How do you manage a crawl space with a dirt floor?

I too have a home with a dirt floor crawlspace, and my advice is to absolutely put down 6 mil poly sheeting. The vapour barrier makes a huge difference inside the space as far as moisture goes, and you’ll find that you will lose that “musty” smell within a week or two.

A few tips: First, make sure the poly covers the floor completely, and both overlap and tape any seams. Second, you’ll need to protect the plastic from damage. The least expensive way to accomplish this is to protect the plastic with the dirt itself. Pull back some of the soil,  lay the plastic, and then bury the sheeting 4” or so under the surface. Another less expensive option is to lay the plastic and then cover it with 3-4” of pea-stone (a small, round aggregate about the size of, well, a pea).

You can also pick the more expensive options, which involve either “framing” the floor with 2×4 lumber laid flat, and then adding a plywood “floor” on top, or bringing in a concrete truck and pouring the floor.

As for the vents: You will get advice either way, and it really depends on the situation. If the space is musty, then cross ventilation will help dry it out, but in my own house I sealed them tight. Your crawlspace, being insulated to R20, can now be considered a warm space in the home. In fact, since you mention your duct work runs through the space, I would consider adding a register or two in the ceiling and heating the space. You’ll find the floors above warmer in winter, and a dry/warm crawl space is excellent for storage.