Wet room ventilation and damp control in Scotland
A wet room in Glasgow dries more slowly than the same room almost anywhere else in the UK. Extraction is a functional part of the build, not an accessory.
What ventilation does a wet room need?
A wet room needs mechanical extraction removing at least 15 litres per second, running on an overrun timer or humidistat so it continues after the shower stops. In an internal bathroom with no openable window, mechanical extraction is the only means of removing moisture.
Why Scotland's climate changes the specification
Glasgow sits at high relative humidity for most of the year. Air that is already carrying moisture removes less of it from a wet room, so surfaces stay damp for longer between uses.
Persistent surface dampness attacks the soft parts of a bathroom first: grout, silicone junctions and decoration. Those are also the parts protecting the tanking membrane's edges, so ventilation is indirectly protecting the waterproofing.
Extraction rate and control
15 litres per second is the working minimum for a domestic bathroom. A wet room puts more moisture into the air than an enclosed shower because there is no cubicle to contain the steam, so we specify above the minimum where the room allows it.
Control matters as much as the rate:
- Overrun timer: the fan continues for 15 to 20 minutes after the light goes off.
- Humidistat: the fan responds to actual humidity rather than the light switch, which suits households where showers are taken during daylight.
- Adequate make-up air: a 10mm undercut on the door, otherwise the fan is pulling against a sealed room and moving far less air than its rating suggests.
Heating as part of drying
Warm surfaces dry faster. Electric underfloor heating under the tile, on a timer that runs after typical shower times, makes a measurable difference to how quickly a wet room floor dries. A heated towel rail sized for the room does the same for the air.
Signs the ventilation is undersized
The room tells you before any damage appears:
- The mirror stays fogged for more than a few minutes after the shower.
- Silicone develops black spotting within the first year.
- Grout darkens permanently rather than drying back to its original shade.
- Paint or plaster near the ceiling begins to blister.