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  1. #1

    a/c duct condensation behind wall insulation

    Hello, I recently moved into a new home. After cutting into the drywall in the garage for another project, I found some mold on the face of the fiberglass insulation and proceeded to tear out a section of drywall and insulation. I found a ton of condensation running down the a/c duct (metal oval pipe). The wall section is 2x6 stud consisting of drywall on both sides, with R13 paper backed fiberglass insulation between the studs. The duct is behind the insulation between the insulation paper and interior drywall of the family room. This leaves about 3 inches air gap between the insulation and the drywall on either side of the duct. I contacted my builder since the house is under warranty and he sent out his HVAC guy. The guy said he didn't know why there would be that much condensation on the inside of the insulation, but said he would wrap the duct in foil backed insulation if my builder would do the drywall work and put back the R13. I previously had a major mold problem in another home that made our family ill so I am paranoid of mold and concerned that this fix won't be good enough long term. I've been advised that hvac ducts should not be on the outside wall like this but I'm not sure if that's true; also someone suggested I remove the drywall and fill all the spaces in the wall cavity with closed cell spray foam. This seems like a good fix but is very costly, and the builder might not go for that so it could be on my dime. Btw, I see from the basement that there are a total of 4 ducts in this wall (I have not removed drywall from other areas to see if the moisture problem is consistent). Any thoughts on the cause of the condensation and good solutions?

  2. #2
    Join Date
    Aug 2007
    Location
    Northern VA 38 degrees N by 76 degrees W
    Posts
    5,060
    Quote Originally Posted by wible1 View Post
    Hello, I recently moved into a new home. After cutting into the drywall in the garage for another project, I found some mold on the face of the fiberglass insulation and proceeded to tear out a section of drywall and insulation. I found a ton of condensation running down the a/c duct (metal oval pipe). The wall section is 2x6 stud consisting of drywall on both sides, with R13 paper backed fiberglass insulation between the studs. The duct is behind the insulation between the insulation paper and interior drywall of the family room. This leaves about 3 inches air gap between the insulation and the drywall on either side of the duct. I contacted my builder since the house is under warranty and he sent out his HVAC guy. The guy said he didn't know why there would be that much condensation on the inside of the insulation, but said he would wrap the duct in foil backed insulation if my builder would do the drywall work and put back the R13. I previously had a major mold problem in another home that made our family ill so I am paranoid of mold and concerned that this fix won't be good enough long term. I've been advised that hvac ducts should not be on the outside wall like this but I'm not sure if that's true; also someone suggested I remove the drywall and fill all the spaces in the wall cavity with closed cell spray foam. This seems like a good fix but is very costly, and the builder might not go for that so it could be on my dime. Btw, I see from the basement that there are a total of 4 ducts in this wall (I have not removed drywall from other areas to see if the moisture problem is consistent). Any thoughts on the cause of the condensation and good solutions?
    Air leakage in the exterior wall is allowing the moisture to condensate on the bare metal. Rapping the metal as suggested will eliminate the problem, but all 4 should be done

  3. #3

    a/c duct condensation behind wall insulation

    Quote Originally Posted by second opinion View Post
    Air leakage in the exterior wall is allowing the moisture to condensate on the bare metal. Rapping the metal as suggested will eliminate the problem, but all 4 should be done
    Thanks second opinion...I didn't mention that since this is a retrofit and the ducts are pressed tight against the inside wall (drywall), that I'm not sure the contractor could wrap them very well, allowing the air infiltration / condensation to potentially still be a problem. Being an oval pipe tapering into the wall seems like it may complicate the problem and prevent a good seal?

  4. #4
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,336
    All insulation on cold ducts need an air tight, low perm rate exterior clading prevents moisture from migrating to the cold duct surface. The insulation needs to be thick enough to stay above the dew point of the surrounding air. The air temperature in the ducts should be no colder than needed to dehumidify the space. 48^F cold air supply is usually adequate to dehumidify the space. Check the temperature of the cooling coil and supply air temp at the cooling coil discharge. VS fans, two speed a/cs, VS a/cs in an attempt to dehumidify may make the supply air/ducts very cold causing the condensation problem.
    Closed cell foam insultion like Rubbitex is an alternative to closed cell spray foam.
    Check your temp/%RH in the home and at the closest supply outlet to start with.
    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"

  5. #5
    Join Date
    Jul 2008
    Location
    Rochester NY
    Posts
    4,721
    DOE/Steven Winter Assoc has recently completed a study and built standards for spray foam insulating duct in attics. I attended a webinar where the author, Bill Zoeller presented their findings. It was fantastic:

    http://bit.ly/sprayfoamducts

    Early adopters will need to be significantly involved in the process to insure quality, probably having to lead the contractors until these strategies and best practices for implementation trickle to the field.

    Attachment 327661 Attachment 327671 Attachment 327681 Attachment 327691

    You can encapsulate unburried duct. It doesn't meet Challenge Home requirements so you wouldn't design this way, but apparently is a significant improvement for retrofit:

    Attachment 327701

    Here's my spray foam picture album for those interested.
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    DO THE NUMBERS! Step on a HOMESCALE.
    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to BPI.org, or RESNET, and find an auditor near you.

  6. #6
    Join Date
    May 2008
    Location
    Missouri
    Posts
    3,476
    Sounds like you also have unconditioned air entering into these wall cavities in addition to not having the ductwork insulated. My guess would be that these cut outs were not sealed at bottom or top.

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