Hello, simplistically you need the under floor vents to be open to allow an air flow through the underfloor region to drive off any damp. Your opening of the vents is a good move. I've just refurbished my son's flat which had the vents blocked for many years and the rising damp problems were horrendous. One years work on total replacement of all flooring timber and plaster on bottom metre of all walls. Too long a story but if you want more details just ask by reply on here. Cheers, Andy