My darkroom sounds a little like yours. It is in an expanded garage with running water. In daytime there are a few scattered leaks from windows and doors. When I enlarge in the daytime I hang some light blocking curtains which has worked well. If I enlarge at night I don't need the curtains as the slight leaks haven't been detrimental. When I load film in the small tank for developing I just make sure I do it at night and have never had a problem. Don't know if this helps just my experience. But the advice let your eyes be the judge is the best advice.