Understanding humidity readings


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Old 06-05-15, 08:41 AM
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Understanding humidity readings

I've got a wet crawlspace plain and simple. Have everything lined up to get it fixed but in the meantime (the ground around here is too wet to dig the french drain at the moment) I would like to do what I can to dry it out.

I have a humidity sensor down there on one of the floor joists.
It read 90% at 68 degrees in the crawlspace
Outside was 68% at 72 degrees.
The vents are open in the crawlspace and I put a box fan in the access hole (it's outside) sucking air out of the crawlspace.

The sensor jumped to 99%.

Again, not discussing my absurdly high levels, just why did it increase when using the fan?
 
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Old 06-05-15, 09:42 AM
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Humidity readings are a combination of moisture and temperature. The resulting 99% needs the related temperature, but it does seem to go the wrong way. To take a wild guess, if the floor of that crawlspace is dirt, perhaps the moving air is accelerating the evaporation, or circulating the more humid air over to where the gauge is.

Like warm air is lighter than cold air and gets pushed up to the higher locations, moist air is also lighter than dry air, so there may have been some stratification placing your meter at a point where the reading was less.

All just speculating, and as you know, either reading is bad.

You said you are addressing this problem from the outside, which is always a great place to start, but if the floor is dirt, it needs to be covered with a vapor barrier, even with better drainage.

Here is a humidity calculator where you can compare the dew point of any humidity and temp reading. That is what tells you which combination has the most moisture.
Temperature, Dewpoint, and Relative Humidity Calculator

Bud
 
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Old 06-05-15, 09:57 AM
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Thanks Bud.

'To take a wild guess, if the floor of that crawlspace is dirt, perhaps the moving air is accelerating the evaporation, or circulating the more humid air over to where the gauge is. '

That was the best guess I could come up with last night while watching the monitor also.

Sorry, it went to 69 degrees while at 99%.

I actually have a roll of 10mil barrier in a box down there that I bought this winter but knew once the ground thawed that there would be moisture issues again. I don't want to put it down until after I have the french drain in there for a few weeks to verify that the french drain is taking care of the bulk of it. Don't want to have to tear up the vapor barrier if I need to add a perimeter drain inside also.

My only consolation is that it's been this way (worse actually because last fall I buried all of the downspouts 50ft-100ft away from the foundation) for 40 years so it would just be bad luck if a major foundation catastrophe strikes after me being there for only a couple of years!
 
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Old 06-05-15, 10:19 AM
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At those levels I'm surprised you don't have a major mold issue.

Ultimately you will need to decide to take one of the two approaches recommended for crawlspaces.
1. Insulate the floor and vent the crawlspace or,
2. Insulate the crawl walls, close those vents and air seal, and then condition that space, heat and ac or a dehumidifier.

I'm sure you have read those options, but just want to complete the thread.

Just a note, a seemingly dry dirt floor is a crawlspace will still release gallons of water per day. Where you may not see any water, moisture vapor will migrate from all around your house trying to reach that dry floor. One of the laws of physics where moisture moves to dry areas.

Bud
 
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Old 06-05-15, 10:28 AM
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I can only guess that introducing warmer air raised the dewpoint in the crawlspace. RH is directly related to the dewpoint.

It would be similar to when you take a hot shower in the bathroom... introducing warm moist air (and warmer air holds more moisture per sq ft) to a room that's "room temperature" but your mirror still fogs up. The mirror gets foggy because on that surface, the humidity is 100%. You introduced warmer air onto a colder surface. Like taking a can of soda out of the fridge, but in reverse.

So I'm guessing that blowing outside air (with a higher dewpoint than the temperature of the basement air) into the crawlspace is causing the RH to rise due to the dewpoints crossing paths.

At 68F and 90% humidity, the dewpoint was 65F. So if something was 65F, it would get condensation on it... (100% humidity on the surface).

The ground temperature is usually cooler than the air temperature. So we really don't know how that figures into the equation, but I would assume that it was simply circulating the air that caused cooler air from the ground to come into contact with warmer surfaces, causing condensation on those surfaces. Plus moving air speeds evaporation. The ground will always be cool and moist, so anytime you pump warmer air into a cool space you will probably get condensation. Kind of like how when you have .. your car A/C on full blast and you step outside, your glasses will fog up. Except in this case, you are introducing the warm air into the cold space... kind of like rolling down the car window when your A/C is on. Same difference.

Dew Point Calculator
 
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Old 06-05-15, 12:21 PM
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Makes sense...or at least as much as it needs to I guess

Bud - I am going with option 2. 2 inches of foamboard already on the walls and rim joist. Vents will be sealed once the drain and vapor barrier are in place. Humidifier will go in after that.

I know there are a lot of articles online and this completely derails this thread but what are your opinions on interior drains vs french drains? If you've covered this before and my forum searching didn't uncover it, please link it and I'll be glad to do the reading.
After a really hard rain the east side of my house near the foundation has standing water (bad grade). The french drain is planned to go along the east side, daylighting into a field south of the house.
The crawlspace after rain gets standing water primarily in the north half (so there is standing water on the west side of that north half). I was putting the french drain on the east side due to knowing the water stands there but it's possible (likely) that water will still puddle on the west side even after that.
Should I instead go with a perimeter drain in the crawlspace going to the existing sump? I just want to get all of this right before I do the vapor barrier.
 
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Old 06-05-15, 01:40 PM
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Yes, standing water at any time is a problem with the plastic cover. I found one link, but didn't seem to offer anything beyond the obvious.
Foundation Drains | GreenBuildingAdvisor.com

My thinking would be to add interior drainage to whatever extent is necessary to ensure there will be no more water surfacing, but that may be difficult depending upon the source of the water. If your water problem is surface water, then landscaping and drainage should help. If it is the water table rising, multiple sump pumps may be needed to keep ahead of it.

Is your current sump pump running full time? Is it near the pond?

Have you considered Radon as well, while you are sealing and adding drainage below the plastic?

Bud
 
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Old 06-05-15, 02:02 PM
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The sump pump that was there when we moved in is near the pond but the ground is pretty 'wavy' down there so it has to get pretty high before it runs into it. Not sure why it doesn't seep into it below surface. I dug it out as deep as I could with a chisel and hammer after our first wet summer but I may need to rent/buy an electric jack hammer and get it deeper (around 2 ft now). I added a sump in the southwest corner (primarily as a backup) but ran into the same issue of not getting past 18-24".

Since the french drain is on hold for awhile anyways, I'll do some serious research on the interior drains and deeper sumps. I've heard of radon systems but never really considered them. I'll do some reading on that also.

As always, appreciate the input.
 
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Old 06-05-15, 04:30 PM
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You should do some digging to understand what is in place now. Is there a perimeter drain around the outside? Is that connected through the footings to a similar perimeter pipe on the inside? And where do any of those drain to away from your house?

When you say you are not getting as deep as you want, is it ledge or compacted soil?

If you dig just inside the foundation wall it should be all disturbed soil from when built and you should be able to get down to the footing. Not sure how deep that would be.

With no water reaching the current sump pit, I'm thinking your infiltration may be surface water coming through the foundation and flooding the top layer of soil. That would be much better than the water table rising.

Bud
 
 

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