We had been experiencing very dry air at home. To the extent that we'd wake up with very dry noses and feel uncomfortable. I should have done this a long time ago, since I know the issue and how to deal with it. But life can derail my best intentions. I finally got down into the crawlspace last week with a bucket of mastic and a can of great stuff. What a great way to spend an afternoon! I got back into the lowest craggiest parts of the crawlspace where I'd never yet gone, and I'm sure I'll never forget.
Basically, I applied mastic to every seam in the air handler and plenum, and I applied it to every part of the duct system that had anything that looked like a seam, right up to the boots. The boot is the part where the duct meets the vents above. The application is not exactly pretty (see photo of my air handler), but who's looking at it? The whole job took me about three hours. Be sure to wear knee pads, a mask and coveralls or very old clothes. One of the initial challenges that slowed me down was simply finding mastic. The big stores around here don't seem to carry it in stock. I eventually asked an HVAC guy where my nearest supply store is and got it sorted out.
While I was down in the crawlspace I found some nice big holes where pipes of one sort or another go up into the house. These were all plugged up nicely with great stuff insulating spray foam. This will help reduce air leakage some more and keep pests out of the wall cavities.
The most easily accessible part of the floor area is insulated with fiberglass batt. I can only guess it was too much trouble for someone to bother with the rest of the area. Where the fiberglass came up against the duct boots it was badly darkened. This indicates that air was being sucked in and "filtered" by the fiberglass. And that just proves there were leaks where the ducts meet the boots.
The end result is, with a week of cold days to test the changes, we're experiencing much more comfortable air. And we're not waking up every morning with dry chapped noses. There's a great description of this very same situation by Arnie Katz at Advanced Energy, here.