I was wondering what I should use to seal around the heat ducting where it joins the wall register. I rent an apartment and am trying to cut down on heat loss this winter and also seal the apt as much as possible to block out secondhand smoke from the downstairs apartment. The converted house is quite old and the heating system isn't in great shape so where the metal duct comes out into the wall register there are pretty big gaps between the wall and the metal duct and the registers no longer will sit flush into wall as a result.