@T Burns I don't have an ATMOX system, but from what I've seen they have one inside and one outside sensor input on their controls. One outdoor should be sufficient, but for a large crawlspace I'd prefer more than one just because humidity CAN be localized if there is bulk water such as a flood or plumbing leak. My home had mold and rot only in certain areas--generally the east (downwind) side. The ATMOX optional louvers don't appear air-tight but passively flap down over the opening when the fan isn't running, like on a dryer vent. It's probably no worse than a typical sliding crawlspace vent. Plotting temp and RH over time is a great idea. From there you can obtain dew point and get an idea how often an ATMOX system would be able to run with just fans to keep the RH below 60%. With tight clearance between floor framing and the ground, the joists will track ground temperature by radiant heat exchange unless you have lots of airflow. You can get an idea of what RH will be at the joist surfaces by looking at those temperatures and figure out what max average dew point should be allowed. From there you can look at historical dew points for your area and get an idea of what percentage of the time fans can actually dry the crawlspace. As the ATMOX folks point out in their literature, it takes an extended problem to elevate wood moisture content enough to cause mold. You may be able to get by with a dehumidifier for half the year or less. But I would definitely not install one of these systems without a dehumidifier as backup, integrated into the control logic. If nothing else, it will come in handy if you get a flood or a fan goes bad and you need brute force drying. There might be some aspects of encapsulation that could benefit your situation, but I'm leery about arbitrarily introducing spray foam, foam board, or plastic-wrapped batts, like the Youtube encapsulation guys do, without really thinking through the moisture transport situation. You could easily end up trapping moisture or hiding rot or termite activity in an old house that wasn't built to modern codes. For example, my home has poly flashing between the rim & band joists and the brick, lacks a brick ledge, and was not built with pressure treated sills. If the brick really gets soaked, the framing gets damp. This is a once-a-year event at most, but it has a way to dry. The last thing I'd want to do is to trap moisture in the lumber. Also, my home's crawlspace vents also admit air to the gap between the brick and sheathing, and blocking those gaps with foam board will reduce ventilation behind the brick. Retrofitting an older home has to be taken case-by-case. I wouldn't trust most one-size-fits-all encapsulation contractors to consider that. I would probably never be totally satisfied with an ATMOX system, since the control logic isn't arbitrarily customizable, but it's an incremental improvement over full-time passive ventilation that we know doesn't work in summer. Automating the ventilation and combining occasional dehumidifier use may be all you need to banish the mold if there are no drainage issues. Thanks for the tip about the upcoming revision. I'll have to get in touch with them to see if they're adding fan diagnostics.

"...good amount of water congregating in the low corner of the crawl below my vapor barrier." It the low point is below the water table water can easily rise up. Adding some fill to the low point (gravel) under the vapor barrier, and then topping it with sand is about the only fix. The sand is needed to protect the vapor barrier if you ever need to work in that area. If the water subsides on its own without breaching the vapor barrier it should be fine. The only long term issue I can think of would be repeated movement of a vapor barrier. Plastic tends to stiffen with age and the movement could eventually cause cracking. A repair would be needed at that point.

Well, luck has it that I was in the crawl when we had a temperature hike, so there was some water action going on. Turns out, the house used to have a well, which was sealed off. When the house was connected to city water, they slid the new piping up the old piping. Looks like I need to do a little digging to figure out where the original piping terminates outside the house, since the other end delivers the water to my crawl. As they say, water travels the path of least resistance and it's all downhill to my crawlspace. I don't think I could have found that if it wasn't for that fluky moment I was in there. Vith, thanks for the great advice. Once I get this water situation under control, I will be conditioning the space and adding a dehumidifier.

Thank you for your reply. I see that you are also a builder, I hope you don’t mine me asking you some builder related questions. We live in the northern part of SC so the ground is mostly red clay. As I said in my original post, our house was completely destroyed by a fire. We had the house torn down, lot scrapped and cleared. The builder is saying because we chose to build on the same lot the soil make up has changed, meaning the clay may not be as compacted. He says the water is coming in from under the ground from the original footings. The original footings were torn out and new ones were poured. While I can understand his logical what I’m struggling with is, I’m sure we are not the first family to build on an existing lot. Have you experienced anything similar to this? I do wonder if maybe the lot wasn’t graded correctly or if having the tankless water heater and air conditioner outlet pipes on the same side isn’t adding to the problem. That side of the house especially in that flower bed stays very wet (soaking wet). He wants to make sure “there isn’t anything from his build” that is causing the problem, which I appreciate. But even if it is the ground make up, shouldn’t he be expected to fix the problem....even if it requires a sump pump (I believe that will be the case), as the home is just over 6 months old? Also, what’s the potential for this standing water to do other damage...to footings, ground fall (I’m thinking moisture)? I can’t see to the other side of the crawl space, so I don’t know if the problem is on the other side as well. Thanks so much.