The two panel boards directly above the heater have gone the colour of strong tea, and at the knots they are almost charcoal. Lower down, the front edge of the top bench has gone soft: press a thumb into it and the fibres give a little, then stay dented. Neither is a finish problem. The dark panel says the heater sits closer to the wood than the manufacturer intended, or that the cavity behind the panelling stopped moving air years ago. The soft bench edge says water has been sitting where it lands, never drying before the next bathing evening. Both are structural messages that most people read as “it’s just old”.
That is the first thing to understand about sauna work. A bathroom fails from water; a sauna fails from water and heat alternating on the same square metre — ninety degrees for two hours, then cold and damp for five days. It also explains a line that catches people out in quotations. Estimates here are headed in Finnish, and under Kylpyhuoneremontti Turku the sauna may sit inside the quoted scope or as a separate priced line beneath it. The difference between those two readings runs to thousands of euros, so establish which one you are holding before you compare it with anything.
The vapour barrier is the actual job
In a washroom, the layer that decides everything is the vedeneristys, the certified waterproofing membrane under the tiles. In a sauna, the equivalent layer is the vapour barrier behind the panelling, and it works in the opposite direction: it stops hot, saturated air from pushing into the insulation and the structure behind it, where it cools, condenses and quietly rots things for a decade.
It has to be aluminium-faced barrier foil, not ordinary polythene, because the aluminium reflects radiant heat back into the room and because polythene is not rated for these temperatures. Every seam has to be overlapped and taped with the tape sold for the purpose, and the barrier has to be carried around penetrations — the heater cabling, the light fitting, the ventilation ducts — and sealed there too. A barrier with one untaped seam behind a bench is not a barrier with a small fault. It is a funnel.
Then — and this is where cheap jobs get cheap — the panelling must not be screwed flat against the barrier. Battens create a ventilated cavity of roughly 15 to 20 mm behind the boards, open at the bottom and top so air can move through it. That cavity dries the back face of the panelling between sessions. Nail the boards straight onto the foil and every board stays wet behind — which is why panels that look fine from the bench are black on the reverse when you pull them off.
Wood, and why the wrong wood burns your shoulder
Aspen and alder are the standard choices for panels and benches, with thermally treated versions of both used where a darker tone is wanted. The reason is not aesthetic. These woods have low density and low thermal conductivity, so at 90 degrees they stay touchable. Ordinary spruce or pine has resin pockets and knots that weep and get genuinely hot; dense hardwoods store heat and will take the skin off a thigh. Bench slats want rounded edges, about 10 mm of spacing so water drains and air passes, and every fixing either hidden underneath or countersunk and plugged. One exposed screw head at bench height is a burn waiting for a bare back.
Heater, clearances and the electrician
Heater output is matched to room volume, not floor area — roughly one kilowatt per cubic metre for a properly panelled and insulated room, with additions if there is glass, tile or stone surface, because those absorb heat the panelling would have given back. Undersize it and you never get proper steam; oversize it and you cook the panelling nearest the stones — one plausible reading of that blackened board.
The clearances are not advisory. Every heater has minimum distances to the wall, to the bench above, to the ceiling and to the guard rail, and they are model-specific and printed in the installation instructions. Electric heaters over the domestic plug-in sizes are fixed wiring: the connection must be made by a qualified, registered electrician, and a documented installation is the only version of this that is legal or insurable. The requirements for heaters and their installation sit with the national safety authority, and tukes.fi is where to read them, rather than a forum thread. If the sauna is being rewired, deal with the lighting at the same time: luminaires must be rated for both the moisture and the temperature at the height they are mounted, and a fitting that is fine on a washroom ceiling may not be fine above a top bench.
The floor, and why the washroom comes with it
Sauna floors are wet rooms too. The floor needs a certified membrane and a fall of roughly 1:100, steeper near the drain, running to a lattiakaivo (floor drain) rather than to a low corner behind the heater. Where the sauna sits directly against the washroom, the membrane, the floor gradients and the mechanical ventilation are one continuous system across both rooms. This is why the two are almost always renovated together. Doing the washroom now and the sauna in three years means opening the same waterproofing twice, and paying twice for demolition, drying and the tiling edge cut back to make the join. The Finnish-language guide filed under saunaremontti goes into the bench and heater side in more depth than a general bathroom page will.
The Turku stock makes this concrete. In the 1960s–1980s blocks in Varissuo, Runosmäki or Pansio, the flat sauna is small, original, and shares a slab and a drain with the washroom; anything built before roughly 1999 frequently has no certified membrane at all. In a taloyhtiö (the housing company that owns the building), a muutostyöilmoitus (the alteration notice a shareholder files) goes to the isännöitsijä (the property manager) before anything is opened up, and the board may appoint a supervisor at your cost. A detached house on Hirvensalo has no such step, but owns its ventilation problem alone.
What actually makes a sauna last
Ventilation, and drying. No coating, no wood species and no heater brand will outperform air movement. The room needs a supply near the heater and an extract low on the opposite wall, and the mechanical extract has to keep running after bathing. Leave the door open, tip the benches up if they lift out, run the fan on boost for an hour, and throw the last water on the stones rather than the floor. A sauna that dries fully within a few hours of every use will outlive one scrubbed with expensive products and then shut tight while still steaming.
