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  1. #1
    Join Date
    Dec 2010
    Location
    Southeastern NC
    Posts
    9

    Help with Mom's 12 year old house, high humidity in and in crawlspace, getting moldy

    Hi folks! Thanks again for the help I got with my new HVAC unit last Winter.

    OK my Mom has a 12 year old house in Laurinburg NC (Flat and hot). Brick exterior, real nice... 2525 SF down, 950 SF up, well insulated, good windows, gutters with extended downspouts- the whole shooting match.

    It's 3,400 square feet, an expanded version of this Southern Living plan:
    http://southernlivinghouseplans.com/plans/SL572A

    She says the blueprints show 3,475 square feet... I'm not 100% on that and too far away to go measure.

    She has always had a problem with moisture in and under the house. She has 2 dehumidifiers going all the time and the interior humidity never goes below about 63%. She has her crawlspace vents open and a large fan under the house going all the time blowing air through the crawl. The crawl humidity level is 70%. The other day she noticed mold on some of her furniture.

    HVAC units:
    xe-1200 trane model twpo18c100a3 upstairs (950 square feet)
    xe-1200 trane model twpo48c100a4 downstairs (2500 square feet down)

    She just got an estimate for crawslapce encapsulation ($$$!)- guy said he'd shut off all the vents and seal it all up, told her to stop using the crawl fan etc.

    She's been told by multiple contractors/ HVAC guys that her unit is oversized and doesn't run long enough to pull the moisture out of the air. In a 12 year old house why not just get the right size unit put in and see if that helps? In conjunction to that should she stop using the fan under the house and close her crawl vents in the summer?

    Any ideas? She's getting desperate.
    Thanks! John
    Last edited by John in NC; 08-09-2012 at 03:00 PM.

  2. #2
    Join Date
    Dec 2010
    Location
    Southeastern NC
    Posts
    9
    O.P. again...

    My thought is get the house professionally measured, get the right size downstairs unit (12 year old house/ original unit) and then maybe seal the crawl and get a dehumidifier installed in the crawlspace? Or maybe just shut the vents and turn the crawlspace fan off with the new HVAC unit and see if it helps with the moisture content.

    Oh and Mom says the crawlspace is usually dry, but it was damp when this guy went under the house, again all the crawl vents are open and the fan sucks air through.
    Thanks again, John

  3. #3
    Join Date
    Aug 2003
    Posts
    906
    Exhaust fans create negative pressure, suck in warm humid air from outside. It's cooled down in the crawl so relative humidity increases. The fan may also be sucking air from the house, which is replaced by humid air from outside. Turn off the fan and close the vents for 24 hours. Make sure that the thermostat fan setting is set to "auto" and not "on". Keep the dehus running. I bet you that the humidity will decrease.

  4. #4
    Join Date
    Dec 2010
    Location
    Southeastern NC
    Posts
    9
    OK... her thermostat fan was indeed set to "ON" so she put it on "Auto".
    She turned off the crawlspace fan.
    She's in shutting the vents today.
    She's keeping the 2 dehumid's running.

    So far the humidity % in the house has dropped from 63-65% (sometimes 70%) to 55%. Looks like we're heading in the right direction!

    Do you believe in crawlspace encapsulation? She has very sandy soil and heavy mil vapor barrier down now. Crawl has been dry when I was under, but this contractor said it was 70%+ humidity....

  5. #5
    Join Date
    Aug 2003
    Posts
    906
    Once you're happy with the humidity level in the house, measure what it is in the crawlspace. You can get water vapor in the crawl space from the ground or from infiltration from outside air. If the humidity is too high then you need to diminish those sources or increase the temperature (heat the crawlspace). A vapor barrier on the ground diminishes humidity from the soil. Sealing the crawlspace will help by diminishing infiltration. People run dehumidifiers in sealed crawlspaces. Dehumidifiers heat the crawlspace and capture water vapor. A sealed crawlspace costs less to dehumidify. But measure first.

  6. #6
    Join Date
    Apr 2001
    Location
    Over Here
    Posts
    1,105
    Power ventilating a crawlspace will only dry it to whatever the ambient humidity levels are. At night, conditons may allow surface temps to reach dew point, causing excessive moisture.

    If not done correctly, powered ventilation applied to any part of a conditioned envelope could create a significant pressure imbalance on the conditioned space, causing what could be happening here. Also, running the A/C blower in 'on' and not 'auto' mode is a major contributing factor to high humidity in homes - especially if the ducts are outside the conditioned space.

    I agree with what has been recommended so far. Seal it up and dehumidify. You will still need ventilation, but controlled, and after a proper analysis is performed. Sounds like rocket science, but it's not really.

  7. #7
    Join Date
    Aug 2003
    Posts
    906
    Consider moving one of the 2 dehus from the living area to the crawlspace. You'll have less noise and it will probably be more useful there.

  8. #8
    Join Date
    Dec 2010
    Location
    Southeastern NC
    Posts
    9
    Mom so far is over the moon... She has the automatic crawl vents? Is there a trick to keep them shut without jamming them up?

    Heres what she wrote:

    Hi, THANK YOU! Just wanted you to know that the humidity in my bedroom this morning was 49 and in the kitchen was 50 (down from the mid 60's + before these fixes). I turned off one of the dehumidifers a couple of hours ago and the kitchen crept up to 53. Not bad. I still haven't closed the under house vents but have figured out a way to do it without tearing them up (automatic vents). That will be interesting. It is shocking to find that the fan running continually in the crawl could cause humidity problems since I had been advised otherwise by so called experts.

    Exciting! Thanks so much!!!

  9. #9
    Join Date
    Aug 2003
    Posts
    906
    Great... However, changing the thermostat fan setting to "auto" is most likely responsible for the majority of the change. Keep in mind also that with the vents closed you'll still need to measure and control the humidity.

  10. #10
    Join Date
    Dec 2010
    Location
    Southeastern NC
    Posts
    9
    UPDATE!

    So Mom is thrilled.
    The humidity level is way down.
    The clincher was she turned off one of the dehu's and just ran one, and instead of emptying BOTH of them a couple of times a day this one unit didn't even fill up the water bucket over the course of 24 hours... so that is pretty good evidence that we're on the right track thanks to the advice of you fine folks...

    THANKS!!! Awesome...

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