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What is the capillary effect of wood in the sauna?
Wood consists of many small cells and vessels (similar to straws). Capillary action is the natural phenomenon where liquids through these narrow spaces are pulled up or sideways by the wood.
In a sauna, this happens in two ways:
Water: If you spill water or sweat on the wood, the “straws” suck the moisture deep into the fibers.
Vapor: The wood absorbs the high humidity (for example after a infusion) and distributes this across the shelf via the capillaries.
Without capillary action, the wood in a sauna would feel piping hot (like metal or plastic). Because wood absorbs moisture and releases it slowly, the surface temperature of the benches remains comfortable for your skin. The wood acts as a species thermal protective layer and also ensures that the wood can set in hot and cold weather.
Controlling capillary action
It is not possible to completely prevent and stop this, but you can control it. If you block the capillary effect of wood 100% (for example by painting the inside of the sauna), the wood loses its unique properties for the sauna. It would get very hot on your skin and the moisture that comes into it can never come out, causing the wood to rot from within.
What you can do is treat the outside of the sauna properly. Like we explain here.