Hi char2020
It could be that the original box became mouldy through lack of ventilation. If the wood is completely waterproofed, it can’t dry out properly. If there are animals inside a box, there will be a certain amount of moisture from their breathing, apart from any moisture they may take in – if they are, say, damp when they go in.
Well made hog boxes will have ventilation holes in them as well as the entrance tunnel. Once a nest is built inside, the tunnel is likely to be partially blocked as well. But if you think that in the wild hogs would build their nest in i.e. the bottom of a hedge They have plenty of ventilation! But they are able to make their nests waterproof and well insulated. Any damp would be able to escape. Ideally, hog houses for nesting would be built from as natural materials as possible. Which is why untreated wood is good.
Personally, I would only put felt on the roof, but make sure the roof overhangs a bit. (that is partly because a hog would probably not be able to make the nest as high as it would ‘in the wild’ and rely to some extent on waterproofing from above).
I would leave that box as it is now – especially bearing in mind it’s already occupied, but if you make any others, maybe provide more ventilation. A few small holes just below the overhanging roof on the opposite wall to the entrance might help.
