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  1. #1

    Crawlspace humidity

    My 45 yr old home in Atlanta,GA has an 800 sf (25x32) crawlspace containing 2600 cu ft. The dirt portion is covered with 2 layers of 6 mil plastic sheeting. Furnace, fan, and evap sit on a 10'x10' slab in the crawlspace. Current crawlspace temp and humidity aver. 68 degrees and 64 percent. Outside temp/humidity 92/41. Dewpoint 67. Trying to reduce musty crawlspace odor and humidity being sucked into a/c system. Can I cover the 4 outside air vents in the concrete block foundation and hook up a portable humidifier? Do I need to worry about how much outside air will be taken in, after sealing the 4 vents. Any other suggestions? Thanks for your response.

  2. #2
    Join Date
    May 2011
    Location
    Southern Indiana
    Posts
    97
    One thing I would do is insulate the parimeter walls and run plastic all the way up to the floor. Make sure the duct is sealed properly. Install a ERV unit along with a whole house dehumidifier.

  3. #3
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,068
    Quote Originally Posted by golfinator View Post
    My 45 yr old home in Atlanta,GA has an 800 sf (25x32) crawlspace containing 2600 cu ft. The dirt portion is covered with 2 layers of 6 mil plastic sheeting. Furnace, fan, and evap sit on a 10'x10' slab in the crawlspace. Current crawlspace temp and humidity aver. 68 degrees and 64 percent. Outside temp/humidity 92/41. Dewpoint 67. Trying to reduce musty crawlspace odor and humidity being sucked into a/c system. Can I cover the 4 outside air vents in the concrete block foundation and hook up a portable humidifier? Do I need to worry about how much outside air will be taken in, after sealing the 4 vents. Any other suggestions? Thanks for your response.
    You are on the right track. Close the vents and any large air leaks from outside that could allow damp outside from entering the crawlspace. It is possible that the best, 70 pint residiential dehu will reduce the humidity to <55%RH. Ventilation is not necessary for the crawlspace.
    Many contractor are fixing crawlspace problems with closing vents, covering the earth with plastic and using the Santa Fe Compact dehumidifier.

    I would suggest that solve all the indoor air quality/comfort problems of the home at one time. Install a whole house dehumidifier with fresh air in the crawlspace that keeps the whole home <50%RH and supply a small amount of dry air from the dehu to the crawlspace. Keeping the whole house dry during outside high dew point weather and introducing a small amount of fresh air to the home, indoor pollutants are purged and oxygen is renewed. Look at the Ultra-Aire 70H for the whole house fix.
    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
    May 2011
    Location
    Southern Indiana
    Posts
    97
    Sorry I didn't get as detailed, but the company installing this should know all the details

  5. #5
    Thanks for your responses. After more research, I'm reading that if your furnace is in the crawlspace, you should not be closing off the outside air vents (something about combustion gases becoming dangerous?). Is this a concern? I hear a local radio home fix-it show with guests promoting encapsulating the crawl space, but don't recall any warning about furnace gases.

  6. #6
    Join Date
    Jul 2007
    Location
    Delaware
    Posts
    3,823
    Shut the vents, you'll be fine. Do you draw outside air into the crawl space for combustion? Is this a high efficiency furnace?
    Always here

  7. #7
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,068
    I missed the furnace vent thing. Certianly provide the vents required by local codes for make-up air if the furnace has a gravity chiminey. If it is sealed combustion with powered exhaust, venting is included or not required.
    The other is important point is that controlling %RH in the crawlspace will dramaticly reduce the moisture in the home. Also eliminate condensation on the a/c supply ducts.
    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"

  8. #8
    The furnace is not a high efficiency(26 yrs old) and there is no powered exhaust, so I'm guessing it draws air from the crawlspace. Also, I have a gas hot water heater open to the crawlspace 10' away from the furnace. Both exhausts are metal flues ducted thru the roof of this tri-level house.

  9. #9
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,068
    Quote Originally Posted by golfinator View Post
    The furnace is not a high efficiency(26 yrs old) and there is no powered exhaust, so I'm guessing it draws air from the crawlspace. Also, I have a gas hot water heater open to the crawlspace 10' away from the furnace. Both exhausts are metal flues ducted thru the roof of this tri-level house.
    Get the specs for smallest vent that meets local codes. Usually, 8" screened inlet will do. A good dehumidifier can handle that kind of opening. Any more thoughts about doing it right and providing a 6" fresh air inlet via an Ultra-Aire ventilating dehumidifier, keeping the entire home <50%RH? Include a small 4" dry air supply to the crawlspace
    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"

  10. #10
    Thanks Teddy Bear. This sure has been an educational process. Thank you for pointing me in the right directions. We keep our temp at 76-78, the RH averages 51-53 when it's in the 90's. We've been in this house for 31 yrs. Since the wife and I are now semi-retired, we're inside quite a bit more than when we worked outside the home. What I've done to the crawlspace, so farut down 2nd layer of 6 mil sheeting, scrubbed all overhead wood. My to do list: mastic plastic sheeting to piers & walls, contract to get all ductwork vacuumed/cleaned, strip old insulaltion off ductwoik, mastic ductwork seams, wrap new insulation. The Ultra-Aire looks like the ideal total fix for our 2800 sf, but we'll need to look at the cost vs benefit over a $150-200 unit. Do you have a guestimate on what I'm looking at for the Ultra-Aire? Also, can you recommend the best system/method for having ducts cleaned?

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