Sealing Ducts in damp crawl space

Why bother? When there's air flow in the ductwork, it's under positive pressure, hence no air infiltration. When air flow stops, the pressure iside and out are the same, so still no infiltaration. HTH

Joe

If you're talking about the seams where pieces of duct connect to one another, I'd use exterior silicone sealer, in whatever color is easiest to see while you're working. Buy a product that doesn't harden completely.

Frankly I don't think you need to. There would be little air exchange between the ducts and the area around them and almost all of that will be from inside to outside, unless they are return ducts.

If you want to you have only two choices. The best is duct cement or calk. It is made just for that. You might find it a little difficult to find, but the pro places will have it. Next best is the metal tape, that looks a little like duct tape, but it is shinny and has a peel off layer over the adhesive. Don't even bother wasting your time with "duct" tape. It is great stuff for some things, but not this one.

No matter what you may use, be sure to clean and dry the surfaces first.

George;

You can seal ducts using approved sealant that meets the 181B standards. If you use a mastic that is not generally intended for ducts, you may be compromising them depending on the type of duct your working with. Also, some mastics really stink.

You can find information for proper residential duct assembly by following this link:

If your ducts are solid sheet metal, than the seams should be sealed using the 181B sealant, Screws and tape. You can find reputable HVAC contractors who can test your duct to see what percentage is leaking....

Do you want to heat the crawl space or the living environment?

Leaks reduce the efficiency of the ducting system.

I was thinking there might be some kind of venturi effect where air flow will tend to pull in air from around it.