Eli5: How come wood stays cool to the touch in high temperature environments (e.g. saunas) when all other common materials would be hot?
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Elianor’s answer is the right one, I just want to add WHY wood is a bad / slow heat conductor. Wood is mostly air, and air is a bad heat conductor, especially when it’s confined to a bunch of little pockets that can’t move or circulate to help move the heat. That’s how clothing, blankets, and even fiberglass and foam insulation work – it’s all about trapping tons of tiny air pockets, creating a material that passes heat very slowly since the energy has to hop from one pocket to the next to the next and all this transferring takes time.
Wood is a similar deal. Dry wood contains a ton of air confined in many little pockets, so it acts like a thermal insulator. That’s why it feels cool to the touch in the sauna – it’s at the same temp as the room, but it transfers that heat very slowly into your finger so it doesn’t feel as hot as metal at the same temp that quickly dumps heat energy into your finger. And that’s also why a wood toilet seat feels WARM to the touch. It’s just as cool as the room, but it transfers heat from your butt slower than plastic, so the wood feels warmer. Again it’s the rate of heat transfer (in or out!) that you feel, not the temp itself.
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