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  1. #1
    Join Date
    Feb 2012
    Posts
    5

    Default Ventilation for crawl space?

    Greetings,

    We moved a 100 year-old schoolhouse to our property and set it on a concrete slab and two courses of concrete block. While we are repairing the floor, we've noticed the slab "sweating" a bit. I don't know if this is moisture passing though the concrete as the frost leaves the ground (happy early spring!) or condensation from the humid air (since there is a 7x7 foot hole in the floor)...

    We have an access door at the back of foundation to get into the crawl space. The schoolhouse will be (mostly) conditioned, but there may be a month or three in the winter when it is closed up and not heated.

    Do we need to be concerned about the moisture on the slab? There is no ventilation in the crawl space and we are worried that over time this could lead to moisture/mold/mildew issues. Thanks!

    -Tony

  2. #2
    Join Date
    Aug 2007
    Location
    The Great White North
    Posts
    4,045

    Default Re: Ventilation for crawl space?

    How long was the slab exposed to the outside frezing temps before the building was palced on top ?

    Is there rigid foam insulating the the slab from the ground frost ?
    "" an ounce of perception -- a pound of obscure "
    - Rush

  3. #3
    Join Date
    Jul 2010
    Location
    SoCal
    Posts
    5,436

    Default Re: Ventilation for crawl space?

    A crawl space needs ventilation and there is an actual way to calculate how much ventilation for what volume of space, no matter where the moisture is coming from. Ask at your local bldg dept.

  4. #4
    Join Date
    Feb 2012
    Posts
    5

    Default Re: Ventilation for crawl space?

    @canuk

    Not long. The slab was poured in August. The building was set down in early November, and the last concrete blocks were set on the last warm day of the season (mid November, above 32F).

  5. #5
    Join Date
    Aug 2007
    Location
    The Great White North
    Posts
    4,045

    Default Re: Ventilation for crawl space?

    Quote Originally Posted by Tony Skalski View Post
    @canuk

    Not long. The slab was poured in August. The building was set down in early November, and the last concrete blocks were set on the last warm day of the season (mid November, above 32F).
    Then it's not likely frost sweating ( unless you have severe ground frost)
    Usually frost sweating would occur when there are sudden extreme temperature differences between the frozen ground and air temp ( or hot sun shinning on the frozen slab.) Unless the space above the crawl space is really warm ,even with the opening in the floor I would be hard pressed to say that's a cause.

    If there is no moisture barrier under the slab it's possible there's wicking going on ( moisture under the slab wicking upward to the surface in the crawl space.)

    Are the blocks used for the cripple walls damp ?
    There may also be moisture infiltration from the exterior from snow melt/rains. Is the grade positively sloped away from the floundation?

    Generally venting a crawl space is only benifical during warmer weather to maintain the same temperature in the crawl space and the exterior and regulating the relative humidty preventing a dew point issue.
    "" an ounce of perception -- a pound of obscure "
    - Rush

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