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  1. #1
    Join Date
    Jul 2004
    Location
    South Alabama
    Posts
    24

    Dehumidfying crawl-space..

    I know this is probably off-topic, but I couldn't really decide where to place it.

    1st off.. I'm a homeowner, and I know the rule about no diy.. but this is not hvac diy, so I'm not sure if it is ok or not. I could not find anything like this for sale or anyone offering this.. so. I'm asking this here, because well.. you guys know about this stuff If this is crossing the diy line, let me know.. and I apologize and will delete my post.

    I've recently put a vapor barrier in my crawl space... and want to do more to keep the humidity down in it. I do not want to seal it. There are only 4 vents in it and I live in the hot/muggy southeast.

    Here's my plan.. This is mostly something put together with stuff laying around. I am putting in some new grills with 12volt computer fans, they are wired to a humidistat for outside the crawlspace and a dehumidistat for inside the crawlspace.

    The fans will come on when the crawl-space reaches a certain humidity level *IF* it is less humid outside.

    My question is should I also take inside or outside temperature into account? I realize this will probably want to kick on when the outdoor temperatures are at their peak and the outdoor humidity is at its lowest. Seems like this would increase my cooling load in the house some.

  2. #2
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,296
    What is your objection to sealing the crawl space? Keeping the moisture out through passive means is easier and cheaper - over the long haul - than control devices and mechanical moisture extraction.

    I will assume you installed a vapor barrier in your crawl space that is in contact with the ground. I will further assume that at some point, the vapor barrier material will assume the surface temperature of the ground below your crawl space. Let's say that surface temperature is 68 degrees, more or less year round. In short, what this translates to is any time the outdoor DEW POINT temperature is above 68 degrees, you should not ventilate your crawl space. If you do, your vapor barrier on the ground may become a water condenser...it will condense moisture out of the air and onto the plastic, only aggravating a moisture problem by leaving standing pools of water around. That is, assuming the plastic material is at ground surface temperature, which itself is below dew point. That is not an unreasonable assumption, IMO.

    Measure this out for yourself. Measure the temperature of the ground below the vapor barrier, then the temperature of the barrier material itself. Then determine the relative humidity and temperature of the air outside your home, and convert that to dew point. If the dew point number is higher than the ground surface/vapor barrier material temperature, you may condense moisture inside your crawl space when you fire up those little muffin fans.

  3. #3
    Join Date
    Nov 2007
    Posts
    14

    Talking Humidity Sucks!

    I have lived and worked in Charleston SC all my life. The humidity in this area is always high, sometimes even during the winter. The best thing to do would be to put as many vents in the crawl space as possible. Fans that help to force air through the space would be a big help. I think lowes or home depot sells stuff for this. I would suggest getting a contractor to help with the installation. But sealing a crawl is a bad idea. Shophound describes how condensation will occur on the vapor barrier. This will also occur unless you completely seal the block wall. Concrete is porous and will allow moisture in and that will cause you block to sweat with the temperature difference between the crawl and outside space. The homes in Charleston that I have seen have this amazing quality. They have open crawl spaces(built on piers)and the air has no restriction. The houses I have seen that have open crawl spaces are 50 years old and older. Some of them up to 300 years old. Guess what. NO ROT!!! At least not unless the water came from a leaking tub or AC drainpan.

  4. #4
    Join Date
    Mar 2008
    Location
    Houston, Texas
    Posts
    352
    Quote Originally Posted by BIG T View Post
    I have lived and worked in Charleston SC all my life. The humidity in this area is always high, sometimes even during the winter. The best thing to do would be to put as many vents in the crawl space as possible. Fans that help to force air through the space would be a big help. I think lowes or home depot sells stuff for this. I would suggest getting a contractor to help with the installation. But sealing a crawl is a bad idea. Shophound describes how condensation will occur on the vapor barrier. This will also occur unless you completely seal the block wall. Concrete is porous and will allow moisture in and that will cause you block to sweat with the temperature difference between the crawl and outside space. The homes in Charleston that I have seen have this amazing quality. They have open crawl spaces(built on piers)and the air has no restriction. The houses I have seen that have open crawl spaces are 50 years old and older. Some of them up to 300 years old. Guess what. NO ROT!!! At least not unless the water came from a leaking tub or AC drainpan.

    I'm not disagreeing with you, it is always better to have it open and well ventilated, if that solves the problem at hand. If open and well ventilated doesn't work......psycometrics will tell you that if you seal it up, and the temp rises, the dewpoint also rises. This means no condensation/moisture. Without moisture you cannot have rot.

    We have very few crawl spaces here, science usually doesn't lie.

  5. #5
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,270
    There are two sources of moisture that cause problems in crawlspaces. The soil is the biggest source and the outside humid air is the other. Outside dew points in your are high enough cause condesation on the floor of the home. High humidity in a crawlspace causes cupping of wood floors. If concerned about mold or cupping floors, a crawlspace dehumidifier is suggested. Try your strategy. Monitor the resulting %RH with a remote %RH meter will verify the results. <60%RH is needed to avoid any problems.
    When dealing with flood planes with home on piers, A good vapor barrier at the bottom of the floor joists, usually stops condensation and cupping of the wood floors. Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  6. #6
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,296
    My last post was envisioning a sealed, conditioned crawl space, with insulation and water proofing at the perimeter walls, as described in this document from www.buildingscience.com:

    http://www.buildingscience.com/docum...ance-and-codes

  7. #7
    (I broke my account I think, and it is tied to an email address I no longer use)

    Shophound.... I have no idea why I didn't want to seal up the crawl-space.... for the life of me I couldn't remember.
    I'm sure I had a reason, but I am not sure what it was. I had plenty of info on doing it.. just for some reason...<shrug>

    I did some calculations and looked at the temp/humidity that the crawlspace is at now. It is ALWAYS humid here. I live just north of Mobile, AL, which actually topped the list for average rainfall @ 67inches/yr.

    Well... I just sealed the 4 vents in the crawlspace, using foam insulation that has a barrier on it & spray foam. I setup a dehumidifier on a kill-a-watt. I'll reset it after it dries out down there, that will let me know how much it has to run in order to keep the humidity down.
    I know this isn't like the "fancy" sealed basements where contractors install the thick insulated white poly-stuff, but I did install my vapor barrier on the walls at least where the block walls make earth contact on the other side, so maybe that will help.

    I'll try it this way and see how this works.. that was actually simpler than setting up the fans, etc.

    Thank you very much for the input guys.

  8. #8
    Join Date
    Mar 2007
    Location
    Myrtle Beach, SC
    Posts
    2,919
    Remember, Air Conditioning begins with AIR.

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