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  1. #1
    Join Date
    Aug 2008
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    66

    Question Best Humidity Control In Closed Crawl Spaces ?

    HVAC Folks,

    I live in the humid Southeast near Raleigh, NC. In this region closed crawlspaces are becoming more common. The idea is to seal out moisture from the ground and simultaneously seal out humid outside air from infiltrating the crawl space, cooling off, and condensing.

    However, once the sealing is done a mechanical means must be implemented to provide a drying mechanism for the crawlspace. This can be accomplished by either a controlled "leak" (outlet) from the supply side of the existing air handler or from a separate dehumidifier. The goal is to keep the humidity in the crawl space below the 70% where mold and mildew start to become a problem.

    The crawlspace has been sealed and I have been monitoring the humidity levels with a remote monitor for the last several months. The humidity has been running between 65-78% using only existing leaks in the duct system.

    The duct system and HVAC/ heat pump is only 2 years old and was well installed and mostly sealed. The main supply side leaks are at the pivot points of the 14 supply dampers. I was rather surprised to find Hart & Cooley didn't make a better sealed product! 28 of these holes adds up to a pretty large leak.

    At any rate the humidity is running too high so am now faced with a decision. Should I add another humidistat controlled damper and outlet in the crawlspace to increase the "leak" or should I install a separate dehumidifier?

    Do any of you have any experience with how well the extra damper can work in controlling the crawl space humidity? Granted this is all dependent on how much the A/C runs. This approach would probably be more efficient and less expensive than a separate dehumidifier.

    Thanks for your thoughts, experiences, and ideas.


    Steve

  2. #2
    Join Date
    Sep 2006
    Posts
    1,253
    Quote Originally Posted by Steve 347 View Post
    HVAC Folks,

    I live in the humid Southeast near Raleigh, NC. In this region closed crawlspaces are becoming more common. The idea is to seal out moisture from the ground and simultaneously seal out humid outside air from infiltrating the crawl space, cooling off, and condensing.

    However, once the sealing is done a mechanical means must be implemented to provide a drying mechanism for the crawlspace. This can be accomplished by either a controlled "leak" (outlet) from the supply side of the existing air handler or from a separate dehumidifier. The goal is to keep the humidity in the crawl space below the 70% where mold and mildew start to become a problem.

    The crawlspace has been sealed and I have been monitoring the humidity levels with a remote monitor for the last several months. The humidity has been running between 65-78% using only existing leaks in the duct system.

    The duct system and HVAC/ heat pump is only 2 years old and was well installed and mostly sealed. The main supply side leaks are at the pivot points of the 14 supply dampers. I was rather surprised to find Hart & Cooley didn't make a better sealed product! 28 of these holes adds up to a pretty large leak.

    At any rate the humidity is running too high so am now faced with a decision. Should I add another humidistat controlled damper and outlet in the crawlspace to increase the "leak" or should I install a separate dehumidifier?

    Do any of you have any experience with how well the extra damper can work in controlling the crawl space humidity? Granted this is all dependent on how much the A/C runs. This approach would probably be more efficient and less expensive than a separate dehumidifier.

    Thanks for your thoughts, experiences, and ideas.


    Steve

    Steve,

    I have sealed my crawl space and installed a vapor barrier, plus put in a sump pump. The final step was to install a dehumidifier. I purchased an Energy Star, low ambient unit that is rated at 35 pints a day. Once it got the space dried out which took several days (it was so humid when the dehumidifier was being installed that every surface had condensation on it) it now only runs on an infrequent basis. A bit more when we have had a big rain and more moisture is migrating through the space.

    I decided to use the dehumidifier for a few reasons. First, the space had different needs than the rest of the house. Trying to tap off my supply ducting would be difficult as it is not in the crawl. Also, pumping conditioned air outside the living space puts the living space under negative pressure which increases infiltration.

    Installing a dehumidifier was an inexpensive option that specifically addressed what the space needed - moisture reduction.

    BTW, you will want to keep the space below about 55% RH to control mold problems.

  3. #3
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,547
    You got a good reference on placing a dehu in the crawlspace. An adequately sized dehu will take care of any sealed crawlspace. Also, you do have an opportunity to make your whole home an perfect humidity even during wet cool weather. By installing a whole house dehumidifier, your entire home will be <50%RH with low/no cooling load, or even with the a/c off. A high eff. Ultra-Aire/Santa Fe 90 pint dehu with minimal ducting is capable of maintaining <50%RH in 2,500 sqft on home plus crawlspace. Check thermastor.com for info. 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"

  4. #4
    Join Date
    Aug 2008
    Posts
    66
    Quote Originally Posted by mchild View Post
    Steve,

    I have sealed my crawl space and installed a vapor barrier, plus put in a sump pump. The final step was to install a dehumidifier. I purchased an Energy Star, low ambient unit that is rated at 35 pints a day. Once it got the space dried out which took several days (it was so humid when the dehumidifier was being installed that every surface had condensation on it) it now only runs on an infrequent basis. A bit more when we have had a big rain and more moisture is migrating through the space.

    I decided to use the dehumidifier for a few reasons. First, the space had different needs than the rest of the house. Trying to tap off my supply ducting would be difficult as it is not in the crawl. Also, pumping conditioned air outside the living space puts the living space under negative pressure which increases infiltration.

    Installing a dehumidifier was an inexpensive option that specifically addressed what the space needed - moisture reduction.

    BTW, you will want to keep the space below about 55% RH to control mold problems.
    Thanks for a voice of reason. You are correct about the negative pressure and different needs. I have seen this already with rising humidity in the crawl when A/C is not needed in the living space. I have tried to improve this with the A/H fan setting but this hasn't done much.

    Can you provide any places to find comparisons of various dehumidifiers that are designed specifically for crawlspaces? You mention low ambient and energy star. Which brand and model did you choose?

    Thanks,

    Steve

  5. #5
    Join Date
    Aug 2008
    Posts
    66
    Quote Originally Posted by teddy bear View Post
    You got a good reference on placing a dehu in the crawlspace. An adequately sized dehu will take care of any sealed crawlspace. Also, you do have an opportunity to make your whole home an perfect humidity even during wet cool weather. By installing a whole house dehumidifier, your entire home will be <50%RH with low/no cooling load, or even with the a/c off. A high eff. Ultra-Aire/Santa Fe 90 pint dehu with minimal ducting is capable of maintaining <50%RH in 2,500 sqft on home plus crawlspace. Check thermastor.com for info. Regards TB
    Thanks for the Thermastor reference. The house is only a small ranch of about 1350sq. ft. so the unit that you suggest is probably way too big. Thermastor has some smaller capacity units but I notice that their efficiency goes WAY down compared to the larger units. Is there a way to get lower capacity and also high efficiency?

    The main A/C does a good job at maintaining the humidity in the living space at less than 50% already so I wouldn't need a ducted whole house unit. We tend to see very little wet/cool weather in this part of the country. If it is cool it is usually dry and if it is wet it is usually warm enough to be using the A/C.

    Thanks,

    Steve

  6. #6
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,547
    http://www.energystar.gov/ia/product..._prod_list.pdf
    This is a list of the best dehumidifiers. Checkout the Santa Fe Compact for the crawlspace. The Ultra-Aire 90H is the whole house dehu. 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"

  7. #7
    Join Date
    Sep 2006
    Posts
    1,253
    Quote Originally Posted by Steve 347 View Post
    Thanks for a voice of reason. You are correct about the negative pressure and different needs. I have seen this already with rising humidity in the crawl when A/C is not needed in the living space. I have tried to improve this with the A/H fan setting but this hasn't done much.

    Can you provide any places to find comparisons of various dehumidifiers that are designed specifically for crawlspaces? You mention low ambient and energy star. Which brand and model did you choose?

    Thanks,

    Steve

    Here is the Energy Star list of dehumidifiers that have that rating:
    http://www.energystar.gov/ia/product..._prod_list.pdf

    Efficiency is measured and listed in the far right hand column. I went with the last one listed (50 pint not 35 as I had remembered). It is not specifically for crawl spaces, but it has worked fine for that use. I made a small platform for the unit to sit on and then hung it from the floor joists with threaded rod. Drains into the sump pump crock with a short section of old garden hose.

    I weighted the up front cost and efficiency for the space with the expected amount of run time of the unit. While there are units that have better efficiency, the cost is much higher and I didn't think I could ever recover it in the amount of run time. The unit does not run much so I believe I made correct choice.

    It is still surprising to me when I go into the crawl and it is so dry.

  8. #8
    Join Date
    Feb 2008
    Location
    Western NC
    Posts
    2,504
    Steve..

    Contact your local HVAC contractor....


    They will be able to guide you best through this process. I live in Eastern NC as well and I know what difference a dehumidifier will do.

    Good luck....
    I fully support the military and the War on Terrorism.


    If you don't know, then don't do. If you don't know and still do, then be prepared to pay someone else a lot to undo what you did and then do it right.

    If you do know, then do. But do it right. Otherwise, you may not be doing it long.

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