Using Humidistat in crawl space

Don't see any reason why not. You will get contamination from critters. I'm always amazed at how creative spiders can be at messing up stuff. Can help that with insecticide in the vicinity...unless that messes up the sensor.

I'm sure people can come up with exceptions, but as a general rule, If you have a wet crawl space, measuring humidity is redundant. Run the fan all the time, and work on making it a dry crawl space.

If you have a dry crawl space, measuring humidity is unnecessary, and you don't need a humidity controlled fan.

I always get confused when talking about humidity. I never quite know what people think they're saying and if they're using terms that accurately describe it.

Most everything is described in terms of relative humidity, cause that's what the gauge you bought at walmart shows.

Most of what I try to "fix" is related to dew point. But that's not what my walmart gauge measures.

Example: Right now, it's

I'm using cheapo remote sensors of questionable accuracy, but if you look up the numbers on the Psychrometric chart, the dew point, or the actual amount of water in the air is about the same.

Blowing air from outside into the crawl space wouldn't change the amount of water in the air at all...or the RH reading...for constant temperature. It would just increase the heat flow thru the floor to warm that outside air.

I bought a humidity controlled crawl space vent at a garage sale. I decided that the cost of getting power to it, while still meeting NEC, was prohibitive. And it wouldn't make any difference anyway.

Bottom line, make sure your fixing the problem you think you have. mike

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do you need a controlled fan instead of passive ventilation?

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It is completely unsuitable for crawl space humidity control- wrong component, wrong algorithm.

More words please. What's the algorithm and how does it differ from one designed for use in the crawl space?

Yes, I do need powered vents. I have seen mildew spots on large areas of the underflooring. There's no standing water that I can see, but the ground is wet. It has stayed wet almost constantly since early October (winter is out wettest season).

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Isn't the ground covered with a vapor barrier under there? I live in Oregon and my crawlspace has 6 or 8 mil black plastic sheeting covering the ground and it is sealed to the foundation walls. I have passive ventilation and no mild, mildew, whatever anywhere. Art

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Make that "... no mold, mildew ..." Art

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You probably don't.

I suspect you need a 'French Drain' which is a peripheral trench around your home that provides an easy way for water to exit into the storm drain before it enters your crawlspace.

--Winston

unless It's succeptible to extreme temperatures that could be present there but not found indoors I can see no problems.

If it were me, the first thing I'd try to do is kill the mildew. And if it's in a place that could migrate up a wall, I'd worry a lot about that from the standpoint of health and long term destruction of the structure. Somebody else can recommend whether you need to seek professional help.

Then I'd do some math. How much volume in the crawl space. How many CFM for the vent fan? Assuming perfect air distribution, how long does it take to change the existing air for outside high-humidity air?

If the crawl space is much warmer than outside air, the RH will be somewhat lower than the outside air...consult the psychrometric chart. But that means you don't have a big enough fan. That also means you've got a lot of heat lost thru the floor.

Then, I'd wire up the fan temporarily. Measure the humidity. Run the fan continuously until the humidity reaches your target value. Turn the fan off. Monitor the humidity as it rises. Plot the graphs. Possibility 1...the humidity doesn't decrease significantly. If the ground is wet, you can expect the humidity to be at the saturation point. A small fan is inconsequential. Possibility 2...the humidity rises so fast that you might as well leave the fan on all the time anyway. Possibility 3...????? I can't think of one.

Unless you have a fan capable of drying the ground, I don't have high expectations.

You need ground cover. Depending on how wet "wet" is, you may need to find out how the wet is getting in there and fix that.

I think the early demise of the humidistat is the least of your worries.

Perhaps a French drain would work. but installing powered vents would be much less expensive and much easier to install. I already have the humidistat, and the powered vents are relatively cheap and easy to install. The majority of the work to be done will be to run power to the crawl space. That will be fairly straightforward as well. Thanks for all the comments.

We had a serious 'standing water' problem in our crawlspace after every rain. After installation of the French Drain and clearing a few obstructed foundation vents, the crawlspace remains quite dry during all but the very worst downpours.

(Even during power failures.) :)

--Winston

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French drain should be required by the local building code. If he already has one, then it's clogged. Another requirement is that the grade slope away from the foundation slope at least 6" for 10'. Although not a requirement, another recommendation is that the rain gutters be channeled a minimum of 10' from foundation, this is done by attaching the same 4" poly tubing either on or under the ground for that distance, with minimum 1/4" per foot of run slope for standard 4" pipe. Many times there is a watershed issue where the foundation is located on the down hill side of surrounding area. This is handled by excavating a swale to divert the water from around the vicinity of the house. A swale is a simple three sided trench filled with no.3 stone to prevent erosion. Lastly, regardless of the drainage, a 6-mil vapor barrier is used to cover the ground inside the crawl space, plastic is staked in place, seams overlapped by minimum 6" and taped or glued, also it is run up the interior walls by 12" and taped/glued. Crawl space ventilation does not work and is no longer recommended.

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That's a serious problem and could end up killing you and the other inhabitants eventually, nearly all mold is toxic to one degree or another. Do you have insulation in the floor joists, and if so does it have a vapor barrier? One seemingly paradoxical problem with most vapor barrier technology is that it is vapor permeable but watertight, and is designed to be used only for vertical installation and not horizontal, TYVEC is a pefect example. This means that the usual high interior humidity during the winter season permeates the flooring, condenses to liquid on the vapor barrier and is trapped there. If you want to install instrumentation then consider a wood moisture indicator. Mold growth begins when all of the following are exceeded: RH in excess 70%, temperature exceeds 70oF, wood moisture content exceeds 17%.

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The crawl space ventilation controllers inhibit activation when outside humidity is higher than the crawl space humidity. The indoor application doesn't have this because the air is juts being circulated through a mechanical dehumidifier, but the crawl space humidifier is drawing air in from the outside and therefore must measure its humidity.

Thanks.

Could you elaborate more on RH as it applies to structural damage?

Virtually everything I read talks about RH. Is that what we really care about?

For example, shouldn't the crawl space humidistat inhibit activation when the dew point outside exceeds the dew point in the crawl space? My crawl space is 55F year round. Outside temperature varies between

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I do not know that any of them do. You would be better off closing off the vents during warm weather, and circulating conditioned air from the interior of the house through the space. It does not require much, the nominal design flow rate is 2 CFM per 100 sf of crawl space. This assumes you have taken steps to seal the space. See

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Yes. It *should*. :)

Code inspection is done on behalf of the seller in my area, not the buyer. Our present house sure showed that to be the case! None of the houses in my area have peripheral drainage. Water in the crawlspace and cracks due to erosion is a chronic problem for many of my neighbors.

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I sure hope not. :)

Before I started a recent repair of a heat duct in this house, I spent two weeks excavating soil to form a path from the crawlspace entrance to the area needing repair.

Removing the vapor barrier before excavation would have been a show stopper. Replacing the barrier would have been a non-starter.

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Do you mean ventilation by itself? I agree that without the peripheral drain, ventilation (active or passive) would not keep the soil in the crawlspace dry. It'd be like fanning a sponge whilst adding water along one side. :)

--Winston

John Ferrell W8CCW