Ventilation for kitchens and baths
Applies to: Assumes rooms where water vapour is generated in quantity, in a house being progressively air-sealed and insulated.
Last worked on 2015-02-27
Moisture generated indoors has to leave the building. If it is not extracted, it condenses on the coldest surface available, which is why sealing a house without ventilating it makes mould rather than warmth.
Extract at the source, not from the hall
Vapour is easiest to remove where it is made and while it is still concentrated and buoyant. Once it has spread through the house it is diluted, it has already reached the cold surfaces, and no amount of general ventilation retrieves it.
This is why an extract fan in the room with the shower does more than an open window in the landing, and why the fan needs to run during and after the event rather than being switched with the light.
A fan needs somewhere to push to
A fan is a pressure device. It can only move as much air as the duct, the terminal and the make-up air path allow. A powerful fan on a long flexible duct with a tight grille at the end moves very little, and the reason is invisible from inside the room.
Short, smooth and straight beats large and powerful. A duct that must run far should be rigid, gently curved rather than elbowed, and terminated at something with a real free area rather than a fine mesh.
Make-up air
Everything extracted has to be replaced. In a leaky house it comes in through the leaks and nobody notices. In a house that has been sealed, the fan simply cannot move its rated flow, or it pulls air from the least desirable place available, which can be a flue or an underfloor void.
Providing a deliberate path - a trickle vent, an undercut door, a wall vent - is part of extract design, not an afterthought.
Where the duct terminates
A duct that discharges into a roof space rather than to outside moves the problem to somewhere it cannot be seen and where the timber is. This is common in retrofitted fans and it is a serious defect rather than a shortcut.
The duct also needs to be insulated where it passes through cold space, because warm moist air in a cold duct condenses inside the duct and runs back down it.
Checking an existing fan
- Run the fan and hold a light sheet of paper to the grille. If it does not hold, the fan is not moving useful air whatever its rating says.
- Trace the duct along its whole length and confirm it reaches an external terminal, not a roof space or a void.
- Count the tight bends and look for crushed or sagging flexible sections.
- Check the external terminal opens, and that the flap is not painted or gummed shut.
- Check that air can get into the room when the door is shut, and add a path if it cannot.
- Check the run-on: the fan should continue after the room is left, not stop with the light.
Extract is the ventilation that has to work. General background ventilation dilutes; extract removes. In a house being tightened up, the extract is what makes the tightening safe.