Saunas built for the climate they stand in

Volume first, then the two corrections

Heater capacity starts at about 0.85 kW per cubic metre of genuinely insulated cabin. A 2.0 × 2.4 m cabin at 2.1 m high is roughly 10 m³, so a 9 kW heater is the starting point — the starting point, not the answer.

The first correction is altitude. Thinner air carries less heat per kilowatt, so a heater chosen from a sea-level chart stalls below temperature. At Nairobi's 1,795 m the correction is real, and on the Ngorongoro crater rim at roughly 2,300 m it is large. At sea level in Lagos, Dar es Salaam or Mahé there is none, and we say so rather than padding the quote.

The second is glass. Every square metre of glazing loses heat far faster than an insulated wall, so we add roughly 0.42 kW per square metre before rounding up to a heater we can actually buy. That is why the glass goes in the drawing before the heater is chosen, never after.

Insulation ahead of kilowatts

Given a choice between a bigger heater and a better-insulated cabin, we take the insulation every time. A well-insulated room reaches temperature faster, holds it on a third of nameplate and costs less every evening for the next decade — and on a property running off batteries that difference decides whether the sauna gets used at all.

Behind the timber: mineral wool at the right density and thickness, a continuous foil vapour barrier taped at every lap, a battened air gap, then the lining. Each of those has a failure mode we have been called to repair, and none of them are visible at handover, which is exactly why they get skipped elsewhere.

Timber, and what it does at 90 °C

A hot room cycles from ambient to 90 °C and back with humidity swinging each time water hits the stones. Timber that moves under that cycle opens joints, cups benches and works fixings loose. Resinous species bleed. Knots conduct heat differently and can get hot enough to burn skin.

So linings and benches are thermo-treated aspen or pine — heat-treated at around 200 °C, which drives out the resins and stabilises the fibre. End grain is sealed in the workshop as it is cut, before any panel goes together, because sealing after assembly reaches perhaps half of it and never the cut ends inside joints where water actually collects.

Locally available species go into frames, plinths and surrounds, where they belong. Eucalyptus is superb for construction and wrong for a bench, and we will decline to line a hot room in it however available it is.

Outside, where the climate lives

On any African coast: A4 stainless for every fixing including the ones nobody sees, drained and vented cavities rather than sealed ones, a ventilated void under the cabin rather than a floor flat on a terrace, deep eaves and a plinth that lifts timber clear of splash.

Inland and at altitude: none of that premium, and insulated floors instead, because highland ground is genuinely cold and a cabin standing on it loses more through the floor than through any wall.

In the harmattan and the dry season: cleanable hardware rather than sealed-for-life, because out here everything gets washed down eventually.

In short

• Heater from real insulated volume, corrected for site elevation, glass counted in
• Insulation, vapour barrier and battened gap specified before the heater is chosen
• Thermo-treated linings, end grain sealed in the workshop before assembly
• Marine, dust or highland package chosen by site, and shown as its own line