Hi all. I’ve just laid 18mm Elliotis Plywood over the top of existing solid floor boards which are suspended in my kitchen diner. There are also a number of air bricks on the external wall for ventilation. Question is, should I have primed the underside of the plywood before screwing down? To protect from rising moisture. My tiler or builder don’t seem concerned but reading various posts, am I better priming now to be safe?