Damp corner on new paint job

[WorrisomeHeart]

Help! DH painted our box room yesterday and this morning the corner which has the two external walls meeting is damp down the seam. I can't swear that it was dry last night but it's very noticeable this morning.
Is this likely to be a case of just being a very cold day for painting yesterday or should we be looking for further causes of damp? There was a small amount of mould on the wall there before painting which is making me worry about the external render or guttering. My main reason for concern, besides not wanting a damp house, is that ds2 is due to arrive in a couple of months and it'll be his room. What should we be looking for?

emulsion paint is mostly made of water, so you will get condensation on the windows and other cold parts of the room. If you open the windows today it would dry out (fresh air is better than heat)

however damp on an external corner suggests that your gutters, or possibly the tiles, are letting water out. Go out on a rainy day (borrow binoculars if you can). There will be either a stop-end or a corner in the guttering, and the joint may have failed or drooped, or the gutter or downpipe might be blocked with leaves.

Brilliant, thanks PigletJohn, I'll go out now to have a look.

You mentioned a small amount of mould being there before. And with this being where the external walls meet, I assume it's the corner furthest from the door. If there's no actual wetness towards the top of the room, another possibility is that the mould was just due to condensation, which was in turn due to poor ventilation.

Like John says, open a window. If it dries out fully you may find there's no external leak, but you'll have to be a bit careful about ventilation in future.

I'm not an expert, but exactly the same thing happened recently in our soon not to be spare bedroom.

Thanks Cheese, I've had the window open all day and it's dried nicely. Have also booked to have the gutters cleaned next week and will get quotes to replace the damaged fascias that were picked up in our survey (new purchase). Hopefully that will do the trick!