Need help with math problem.

[Poindexter]

I installed a dehumidifier in my crawlspace this weekend.

Crawlspace floor is 20x30 feet, and average 4 feet tall, so 2400 cubic feet.

Pressure, thankfully, was stable at 29.9 inches through out the sampling period.

When I started the dehumidifier, crawlspace temp was 64 degrees F, RH was 47 %.

18 hours later, 29.9 inches, 64 degrees, 40% RH.

I recovered 6.5 pints of condensate from the dehumidifier.

I thought would just convert to absolute humidity for start/ stop to get a feel for how much water I am sucking out the poured concrete walls of the crawlspace. It is starting to look like maybe working in specific humidity might make more sense....

Help, help, I are well and truly stuck.

 

[Poindexter]

Have a third data point. After the above two measurements, the dehumidifier ran in continuous mode for 24 hours in the crawl space with the trap door closed.

At the end of 24 hours, 64dF, 30.3", 38% 5.3 pints recovered.

All three points, neatly

09-03-18, 1320, 64dF, 29.9", 47% RH, begin
09-04-18, 0700, 64dF, 29.9", 40% RH, 6.5 pints recovered
09-05-18, 0700, 64dF, 30.3", 38% RH, 5.3 pints recovered.

I have the dehumidifier set now to maintain the RH in the crawlspace at 40%, and will check it again tomorrow morning.

I suspect, now more strongly, that I am sucking water out of the dirt outside the house through the poured concrete wall of the crawlspace and recovering it from the air in the crawlspace.

x is the date in the future I need to rent a backhoe and dig a moat around the building. Trying to solve for x.

 

[NateB]

Not only a moat, but under the floor also. I wish I would have seen the following video before I build my house, but I did get a vapor barrier in the floor. I decided to use a heatpump water heater to make domestic hot water, and cool and dehumidify the basement. Here is a good video on insulating a basement or crawl space. . If I ever build again the insulation and vapor barrier will be on the outside.

 

[Poindexter]

Thank you @NateB . I only watched about three minutes of this so far. I will watch the whole thing after work tonight. He had, in the first three minutes, already described a lot of what I think I am observing.

 

[Poindexter]

Poked around on engineering toolbox dot com a bit on my lunch break today.

Rough SWAG estimates I started with 194 pounds of dry air in my 2400 cubic foot crawl space and the dry air should have had in it, total for the entire volume at 47% RH, 1.08 pounds of water.

If the crawlspace was air and vapor tight; I know it isn't, but if it was, at 40% RH there should have been 0.8342 pounds of water left in the same 194 pounds of dry air.

So if I didn't have water vapor entering, I should have recovered about a third of a pound, one third of one pint of condensate - but I got six and a half pints.

My suspicion that I had water vapor entering is confirmed. Intuitively that six and a half pints was too much for the small change in RH.

I don't really look forward to doing calculations like this repetitively, but I don't see a way around it.

 

[Poindexter]

I am about halfway through the Minnesota video Nate posted above, and coming to a familiar cold climate crossroads.

I can burn a little more fuel to keep the structure dried out, or I can insulate to save a little fuel but have moisture which leads to mold, or I can build it freaking state of the art perfect and save a little fuel until the perfect system springs a pin hole leak.

I suspect the barely discernible musty odor in my crawlspace is from water trapped behind the insulation in the rim joist cavities.

I am going to wait and restart the video when the wife gets home. Looks like I am stuck running the dehumidifier until I can get the exterior foundation wall excavated and sealed. Oh joy.

I do wish I had noticed this before we dumped thousands of dollars into refurbing the lower level, but the musty odor in the crawlspace just wasn't detectable until we got the lower level carpet out and sealed all the existing drywall with BIN primer and on and on and on.

Thanks again @NateB .

 

[Ashful]

Just saw this thread title and got excited. But... no Bessel functions, no Laplace transforms, not even a stinkin’ integral. What you had, my friend, was an arithmetic problem... not a math problem.

 

[AlbergSteve]

I was hoping for a Fourier transform problem. Oh how I don't miss those DSP classes...

 

[zrock]

It can take up to a week or 2 to climatize a area with a dehumidifier especially when your dealing with concrete.. Your overthinking things just set it and forget it. Yes you will be pulling some moisture from the ground once things freeze up for the winter that will stop.. I used to run a dehumidifyer in my house it would take over a week to get things down. I would run it all winter as it would also heat the space for a 1/4 of the cost of a heater. Now with wood heat i no longer need it..
A small fan in a crawl space will do wonders as their is no air movement and thats where the musty smell comes from