Will leaving a gl of water by baby's crib at night actually impact humidity?

currently baby sleeps in a room with us. She is a little stuffed up and we are often told to leave a gl of water by her crib to increase the humidity. The room is circa 25 metres cubed, temperature about 21 degrees Celsius, minimal air flow, 1 atmosphere pressure. I don't know what current humidity is but we live in a mid terrace in Southern England if that offers any clues. Personally, I can't believe it's going to make a difference. Please keep answers scientific (ie. Not "it worked for my baby" or "midwife told me the same". Thanks in advance!