Dehumidifier in Basement or Crawlspace


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Old 01-14-05, 02:49 PM
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Dehumidifier in Basement or Crawlspace

We have a home with 2 basement rooms and adjoining crawlspace. There is no direct access between the 2 but we have excess humidity in the basement areas which have concrete slab floors. Years ago I used a Sears portable refrigeration type humidifier but I had to run a hose for the condensate to remove the water collected.

I am now looking to install something more permanent. It has gotten to be a bigger problem this year because of unusually wet weather and ground water event though we have good exterior water extraction measures. We have a large French Drain installed entirely across the uphill side of the home which runs freely during wet weather.

Would it be best to install a dehumidifier in the crawl space (uphill of the basement rooms which are at grade level on the downhill side) or in one or both of those rooms?

For disposal of the condensate should I tie into the sewer line or just run a line outdoors into planted areas?
 
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Old 01-14-05, 03:22 PM
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Question More info is needed.

Hello Joe,

In order for someone to be able to give the best advice it would help to describe your problem in a little more detail.
It is usefull to know roughly where you live and will need more info on the details of your home and basement.
Two rooms separated but with adjoining crawlspaces make it sound as if you have a part basement.

More detail please.
 
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Old 01-14-05, 04:31 PM
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You are Correct, we have a partial basement. Our house is multi-level on a fairly steep site. We live in San Diego county only about 600 ft elevation but on a hill where almost everyone up the hill from us is on septic sytems and the hillside is mostly fractured granite and decomposed granite. While this is very stable from a foundation standpoint, it does allow some water seepage.

The Basement rooms are on the downhill side of the house and directly above are the living and family rooms which connect to the basement by a stairway.
The Living and Family Rooms are open to the Entry, Den, Dining Rm, Kitchen and Breakfast Rooms which are above the Crawl Space directly upslope of the basement rooms. Up Slope from that is another Crawl Space which has a 3 car garage above it and above that is the Master Bdrm/Bath.

I could go on describing the intermediate level above the Entry/Dining/Kitchen but I think it isn't that relevant. The main thing is that the home is a very open plan - there is pretty much a free flow of air from one level to the next because there are few walls and the Living Room which is 2 stories in height is open to the levels above. For that reason, the house is zoned with 2 FAU/AC units. One for the top portion of the house (bedrooms) and the other for the bottom levels (living areas). The basement rooms are basically hobby/shop and are handled by the same FAU/AC system as the living areas.

The high humidity problem is only in the basement and crawl space. My thinking is that putting a efficient dehumidifier in the main crawl space will drastically reduce the problem in the basement since the soil in that area would be much drier and the basement slab would not then wick very much moisture into the air.

Is this sufficient detail?
 
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Old 01-14-05, 04:42 PM
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On thing that I mentioned earlier is that when we built the house we had a French Drain installed across the entire uphill side to intercept subsurface water and direct it around the house. This was done because of seepage that occurred during foudation excavation. It works extremely well, but as with any soil, there is a natural amount of "wicking" that occurs when the ground outside is subjected to excess moisture.

As you know, we have had a lot of rain recently and the ground in most areas of California are totally saturated. We are currently experiencing just about 100 % runoff when it rains.
 
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Old 01-14-05, 04:51 PM
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It's still not clear if the crawlspace area is dirt or soil that may be a source of the moisture.
If so then rather than worrying about a dehumidifier at this point I would first look at using some heavy poly to cover all exposed surfaces.
I would then look at ventilation in the space.
 
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Old 01-14-05, 05:25 PM
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The soil definitely is the source but covering it with poly is not possible nor would it eliminate the problem. Most of the humidity in the basement results from the dampness wicking thru the concrete slab floor which is lower than the soil in the adjoining crawl space. There is a retaining wall between the basement and crawl space, but no way to prevent some humidity. I guess that's why I asked if it would be best to put the dehumidifier in the basement room(s) or try to decrease the upslope soil moisture content (and consequently the moisture content under the concrete slab of the basement by dehumidifying the crawlspace area.
 
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Old 01-14-05, 07:11 PM
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Dehumidifying will not be much help.
You cannot remove moisture from the soil by dehumidfying the air.

There must be weeping tile installed around the basement area that is connected to a floor drain.
Do you also possibly have a high water table that is providing a source of moisture.
 
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Old 01-14-05, 08:48 PM
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No, there is no flloor drain or "weeping tile".

As I said previously, this is a rocky hillside. Huge Granite Bolders with cracks that allow water from up slope to percolate down. We intercepted the major underground streams with the French Drain when we built the house. However, there is (depending on the season) either a tremendously dry condition or a damp condition - not wet, just moist - of the soil under the house. All homes in Southern California with crawl spaces are well ventilated by code and I have never seen one where any sort of plastic was placed on the ground except directly under a slab. In any event, the crawl space has sloping ground and is mostly rock - some large (3-6 feet in diameter). The foundation was formed mostly over the rock with rebars epoxied into holes drilled in the rock.

In the 14 years since we built the house we have never had water actually appear either in the crawl space or in the basement. Note that the basement is at grade along the downhill side with sliding doors out to a deck. This humidity problem is not huge, but can cause a musty odor at times.

I still think that minimizing the amount of moisture that evaporates up from the soil or thru the slab is the way to deal with it.
 
 

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