We have a small wooden den/ summer house in the back garden.
There is dampness on the ceiling, which is turning black. We're pretty sure it's not coming from the outside because we painted the shingles with a thick, gloopy tar stuff.
The wood is pretty soft where the dampness is.
The den window is always ajar, even throughout winter, and in the summer it door is left open most of the time. There are 3 units of condensation crystals inside too, and they do take a lot of water out of the atmosphere.
Any ideas?