Sealing Ducts in damp crawl space

I have all of my ducts and furnace in a "damp" crawl space. To prevent moisture from coming into the house via the HVAC system I want to seal my ducts as much as possible and would like some advice on the best way to do this. I did have someone seal all of my return ducts with mastic and was wondering if it made sense to do the same with the supply ducts. Is there a better way, etc?
Any advice is appreciated.
George
PS.. BTW.. I did put down 6 mil plastic on the floor of the crawl space to help bring down the moisture it the area.. I also plan on putting a dehumidifier down there with the goal of drying the area somewhat.
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<< I did have someone seal all of my return ducts with mastic and was wondering if it made sense to do the same with the supply ducts >>
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
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snipped-for-privacy@aol.comtosspam (Joe Bobst) wrote:

Do you want to heat the crawl space or the living environment?
Leaks reduce the efficiency of the ducting system.
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I was thinking there might be some kind of venturi effect where air flow will tend to pull in air from around it.

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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.

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George Sambataro wrote:

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.
--
Joseph E. Meehan

26 + 6 = 1 It's Irish Math
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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:
www.flexibleduct.org
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....
http://www.comfortinstitute.org
http://www.comfortinstitute.org/consumer/report3.htm
--
Zyp
"George Sambataro" <george snipped-for-privacy@pcwp.com> wrote in message
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