If you do unscrew one of the floor vent covers and set it aside.
Floor vent seal.
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To properly seal around air vents apply caulk or expanding foam between the ducting and the sheetrock.
This is one of those things that can go unnoticed but have.
If you live in a house with a crawl space there is a good chance the floor vents are not sealed.
Is there a gap between the metal duct and the floor.
While it certainly is possible to abandon the under slab ducts and to put ducts in soffits or an attic this.
This will further improve efficiency and prevent energy losses.
Look into the vent right where the ductwork meets the floor.
Next apply weather stripping around the underside of the vent cover.
Check to make sure you have a crawl space and that you have air vents coming up through the floor.
The goal is to prevent conditioned air from escaping or unconditioned air from entering the thermal envelope.
The good news is that an alternative method of cleaning and sealing air ducts exists.