With winter comes condensation. Sometimes, it's obvious, like when it occurs on a window. Other times, you have to look for it, as when water vapor condenses on roof sheathing or HVAC ducts in the attic. But the potentially most destructive condensation occurs out of sight on the back side of wall sheathing.
Building science blogger Allison Bailes explains the simple physics behind what he calls "accidental dehumidification" and offers three solutions. Two of them depend on careful detailing of the building envelope.