A big Shout-out to energy rater la
Energy rater la has repeatedly suggested that heating and cooling dollars are wasted because of gaps and leakage in supply outlets. She has posted this photo to demonstrate how to fix one aspect of the problem using Hardcast #1402 mastic tape.
My single story house has a gas furnace and an unconditioned crawlspace. Out of curiosity, I decided to lift a few supply registers to see the ductwork. I was shocked to find that every outlet had huge gaps between the sheet metal and the edges (some up to 1/2 inch), through which flowed a very steady stream of cold air up from the crawlspace. I got a roll of the Hardcast tape (about $27 from Ferguson) and have followed energy's advice.
Energy, thank you for an awesome and easily followed suggestion.