Bathroom ventilation and what the building regulations require

A shower puts a remarkable amount of water into the air. Somewhere between one and two litres per person per day ends up as vapour in a bathroom, and if it is not taken outside it condenses on the coldest surface available — which is usually a window reveal, an external wall, or the underside of a roof.

That is why ventilation is a building regulations requirement rather than a preference, and why the way an extractor is installed matters more than which one you buy.

What is required

Part F of the building regulations covers ventilation. For a new or refitted bathroom it means mechanical extract, at a minimum rate, ducted to outside.

The rate is expressed in litres per second. A bathroom or shower room has a minimum intermittent extract rate; a WC without a bath or shower is lower, and a kitchen higher, with the kitchen rate depending on whether the fan is over the hob or elsewhere. Continuous-running systems are rated differently again.

Two practical points that matter more than the exact figure:

A fan’s rating and its actual performance are different numbers. Fans are rated in free air. Put one on the end of five metres of flexible ducting with two bends and a fly screen and the real throughput can be a fraction of the rating. The duct route is part of the design, not an afterthought.

Rigid duct outperforms flexible. Concertina flexible duct is convenient and it is the standard reason a fan underperforms. Where you can use smooth rigid duct, do — and keep the run short and the bends few.

The mistake that causes actual damage

Ducting into the loft.

It is extremely common, it is quick, and it is a genuinely destructive thing to do. Terminating a bathroom extract in a roof void does not remove the moisture; it relocates it from a room you can ventilate into a space you cannot, full of cold timber. The vapour condenses on the underside of the roof and on the rafters. Over a winter or two you get mould, then rot, then a structural problem in the most expensive part of the house to repair.

Extract has to terminate outside — through a wall, through a tile vent, or through a soffit. If it goes through a cold loft to get there, the duct itself must be insulated, otherwise the moist air cools inside the duct, condenses, and runs back down into the fan.

If you take one thing from this: ask where the duct terminates, and go and look.

Overrun timers and humidity sensing

An extractor that runs only while the light is on removes almost none of the moisture, because the moisture is still evaporating off wet surfaces after you leave.

An overrun timer keeps the fan going after the light goes off. A humidity sensor runs the fan when humidity rises regardless of the light, which handles the case of a bath taken with the light off during the day, and stops it running pointlessly at 3am for someone using the WC.

Neither is required. Both make the difference between a fan that works and a fan that is technically present.

Electrical zones

Bathrooms have defined zones around the bath and shower, and what may be installed in each is restricted — how well protected it needs to be against water ingress, and what voltage. The practical consequences: fittings inside the shower area need appropriate ingress protection, and standard socket outlets are not permitted within a defined distance of a bath or shower, with shaver supply units the usual exception.

Bathroom electrical work is notifiable under Part P. Either a registered electrician self-certifies it, or it is covered by a building control application. Get the certificate at the end — it is one of the documents a buyer’s solicitor asks about.

Supplementary bonding of pipework and metal fittings may also be required depending on the installation. That is a question for the electrician, not the tiler.

Windows do not count on their own

An openable window is background ventilation, not extract. It does not satisfy the requirement for mechanical extract in a room with a bath or shower, and in practice nobody opens a window in February, which is precisely when condensation is worst.

Where it goes wrong in a refit

Bathroom refits are where this is most often missed, because the room already exists and the existing fan gets reused without anyone checking it.

Worth confirming before the tiling goes on:

• Is there mechanical extract at all, and does it actually move air? Hold a sheet of paper to it.
• Where does it terminate? Outside, or in the loft?
• Is the duct insulated where it passes through cold space?
• Is there an overrun timer or humidity sensor?
• Is the electrical work being certified, and by whom?

Every one of those is cheap to fix while the walls are open and expensive afterwards.

What to do next

If you are refitting a bathroom, ventilation is the item most likely to be skipped and the one most likely to cause damage. It should be a line in the quotation, not an assumption.

Tell us about the room and we will tell you what it needs.