Bathroom ventilation: how to keep your bathroom dry and free of mould

Why a bathroom needs extraction

A hot shower puts a lot of water vapour into a small room in a short time. Warm air can hold a lot of moisture. As soon as it meets a cold surface, such as an outside wall, a window or a poorly insulated ceiling, the moisture condenses. You see droplets on the mirror and tiles.

If that happens every day and does not dry quickly, mould grows in the silicone joints and on the ceiling. On a timber floor or in an attic bathroom, moisture can also get into the structure. Good ventilation removes the moisture before it settles and dries the room quickly after a shower.

What the Dutch rules require

The Dutch Building Decree, the Besluit bouwwerken leefomgeving or Bbl, sets ventilation requirements for bathrooms and toilets. According to IPLO (in Dutch), the government information point for these rules, a bathroom in an existing home needs a ventilation capacity of at least 14 dm³/s and a toilet room at least 7 dm³/s (article 3.67, paragraph 6). If the toilet is in the bathroom, the higher figure of 14 dm³/s applies.

New builds have the same capacities (article 4.122, paragraph 5). In addition, according to IPLO on new-build ventilation, air from a toilet and bathroom must go straight outside, not through another room (article 4.128). A duct that runs through other rooms is allowed.

For renovation, the principle is the legally obtained level: the building may not be worse after the work than before. If an installation is completely replaced, new-build requirements apply. For a bathroom that comes down in practice to the same 14 dm³/s, venting outside.

The systems you will find

In Dutch homes you come across four situations. Which one you have decides what needs doing during the renovation.

• Natural ventilation: a window or vent in the outside wall, without a fan. In most cases that is not enough for a bathroom used daily.
• Mechanical extraction with a central unit: an extract valve in the ceiling of bathroom, toilet and kitchen, with a fan unit in the attic. Fresh air comes in through trickle vents.
• Balanced ventilation with heat recovery: extract and supply through ducts, with a unit that recovers heat from the outgoing air.
• A separate fan in the wall or ceiling, with its own duct to the facade or roof.

Air also has to come in

A fan can only pull air out if air can get in somewhere. For a bathroom that air usually comes from the hallway, under the door. In its page on mechanical extract ventilation (in Dutch), Milieu Centraal, the Dutch environmental information centre, mentions a gap of at least 1.5 cm under the door, or a grille in the door.

This often goes wrong during a renovation. The new floor is thicker, the old door is trimmed until it just clears, and the gap is gone. The fan still runs, but moves little air. When setting the floor height, keep an eye on the door and the gap underneath.

The duct and the fan

A fan is only as good as the duct behind it. A long duct with many bends and a small diameter creates resistance, and the fan will not reach its stated capacity. Keep the duct as short and straight as possible and have a non-return valve fitted, so cold air does not blow back in when it is windy.

If the duct runs through a cold attic, the damp air condenses inside it and the water runs back to the fan. An insulated duct prevents that. Choose a quiet fan and fix it free of vibration. For renovation, Dutch rules also set limits on noise from new installations in habitable rooms, see IPLO on installation noise (in Dutch).

A fan with an overrun timer or humidity sensor keeps running after your shower until the moisture is gone. That works better than a fan that switches off with the light.

Ventilation during a bathroom renovation

A renovation is the moment to get ventilation right, because the ceiling and walls are open anyway. During the survey we check whether there is an extract point, where it goes and whether the capacity is enough. If there is a valve from a central unit, check that the duct is clean and clear.

In an apartment the extract often runs through a shared duct belonging to the owners' association, and you cannot simply connect your own fan to it. More in renovating a bathroom in an owners' association apartment. In an attic bathroom the duct usually goes through the roof, see attic bathroom.

The electrician runs the power supply for the fan before the wall closes. Where the fan fits in the order of work is in bathroom renovation step by step.

Using it well after the renovation

Even the best ventilation only works if you use it. Milieu Centraal advises setting mechanical ventilation to its highest setting while you shower and letting it run on afterwards. It advises against leaving a window open in winter. If you want to air the room, open the window for no more than 10 minutes, straight after showering.

After showering, squeegee the floor and walls. That removes a lot of moisture that would otherwise have to be ventilated away. Clean the extract valves once a year with soapy water and put them back at the same setting. Do not use latex paint on the ceiling, but a moisture-resistant paint.

Ventilation and mould: when it is more than that

If mould keeps coming back even with good ventilation, the cause is often deeper: a cold outside wall, a leaking pipe in the wall or waterproofing that has failed. A stronger fan will not help then, and the cause has to be fixed. At Parus you have one point of contact for the whole bathroom, including plumbing and electrics. During the survey Stas looks at where the moisture is coming from.

Frequently asked questions