Why ventilation is the whole game here

The physics of a room that will not dry

Drying happens when the air in a room can absorb more moisture than the surfaces are giving up. Warm air holds more water than cold air, but warm air that is already near saturation holds almost nothing more.

Mauritian air is warm and close to saturated for much of the year. Open a window in a wet bathroom in February and you are exchanging damp air for slightly different damp air. Nothing dries.

What a mechanical extract actually has to do

It has to move enough air, for long enough after use, and put it outside rather than into a roof void. Three things, all of them commonly got wrong.

Enough air means sized on room volume with a target of several complete air changes after a session — not a token bathroom fan. Long enough means a run-on of thirty to sixty minutes, not five. Outside means ducted to atmosphere, because dumping humid air into a ceiling void moves the problem into the structure.

Why we specify humidistats rather than timers

A timer runs for a fixed period regardless of conditions. On a dry day that is wasteful; on a wet one it is not enough.

A humidistat runs the fan until the room is actually dry and then stops. In a climate where the outside air varies as much as it does here, that is the difference between a room that recovers and a room that is slightly damp all the time and eventually smells.

It costs a little more than a timer and it is the single best-value component in a Mauritian wet room.

Vapour barriers, and the gap that ruins them

A vapour barrier belongs on the warm side of the insulation, continuous, with every penetration — cable, pipe, light, vent — sealed properly.

A barrier with gaps is worse than no barrier. Moisture gets in through the gap, cannot get out through the barrier, and condenses inside the wall where nobody will see it until the cladding fails.

This is the single most common construction error we find when we open up somebody else's work on this island, and it is invisible until it is expensive.

Cabins need a void underneath

A sauna sitting flat on a timber deck traps moisture between the two surfaces and neither dries. We set cabins on feet with a ventilated void underneath, so air moves and water that gets in can leave.

The same applies to plant. A chiller in a sealed cupboard with no through-ventilation runs hot and corrodes; a sealed and filtered enclosure with designed airflow is a different thing entirely from a box.

Materials that forgive and materials that do not

Oiled teak, thermo-treated timber and charred timber all handle continuous humidity. Untreated softwood does not, and on this island it fails faster than most people expect.

Basalt, coral stone, microcement and glass are all inert. Metal is the vulnerable part, which is why everything is A4 stainless and why a corrosion inspection is part of every service visit.

The general rule we work to: choose materials that do not care about water, and then design the air so that the ones that do care never stay wet.

For properties that stand empty

A villa closed for six months is the hardest case. Nobody opens a window, nobody runs a fan, and the building slowly equalises with the outside air.

So we specify the humidistat extract to run unattended, we leave a written shutdown routine that includes draining and isolating what needs it, and we ask that a caretaker opens the building periodically. Where owners want it, a cheap remote humidity and leak sensor reporting to a phone catches the rest.

It is not an exotic requirement. It is simply designing for how the building is actually used rather than for how it looks on the day of handover.

Sizing an extract properly

The calculation is straightforward and almost never done. Take the room volume, decide how many air changes you want after a session — we work to a target that clears the room several times over the run-on period — and size the fan for that against the resistance of the duct.

Duct resistance is where specifications quietly fail. A fan rated for free air, connected to six metres of flexible ducting with three bends, delivers a fraction of its nameplate. We use rigid duct where we can, keep the runs short, and size the fan against the actual system rather than the box it came in.

And the termination matters: outside, with a cowl that keeps rain and wind out without strangling the flow.

The smell, and what it is telling you

The complaint always arrives as a smell, and the smell is always late. By the time a room smells, moisture has been sitting somewhere for months.

The three usual causes, in order: a trap that has dried out, so the drain is venting; an extract with no run-on, so the room never dried; and water standing where the floor has no fall. The first is a cupful of water, the second is a control change, and the third is lifting tiles.

Diagnosing which one it is takes twenty minutes. Guessing can cost a floor.

Cavities, voids and giving water a way out

The principle behind everything in this article is that you cannot keep moisture out of a building on this island. You can only give whatever gets in a route back out, and then make sure that route stays open.

In practice: drained and vented cavities behind cladding rather than sealed ones; a ventilated void under every outdoor cabin rather than a floor sitting flat on a terrace; weep paths at the bottom of any cavity that are large enough not to block with dust and small enough to keep insects out; and an insulation build-up that is never in direct contact with a cold impermeable surface.

None of it is visible at handover and all of it is why one cabin looks right after ten years and its neighbour does not.

The ceiling void, which nobody looks at

Extract ducting that terminates in a roof or ceiling void instead of outside is depressingly common, and it is the worst possible outcome: you have moved all the moisture from a room you can see into a structure you cannot.

On this island that void will never dry either. We check the termination on every survey of somebody else's work, and when we find one we say so immediately, because the damage compounds silently.

If you take one thing from this article, go and find where your bathroom extract actually goes. It takes ten minutes and a substantial minority of readers will not like the answer.

The same applies to the sauna cabin itself. A hot room needs an inlet low and an outlet high, sized and positioned so air actually crosses the room rather than short-circuiting between two adjacent grilles. Get that wrong and the cabin is stuffy at the top and cold at the feet, and people blame the heater. We set the vents at commissioning with the room hot, not from a drawing.