Are air vents in a house intended to let some air through when they're closed? Would it be bad to completely seal it?
Sealing vents off can damage the duct work and shorten blower life. I have had to open walls where the owner closed a couple of bedroom vents this caused the main duct to split inside a wall I would not totally seal it off it may not sound like much but the main duct has a large area and the flex lines can break loose or metal trunk lines can split I have seen both.