Sauna wood in Rwanda: why not eucalyptus, and what to use

What a sauna actually does to timber

A hot room cycles from ambient to 90 °C and back, over and over, with humidity swinging sharply each time water hits the stones. Timber that moves under that cycle opens its joints, cups its benches and works its fixings loose — not dramatically, but steadily, over two or three years, until a cabin that was tight at handover rattles.

Resin is the second problem. Resinous species bleed at sauna temperatures, and resin on a bench is unpleasant, sticky and permanent. It also darkens unevenly, so a bench that bleeds looks neglected long before it is.

Knots are the third. A knot conducts heat differently from the wood around it and can reach a temperature that will genuinely burn skin. On a bench top a knot is not a character feature but a hazard, which is why clear grades cost what they cost.

Why eucalyptus fails all three

Rwandan eucalyptus is resinous, generally knotty, and moves considerably as it dries and re-wets. It is an excellent timber for construction, framing, poles, scaffolding and firewood, and the country is full of it for good reasons.

It is simply the wrong material to press your back against at 90 °C. A hot room lined in it produces all three failures within a few seasons — resin bleeding onto the benches, knots too hot to sit on, joints opening as the boards move — and the only real repair is to strip the lining and start again.

Most other locally available softwoods fail for the same three reasons. That is no slight on Rwandan timber; it is a very specific set of requirements that few species anywhere in the world meet.

What we use instead, and why it is imported

Thermo-treated aspen for linings and benches: heat-treated at around 200 °C, which drives out resins and stabilises the fibre. It stays comfortable to the touch at sauna temperatures, does not bleed, and holds its dimensions through the cycle. It is the default bench timber in serious saunas worldwide for exactly these reasons.

Thermo-treated pine where a darker, warmer tone is wanted — in a cabin with a lot of glass it is often the better-looking choice, because it reads as a room rather than as a box.

Both are imported, and that is the honest trade-off: the hot room is the part of the build that cannot be sourced in Rwanda. How we size the rest of the cabin around it is on our custom sauna page.

How the timber is worked before it travels

Thermo-treated boards leave the kiln at a low, stable moisture content, and our factory in Istanbul cuts, machines and assembles them under controlled conditions, as it has since 1987. End grain is sealed as it is cut, before any panel goes together; sealing after assembly reaches perhaps half of it, because the cut ends inside the joints — exactly where moisture collects — are out of reach.

Every cabin is then dry-assembled and checked in full before it is numbered and crated with its fixings, seals and a spares allowance, so the joints are proven before the cabin crosses an ocean. Benches are fixed from below, with no visible fasteners and no metal where skin lands. It sounds obvious. It is skipped constantly.

What comes from Rwanda

Volcanic basalt for plinths, plunge surrounds and carved basins. It is local, genuinely beautiful and thermally useful, and a carved basalt basin is the most site-specific thing on any of our projects. It also has real mass, so anything holding water on a suspended structure gets an engineer's opinion before we quote it.

Rwandan brick for surrounds, terraces and plinths — warm, textured, locally fired and entirely at home next to dark timber.

The civil works around the cabin — the plinth, the drain, the cable trench, the deck — are usually built by the client's own contractor to our drawing, from local materials, before the crates arrive. Our own crew then installs, tests and commissions the cabin itself.

The imported share, honestly

On a typical project the imported value sits in three items: the cabin with its hot-room timber, the heater and, where there is one, the chiller. The plinth, the surrounds and the site works are local.

Those three imported items are exactly where buying cheap costs you twice. A cabin lined in the wrong timber has to be stripped. An undersized heater has to be replaced. A chiller sized for a smaller tub than the one it was sold with runs continuously, never reaches setpoint and fails early.

So the honest way to save money on a Rwandan sauna is not to substitute the timber. It is to build a smaller cabin, use a brick plinth instead of basalt, leave out the glass wall, and keep the specification of those three items exactly where it should be.

Stones, which are their own small subject

Sauna stones are not decorative gravel. They must be dense, non-flaking and free of anything that cracks under thermal shock, and they need re-stacking or replacing every two to four years, because they pack down, choke the airflow and eventually burn out the elements.

We supply proper heater stones with every cabin and re-stack them at every service visit. Stones picked from a riverbed occasionally work; more often they crack and take an element with them.

It is a small item, and it is the one piece of maintenance that most affects how a cabin performs a few years in.

Insulation and the vapour barrier: the invisible half

Behind the timber is the part that really determines how a cabin behaves: mineral wool of the right density and thickness, a continuous foil vapour barrier on the warm side with every joint lapped and taped, a battened air gap, and only then the lining.

Each layer has its own failure. Insulation squeezed into a frame that is too tight loses much of its value. A vapour barrier taped with the wrong tape lets go within two years in this humidity. No battened gap means the lining sits directly against the foil and traps moisture behind boards that are supposed to dry between sessions.

None of it is visible at handover, which is exactly why it is where corners get cut — and why we build the whole shell ourselves rather than lining somebody else's.

Outdoor cabins and Kigali's rain

An outdoor cabin here faces two wet seasons a year and afternoon downpours that arrive faster than anyone can cover anything. That changes the outside of the building without changing the inside.

Deep eaves rather than a flush roof edge. A drained, vented cavity behind the cladding so water that gets in has a way out. A ventilated void beneath the cabin rather than a floor sitting flat on a terrace. And a plinth that lifts the timber clear of splash, which a brick or basalt plinth does for free. The details are on our garden sauna page.

Charred cladding — shou sugi ban — performs genuinely well here and is more than a look: the charred layer is dimensionally stable and sheds water. It costs more, and we recommend it on exposed ridge sites where a cabin takes weather from every direction.

The timber rule for Rwanda, in brief

Eucalyptus for the frame of the house, never for the hot room. Thermo-treated aspen or pine inside, end grain sealed before assembly, every cabin dry-assembled and checked before it ships. Basalt, brick and site works local. Proper stones, re-stacked on schedule.

Get those right and a cabin here lasts many years looking the way it did at handover. Substitute the timber to save a few per cent and you will find out why nobody else does.