What The Standards Require
Scottish building standards deal with ventilation under Section 3, and for a bathroom or shower room the requirement is mechanical extract ventilation capable of at least 15 litres per second, or 54 cubic metres per hour.
Where there is no openable window, the fan must also run on an overrun timer after the light goes off. Even where there is a window, an overrun is strongly advisable, because nobody stands in a cold bathroom holding the door shut waiting for the steam to clear.
The number that matters is at the grille, not on the box. A fan rated 15 l/s in free air will deliver considerably less through four metres of flexible ducting with two bends in it.
Why Fans Underperform
- Flexible ducting. Corrugated flexible duct has far more resistance than rigid. Long runs of it kill performance.
- Too many bends. Each bend costs airflow. A straight run to the nearest outside wall beats a scenic route to a soffit.
- Squashed duct. Duct crushed between joists or under insulation in a loft is extremely common.
- Duct terminating in the loft. Still happens, and it dumps every shower into the roof space.
- No make-up air. A fan cannot extract air it cannot replace. A 10mm gap under the door is needed.
- Blocked external grille. Painted over, or the flap seized shut.
Choosing The Right Fan
Axial fans are fine for short duct runs, up to about 1.5 metres straight to an outside wall.
Centrifugal or mixed-flow fans are what you need for longer runs or where the duct goes up through a loft. They maintain pressure where an axial fan simply stalls.
Humidity sensing models run automatically when moisture rises, which handles the bath nobody switched the light on for.
Continuous running low-rate fans, boosting when needed, are increasingly the better answer in a well-sealed modern home.
Older Scottish Properties
Tenement flats and pre-war houses were never designed with mechanical ventilation, and retrofitting it is where the complications come.
In a tenement with no external wall in the bathroom, the duct may need to run through a hall ceiling to an outside wall or a vent stack. That is a longer run, so it needs a centrifugal fan, and it needs planning.
Solid walls also mean core drilling rather than a simple hole through cavity blockwork. Worth knowing before someone quotes you an hour's labour.
Signs Your Ventilation Is Not Coping
- The mirror stays fogged more than a few minutes after a shower.
- Black mould in the ceiling corners or along the silicone.
- Paint peeling or wallpaper lifting near the ceiling.
- A musty smell that returns however often you clean.
- Condensation on the outside of the cistern or on the walls.
A simple test: hold a sheet of toilet paper against the fan grille with the fan running. If it does not hold itself there, the fan is not moving anything like enough air.
Bathroom with a damp problem?
We will measure the room, check the existing extract rate and tell you whether it is a ventilation issue or something behind the tiles.
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