I need to fix humidity in my crawlspace. I'm aware of the expensive option, i.e. conditioned and sealed crawlspace, but I hope that's a last resort. I'm wondering if I can get away with the dehumidifier that we already have if we seal the crawlspace vents up and the only remaining source of moisture would be the soil (which does have plastic tarp on it, but hardly airtight).
Sealing up the crawlspace vents are the last thing you want to do (IMHO).
If anything you want to improve the "natural" ventilation to the crawl space, doing so will (greatly) aid the evaporation of moisture and prevent mold taking hold. A sealed (stagnant air), moist space, is a pretty good environment for mold to take hold and grow (spread) in.
If you really want to stop (or at least greatly reduce the) moisture entering the crawl space from the soil underneath, you need to put down some kind of physical barrier eg
Having read up a little about "Conditioned Crawl Space", I see that it be should designed and constructed to be dry, almost like a "traditional" useable basement:
The above quotes come from, Conditioned Crawl Space Construction, Performance and Codes (direct link to PDF).
I stand corrected, if the crawl-space has been designed and built as a "Conditioned Crawl Space" in the first place, or has been retrofitted to act as one (though I would imagine the "retrofit" option comes in at a reasonable cost, therefore makes me wonder if it's a worthwhile project to undertake), then it would be dry enough and ventilated properly to prevent the space from developing problems...
Example related to above deleted text: In the UK during the 60's, 70's & early 80's "everyone" was sealing up their homes, covering over brick vents, blocking roof vents with insulation, etc
Then in the mid to later 80's they finally discovered, realised that such practices were bad for the building itself and its inhabitants.
Nowadays in the UK under Building Regulations any kind of building ventilation system shouldn't be blocked, covered over...
In fact, the Building Regulations have increased the amount of "natural" ventilation a building needs, along with greater thermal insulation requirements ie
It's all about striking a good balancing act between ventilation and insulation that is vital to a healthy building and providing a healthy environment for the inhabitants within...
Sorry, I don't believe anyone can tell you that, without first conducting some tests on the soil under your crawl space or at least carrying out a thorough evaluation of that soil and the surrounding soil/environment eg
I honestly believe there are currently too many variables (unknowns) to give you a realistic answer to that particular question.
One thing that you can get to help with the moisture in your crawlspace are some vent fans. They usually run anywhere between $80 to $120 depending on what you want. These will help greatly reduce moisture in your crawlspace. These will help pull the moisture out of the soil as well. If you do decide to go this route, you can remove the plastic from your crawlspace and let the fans run for a few days to help pull the moisture out of the soil. Then be sure to put it back.