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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).

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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

  • Concrete slab poured on top of a DPM.

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:

Crawl spaces must always have a drying mechanism. One of the most effective ways to provide a drying mechanism to a crawl space is to condition a crawl space by heating and cooling the crawl space as if the crawl space is included as part of the home. Air must be supplied to the crawl space from the home in order to provide this conditioning. This air can be returned back to the home or it can be exhausted.

Crawl spaces must always have a ground cover that prevents evaporation of ground moisture into the crawl space. There are many ways to provide a durable ground cover or liner. The option used depends on the resources available, the frequency of people entering the crawl space to either store possessions or to maintain equipment. One of the most effective is 6 mil sheet polyethylene that has taped/sealed joints and that is attached to the crawl space perimeter walls. This ground cover must be continuous through piers and supports.

Crawl space perimeter walls should also be insulated.

The above quotes come from, Conditioned Crawl Space Construction, Performance and Codes (direct link to PDF).

Comment by Matt Chambers: Obviously a sealed moist space is bad, but what's bad about a sealed dry space, kept dry by a dehumidifier which drains outside the crawlspace?

I think you will (or anyone else in a domestic situation would) find it nearly impossible to seal & keep dry enough a space so that problems don't develop, occur...

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...

Comment by Matt Chambers: But in that case I still want to know how fast moisture comes out of dry ground.

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

  • What type of soil do you have.

  • How high or low is the water table in your area.

  • How much annual rainfall do you get in an average year.

  • What are your average weather patterns in an average year.

  • Etc.

I honestly believe there are currently too many variables (unknowns) to give you a realistic answer to that particular question.

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Obviously a sealed moist space is bad, but what's bad about a sealed dry space, kept dry by a dehumidifier which drains outside the crawlspace? –  Matt Chambers Aug 9 '11 at 19:24
    
@Matt Chambers, please refer to my edited answer. –  Mike Perry Aug 9 '11 at 19:56
    
Your answer is a good one, and I agree with you Mike. Crawl spaces need to be ventilated by outside air. Most places require it by code. +1 –  lazoDev Aug 9 '11 at 19:59
    
OK, then I'll have to consider the crawlspace conditioning (it's still ventilated, but with the A/C instead of outside). But in that case I still want to know how fast moisture comes out of dry ground. Because doing an airtight seal that's normally recommended for a conditioned crawlspace is the most expensive part. So if the house A/C plus the crawlspace dehumidifier we already have could easily handle the moisture from the ground, then we'd be fine without sealing it up. –  Matt Chambers Aug 9 '11 at 21:12
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I think I will try to determine it empirically on the hottest, most humid day after it rains (i.e. worst-case short of flooding). Then, after sealing vents (temporarily), assuming my dehumidifier can handle the moisture from the ground, I'd wait until the output from the dehumidifier stabilizes (i.e. after the air is dehumidified). The amount extracted per day after that is the amount coming from the ground. If the output never stabilizes then obviously the dehumidifier doesn't have the capacity. :) –  Matt Chambers Aug 9 '11 at 22:50
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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.

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I don't understand how this could work when the relative humidity outside the crawlspace is 70% or higher. –  Matt Chambers Aug 9 '11 at 19:25
    
Because it pulls moisture out and does not let any back in that is the point of the fan. –  lazoDev Aug 9 '11 at 19:30
    
That's impossible if there are open air vents - the moist air would just get sucked in from the other vents. Are you suggested that EVERY vent would have a fan blowing out? –  Matt Chambers Aug 9 '11 at 19:35
    
He's asking questions. Not back talking. –  DA01 Aug 9 '11 at 20:38
    
Matt: the odds are that even if it's humid, it's more humid under your house where the air is cooler and in contact with moist ground. As such, the fan will likely be pulling in less humid air than what would be in the air if it were just sitting there. –  DA01 Aug 9 '11 at 20:39
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