Ventilation in the crawlspace

Old 06-18-03, 04:18 PM
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Ventilation in the crawlspace

Someone here mentioned a while back that they put in a ventilation fan or something in their crawlspace to help air flow. In the past I have opened our vents and so on to let air flow in the summer. Well so far here in Chattanooga, TN we have had record rains. A month ago we had some flooding. I got just a little and what did get in my sump caught.

Now when it was done raining the water that did get in obviously evaporated. Well thing like the outside of my air vents and pipes and so on on got moisture on them. I tried running a fan, but I found that all the humidity outside was being drawn in. So I went to Lowe's and bought a basement grade dehumidifier. I put in the crawlspace, installed a drain pipe and shut all the vents to the outside (per the instructions). That got rid of the moisture on the surface stuff and the ground areas that were wet dried much quicker.

Well I have had it running a month now on auto. It never cuts off, but I have heard the compressor or what have you cycle down to where it is off. My concern is the electrical bill. It has gone up slightly, but not significantly. I still have the vents shut and compared to outside temperature its cooler inside, but much drier.

I was wondering several things. First, is it proper to use a dehumidifier like I am in a crawlspace? Second, If its on all the time, should I look an additional one? I have this one in the center of the house, but the crawlspace is about 1900 square feet. I was thinking one on each end. Third, should I ditch the demumidifier and go with a ventilation fan and thus open the vents back up? If so, where do I get them or who do I call to install them for me? Fourth, I have plastic all over the floor. I see sometimes moisture on it. Should I pull the plastic up to let the ground dry? Especially for those areas that have gotten wet from seepage.

Any suggestions would be appreciated. My ultimate goal is to dry out the places that are still moist. Luckily I have no mold or mildew and the lowes point of the crawl space is about 3 1/2 to 4ft. The wood appears to have no moisture damage.

Thanks in advance.
Old 06-18-03, 08:38 PM
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I assume that where you live, humidity is an issue. I would open your vents to allow for air flow as this will assist in air movement which is essential to relieving any moisture.

I would not remove the plastic over the soils as this prevents more moisture from penetrating the space from the soils below.

A larger dehumidifier might be in order and yes, this takes electricity but you really shouldn't have to nor should you use this.

The vents should be adequate as this is not a habitable space. I think that trying to do more in a crawl space is a waste of money. As long as your foundations is solid, no liquid water getting to the floor joist system, things are fine. The rains that have been coming lately have caused some problems but this considered a infrequent thing. If necessary, add more vents to the foundation.

Code says 1 SF opening per 150 SF of underfloor space. At least one vent should be placed within 3 feet of each corner. This will provide for good cross ventilation. A small fan to assist in ventilation is an option that is viable but no need for anything more than this.

Hope this helps!
Old 06-20-03, 12:07 PM
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crawl way

Sorry I have to go 100% the other way. Dont vent the crawl way or fans to bring more air in. The ground there is cool and if you bring in the warm air from out side we fine it just makes more water there. You said you had a VB down on the ground is it good? The gov. calls for 6 mil poly with a 2ft overlap and up the walls 2' all seams taped. Let that dehumidifier run to help get the water out of there. It will help so your floors dont swell up on you .Have had this happen. Have a boy there by you so been there done that
Old 06-22-03, 07:00 PM
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Site conditions determine appropriate action.

There is a simple rule when it comes to moisture control that one should follow. That is warm air condenses against cooler surfaces. This makes crawl space susceptible to condensation. With that in mind venting a crawl space is determined by which area has the greater humidity level. For example, if the air outside contains more humidity than the crawl space, you would not ventilate the crawl space. On the other hand if the air outside contains less humidity than the crawl space, then you would want to ventilate the crawl space. Naturally this will vary from time to time. You base this on the majority of time. For example if you lived in an area where it was hot and humid most of the summer, then you would not want to ventilate the crawl space and vice-versa.

Insulating the crawl space is determined by the way you decide to ventilate the area. For example, if you don't ventilate the crawl space, then insulate the walls of the crawl space. If you ventilate the crawl space, then insulate the ceiling of the crawl space.

There are instances when both are done. Meaning to say the walls and ceiling in the crawl space are insulated. An example here is lake shore property in an area that experiences cold winters and hot humid weather in the summer. In the summer the crawl space is not ventilated and in the winter it is. Hence the need for insulating both the walls and ceiling in the crawl space.

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