I have two crawlspaces, separate from one another, each with access from the basement and also a grate on the exterior, allowing airflow. If I take steps to encapsulate them, should I block off the basement access and put a separate dehumidifier in each crawlspace, or should I leave the openings to the basement and install a dehumidifier in the basement itself? Or maybe put fans to further circulate air between the crawlspaces and basement? One of my concerns is that,if I encapsulate both crawlspaces, I might not have any air circulation in the basement. I should also mention the climate, I'm in northwest Ohio.
Your climate in NW Ohio is not a lot different than mine. Lots of hot, humid days in the summer, I bet.
Encapsulating the crawl spaces is the way to go. Along with that, you want to block off the external vents. I did a partial encapsulation (6 mil plastic on the dirt floor), but I open the vents on days (like today) when the dewpoint is in the low 50's or below. Leaving them open on hot, muggy days is just going to pull hot, humid air into your crawl space, which is what you don't want.
Moisture/humidity will migrate from a higher humidity area to a lower one. So I would put the dehumidifiers in whatever area you think will be the source of your moisture when everything is done.
I also have a dehumidifier in my crawlspace, one with a WiFi connection so I can control it and see what the humidity level is on my smart phone, without having to go down into the crawl space. That's working out well.
I I have had a home with a partial basement and the basement was heated and cooled so the crawl space was isolated and not dehumidified.
most homes not only in Ohio but in the US do not dehumidify air spaces below the home that is not conditioned.
I do know of a few that are and think this sounds like a big expense with no payback. If the space is encapsulated that will reduce the ground based humidity.
the basement I had in Ohio had a real humidity issue until I epoxy sealed the floor after that a small unit worked fine and we could even have carpet on the floor without mold. Prior to that we could not have cardboard boxes as they developed mold within a month.
If the space is not conditioned I would seal the floor to reduce the ground based moisture and use a dehumidifier if I wanted dry storage but I would isolate the “crawl” space from this area and let normal air circulation take care of things or if I had the extra finances encapsulate it and still seal it off from the area that I wanted to use and keep dry.