This sounds like a pretty unusual installation. In 20+ years of residential remodeling work in the mid-Atlantic area, I've never seen one like this.
In any event, there is a product / process which might solve this dilemma for you. It's a proprietary product called AeroSeal and it can only be done by factory trained companies with the right equipment.
- The best way to describe it is aerosolized forced-air rubber cement. Basically the same stuff that holds all of those fake credit cards to the paper in the junk mail you get.
- The installers cut a hole into the supply or return trunk at your furnace location. Then they meticulously seal every supply and return vent throughout your home with foam plugs.
- Once that's done, they hook up a "blaster" attachment which begins to blow the sealing material through the entire duct system.
Test the leakage rate of your system first
The better installers will test the leakage rate of your system first to establish a baseline (often times as high as 20-30% or more in typical builder grade homes).
- Then once they're done with the sealing, they will re-test to confirm a successful install.
- We've seen these numbers drop down as low as 4 or 5 percent. That's a drastic improvement in efficiency. And this will improve your entire duct system, not just the problematic slab portions.
In your particular case, this should also serve to seal out the moisture problems you're having. That is assuming your moisture is mostly vapor driven and not large amounts of bulk water. If the hydrostatic pressure of the sub-slab moisture is too great, it might still breach this type of solution. But in that instance, you clearly have larger water management issues which likely go beyond the duct work and will probably involve some concrete removal.
Consider duct cleaning before sealing
The other thing you may want to think about is the age of your duct system and it's past usage patterns.
- Specifically, if the system is more than 10 or 15 years old, and/or if you (or any prior occupants) have pets, then you may want to get the ducts cleaned first so that you aren't also sealing all of that dust, hair, etc. in place.
- If you decide to clean them, be certain to only use a provider who is NADCA certified (North American Duct Cleaning Association )
- And whatever you do, avoid the "$79 whole house" specials you see advertised on flyers. Those guys have no training, use inferior equipment, no HEPA filtration, and often make more of a mess than they actually clean.
There are several solid providers in your area that we've used on our projects, but I don't think I'm allowed to mention specific company names. However, both of the web sites linked above have tools where you can search for certified providers close to you. If they're on that list, then you should be fine. But as always, check references, insurance, etc. before you sign a contract.
It won't be cheap
The process won't be cheap (probably several thousand dollars depending on the size of your home's system), but it will be money very well spent.
Aside from the efficiency gains you'll get from an air delivery standpoint, you are also addressing a pretty serious indoor air quality issue.
- A forced air system is designed to remove moisture from your home.
- When you have water remaining in that system, then you run the risk of everything from airborne pathogens (which survive because of the water) to excessive vapor driven moisture getting into your walls during the winter months (hot air seeking cold).
When that overly moist air gets into your walls, it can condense on the insulation or exterior sheathing creating a whole host of problems you don't want to think about.
For more information:
Read Rick Goyette's Q&A "Should I replace metal duct in a 1950s home with the current flex duct? Or is cleaning better?"