Hello. I was curious to see what concerns (if any) there might be with this. A permanent opening by way of an air duct grille is installed below and water heater and adjacent to a furnace and behind an unsealed bi-fold door. I can see directly into the crawlspace which has bare earth and is not sealed off by any means. In a previous post, a duck gained entry through an unscreened vent in masonry block. Anway, if a duck can get in then a snake can get it or a mouse, spider or a big roach. You get my drift. There is one opening in the top of the ceiling from the attic space. It is my understanding tht the opening in the attic should not be screened, but how about the one from the crawlspace?