Its actually a function of the wood and the moisture in the air, not the heat. The heat only excites that water vapor.
From a chemistry standpoint everything moves along a gradient, from high to low. So if wood is dried completely then built into a house or a door or whatever. When you end up in a situation of moist air, the vapor naturally want to move into a space where their is "less". Think of it as 2 rooms, 1 room is full of people, the other room is empty, people will naturally move into the other room until it equalizes. Now with wood there is a limit to equalization but thats why water vapor(moisture) wants
to move into the wood - its a lower concentration.
If you think of natural trees, they have tons of vasculature for transporting water and nutrients and shuttling all different things.
When they manufacture wood, they always need actually dry the wood so that it warps less and is more stiff compared to only partially dried wood. As simple as it sounds, the same thing happens to potato chips and corn chips, oatmeal. Anything that is dried when its manufactured "wants" to reabsorb moisture. This is one of the reasons pressure treated wood is preferred over non pressure treated - it lasts considerably longer and wears much better. These are the reasons we stain wood decks, paint wood and wood shingles, it provides a barrier to slow that moisture absorption process.
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