output keeps surfaces warmer and dries towels quickly.
- Underfloor heating warms the room evenly and helps floors dry after showering.
- Heated demister pads behind mirrors keep them clear.
- Porcelain tiles and well-sealed grout resist moisture far better than painted plaster in splash areas. Our guide to bathroom materials compared explains the options.
- Bathroom-grade paint on ceilings resists mould growth.
In a wet room, where the whole floor gets wet, good ventilation and underfloor heating are especially important.
Plan It Properly From the Start
Ventilation is part of the design, not an accessory added at the end. Our designers consider airflow, ducting routes and heating alongside the layout, and our in-house installers carry it out as part of a fully project managed service with a fully inclusive quotation. If you are planning a new bathroom, book a free design consultation at one of our London showrooms and we will make sure it looks as good in ten years as it does on the day we hand it over.
Frequently Asked Questions
Does a bathroom need an extractor fan if it has a window?
In most cases, yes. Building Regulations expect new and refurbished bathrooms to have mechanical extract ventilation, because windows are rarely opened while the room is being used, particularly in winter.
What size extractor fan do I need for a bathroom?
Approved Document F sets a minimum of 15 litres per second for an intermittent bathroom extractor fan. Long or complex duct runs reduce performance, so it is sensible to allow some extra capacity.
Where should a bathroom extractor fan be positioned?
Ideally as far from the door as possible and near the main source of steam, such as the shower, so air is drawn right across the room. Fans in wet zones must be suitably rated and installed.
Why does my bathroom still get mould with a fan?
Common causes include a fan that is too weak, ducting that does not reach the outside, no gap under the door for replacement air, or a fan that is switched off too soon. A humidity sensor or overrun timer often helps.
How can I stop condensation in my bathroom?
Remove moist air quickly with a well-positioned extractor fan, keep surfaces warm with suitable heating, use moisture-resistant materials and allow the fan to run after showering.
