I have a 1400sqft home on a crawl space in southern Ohio. The crawl was configured with 6mil vapor barrier with about 4" gravel on top. The crawl walls are insulated with 2" foam board and rim joist spray foamed. HVAC duct lines run underneath the house and are well insulated. The house was built with passive ventilation for design and has a sump pump underneath. I Have humidity issues in the crawl and need to address as there is growth forming on floor joists. I want to keep solutions simple as possible to begin with and work my way to more complex solutions if necessary. The two ideas I have are as follows..... 1. Seal off the outside vents and install a 250-350cfm bath fan near the middle of the crawl, vent the fan to one of the outside vents and wire the fan to a humidistat based switch... essentially removing any stale dampness from the crawl like you would in a bathroom. 2. Install a dual powered crawl vent fan at 220cfm leave all vents open and let that fan move air into and out of the crawl. I understand this may cause outside air to condense in the crawl. Alternatively I could use this same fan instead of the bath fan with all other vents sealed. My thought on the bath fan was being able to locate it more into the middle of the crawl instead of a far corner distant location. I'm hoping I can remedy with simple ventilation and not have to add additional vapor barrier protection. I've read about adding HVAC vent to the crawl, but I'm not a fan of that concept. Anybody with experience on this issue opinions are appreciated. I'm looking for a simple stable solution.

Where do you think the moisture is comming from? Is the sump to drain the gravel above the 6mil poly or is it configured to drain lines below the poly? If below, that could be the source of the moisture. Lived in a house like this in NY. Finished basement, sealed walls and a sump in a room in the corner with the water heater. Even with a de-humidifier running full time, had trouble keeping up with the moisture untill I sealed the sump lid to the floor with tape to keep the moisture in the sump untill the level rose enough to cycle the pump. Then the de-humidifier output dropped to next to nothing. IF the sump is the source, a ducted exhaust from the sump to the outside, and sealing the sump up as best as you can might force that moisture outside instead of allowing it to raise the humidity in the crawlspace. If you can eliminate the source of moisture, anything else you do will be more effective, including allowing the natural ventilation to perhaps deal with the situation. A de-humidifier might also help to get things under control along with modifying/controlling your ventilation as you described above. A remote temp/humidity sensor might help you gauge if anything you try is helping... Two sensors one outside might also help you get a handle on wether it is drier outside than the basement and wether outside air ventilation even has a chance of helping...

Good luck.

Why do you have vents in your insulated crawl space? That is likely where your moisture is coming from - moist air from outside comes in the vents and condenses on cool surfaces in the crawl space. Close and insulate all of the crawl space vents. You may also need to install a dehumidifier to get the moisture that is already in there out.

The problem with fans is that you don't always have a source of low dew point air - certainly not the outside air in the summer and sometimes not the inside air (eg, a mild but very humid day where you elect to leave the windows open). So the sure thing is to seal it up (blocking moisture and air) and use a dehumidifier. With an alarm (things will go bad quickly when the dehumidifier fails).

That being said, with enough outside air flow and depending on climate, an outdoor ventilated crawl space can work OK (think house on stilts - they don't get mold). Passive vents require no electricity and fans don't require much.

I had the rim joist all spray foamed and had closed off my vents thinking I was doing the right thing.... but they wernt totally sealed and I notice it still very humid down there. The sump pump is in a ground well surrounded by gravel... its not on top of the vapor barrier. I notice moisture around the foundation walls footer, not really puddles just dampness.... hydrostatic pressure from wet ground outside is my guess. I talked to my brother who lives in similarly swappy ground and he said he just leaves his vents open all year round and has no moisture issue... perhaps I need to try that, either way my goal is to get a temp and humidity data logger so I can gather some information about the climate underneath. It seems like a lot of ppl have success just simply adding a fan at one of the vents to move moist humid air out of the space. No matter its a complex issue and it seems like solutions vary greatly depending on lots of factors. Keep the thoughts coming. Thanks.