I put up a 10x7 shed last summer (https://www.onegarden.co.uk/garden-buildings/garden-sheds/wooden-sheds/shire-guernsey-shed-10x7). I've hardly been in there over the winter, but when I have I've noticed condensation on surfaces, and the windows misted up on cold days.
I've never known condensation in a wooden shed before, so I'm curious to know where it's coming from. I was worried it was coming up from the ground, but I was in there yesterday tidying up, and the floor was nice and dry, even under items that have been sitting on the floor. There is damp/mould along the bottom of the windows (presumably from the condensation running down), and in a few places where the walls meet the floor - mainly the corners and the joins between the wall panels. This suggests the rain is getting in behind the cover strips, running down the join, and soaking into the wood at the bottom. Strange though, as I ran frame sealant down these joins before nailing on the strips.
Unfortunately the roof has hardly any overhang, so it all runs down the walls, and there isn't enough to fit guttering.
The general advice for condensation/damp seems to be to install ventilation. There is a 1/3" gap along the top of the double doors, but otherwise the shed is very "air tight". I'm going to fit a 6"x3" grill in the eaves opposite the door, but wonder if that will be enough or if I need to think about adding heating?
Is there anything I can do about the damp wood already in the shed? Assuming it dries out in the summer what can I paint on it to a) kill the mould and b) protect it from getting damp again? Presumably anything I use on the inside isn't going to soak in enough to protect it from the moisture that's soaking in from the outside.