Maybe pee is seeping up through the wood?
Is that side against anything so it can’t breathe properly?
At 13 degrees outside, what is it in your shed?
For mine, 13 degrees outside At night, means my shed is going to be about 18
In the shed, at this time of year and at that outside temperature, I don’t do anything - no hutch covers (not had any for about four weeks), no snugglesafes (not been used since the end of March). They aren’t exposed to any of the elements and they’ve got hay in hides if they do want to snuggle up (I find them asleep out in the middle of the shed most mornings so they aren’t cold!).
If they were actually outside, I’d be doing it differently but the way my shed is, they don’t need it