Results 1 to 11 of 11
  1. #1
    Join Date
    Jul 2013
    Posts
    4
    Post Likes

    Condensation on air ducts between floors, causing mold

    We have a 2-floor house built in 1978 in the DC area, which is hot and very humid in the summer. We've recently had a slight moldy smell, and water on the 1st-floor ceiling. One of the upstairs closets has a hatch in the floor that lets us look in between the lower ceiling and the upper floor. It turns out that the air-conditioning ducts in that space are covered in condensation which is dripping down onto the drywall and wood below.

    I have measured humidity in this inter-floor space to be 80+% and the temperature is in the high 70's which is hotter than the air inside the house. This leads me to believe that hot, humid outside air is getting into this space somehow, and this is causing the condensation. There are a couple issues that an HVAC guy and a contractor noticed that may be causing outside air to get into this space:

    -the 2nd floor overhangs the 1st floor a little in the back and there are a couple vents under the overhang. They appear to vent into the inter-floor space. An HVAC guy said he thought that was unusual, that attics normally have vents but he doesn't think there should be vents between the floors. He covered these vents with duct tape as experiment to see if that stopped it, but it hasn't stopped the condensation problem after a day and a half.

    -there is an air return duct that runs down from the 2nd floor though the inter-floor space, right next to the hatch that lets us look down in there. A general contractor noticed that there is an opening in this return, covered by a grating. He thought this was unusual. This vent is sucking air from between the floors, into the return duct and the AC system. If it's sucking air out of this space, air must be coming into this space somewhere else. So, it seems likely that the negative pressure from this return vent is sucking air from the outside into the space between floors. The contractor suggesting having this vent covered up.

    For a little history:
    Last year we had big mold problems on the ceiling of the first floor. We had mold remediators and other come to the house and they believed it was caused by a leak in the roof around a bathroom exhaust vent. They re-did some bad flashing around that vent and fixed other roof issues that they thought were causing the mold. The first floor is slightly bigger than the 2nd, so it has a roof area above part of it and that's where they thought water was leaking in. It appears that they were wrong about the source of the moisture, or that there may have been a roof leak but it wasn't the only source of moisture. We spent a bunch of money on mold remediation.

    We need to definitively identify and fix the source of this moisture so don't end up repeatedly spending thousands of dollars on repairs every summer. Nobody seems to be an expert at finding the causes of moisture that lead to mold. The mold remediator does repairs but doesn't seem to have much knowledge of how to determine the cause of the moisture other than obvious liquid water leaks. He told as to consult an HVAC company. The HVAC guy and a general contractor were basically scratching their heads and making guesses.

    I've read some stuff about insulating AC ducts, but to do that would require ripping apart much of the house. This condensation is happening in several places over a large area, and it may be happening in other areas that we can't see. It seems like a better approach would be to fix whatever is causing the humid air in this space.

    Does anyone have any ideas? Should we close up that return vent between the floors, like the contractor suggested, or do something else?

  2. #2
    Join Date
    Mar 2009
    Location
    Mount Holly, NC
    Posts
    7,707
    Post Likes
    Blower door test.

    it'll measure the amount of outside air entering the house.

    a good company should be able to find the leakages too.
    The TRUE highest cost system is the system not installed properly...

    Find a HVAC-Talk Contractor by clicking here

    Click below to BECOME a pro member
    https://hvac-talk.com/vbb/forumdispl...ip-Information

    Do you go to a boat repairman with a sinking boat, and tell him to put in a bigger motor when he tells you to fix the holes?

    I am yourmrfixit

  3. #3
    Join Date
    May 2004
    Location
    south louisiana
    Posts
    3,790
    Post Likes
    when you wrote that the 2nd floor overhangs the 1st floor
    it brought this article to mind:
    http://www.energyvanguard.com/blog-b...lding-Envelope

    if between floor areas are open to the outdoors under the cladding of the house
    it causes things like you describe.

    what type cladding do you have?
    can you make an access @ this area to see how/if the transition was sealed?

    could you post some pictures of the overhang of 2nd floor from exterior?

    sorry to hear about $$$ spend on mold remediation with no correct cause
    of moisture defined. I hear this often.
    my plan of action has always been:
    find moisture source.
    eliminate moisture intrusion...then
    determine extent of remediation.

    best of luck
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  4. #4
    Join Date
    Jul 2013
    Posts
    4
    Post Likes
    Thread Starter
    Quote Originally Posted by vstech View Post
    Blower door test.

    it'll measure the amount of outside air entering the house.

    a good company should be able to find the leakages too.
    What kind of company? HA or something else?

    I'll try to get some pictures tomorrow.

  5. #5
    Join Date
    Jul 2013
    Posts
    4
    Post Likes
    Thread Starter
    Quote Originally Posted by hvactert View Post
    What kind of company? HA or something else?

    I'll try to get some pictures tomorrow.
    That should have read "HVAC or something else?"

  6. #6
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    17,107
    Post Likes
    HVAC or an Energy auditing company. www.resnet.us www.bpi.org www.comfortinstitute.org

  7. #7
    Join Date
    Jul 2013
    Posts
    4
    Post Likes
    Thread Starter
    I scheduled an energy auditor to come on Monday for a free consultation. People don't seem to work on weekends.

    I just turned off the attic fan that was installed last year. I'm reading a lot of bad stuff about attic fans drawing outside air into houses.

  8. #8
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    17,107
    Post Likes
    Quote Originally Posted by hvactert View Post
    I scheduled an energy auditor to come on Monday for a free consultation. People don't seem to work on weekends.

    I just turned off the attic fan that was installed last year. I'm reading a lot of bad stuff about attic fans drawing outside air into houses.
    Not only do they pull hot outside air into houses but they pull air conditioning out of the house and into the attic.

  9. #9
    Join Date
    May 2004
    Location
    south louisiana
    Posts
    3,790
    Post Likes
    I have an 8 am mold house saturday morning...not everyone takes off weekends.
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  10. #10
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    9,621
    Post Likes
    Quote Originally Posted by hvactert View Post
    We have a 2-floor house built in 1978 in the DC area, which is hot and very humid in the summer. We've recently had a slight moldy smell, and water on the 1st-floor ceiling. One of the upstairs closets has a hatch in the floor that lets us look in between the lower ceiling and the upper floor. It turns out that the air-conditioning ducts in that space are covered in condensation which is dripping down onto the drywall and wood below.

    I have measured humidity in this inter-floor space to be 80+% and the temperature is in the high 70's which is hotter than the air inside the house. This leads me to believe that hot, humid outside air is getting into this space somehow, and this is causing the condensation. There are a couple issues that an HVAC guy and a contractor noticed that may be causing outside air to get into this space:

    -the 2nd floor overhangs the 1st floor a little in the back and there are a couple vents under the overhang. They appear to vent into the inter-floor space. An HVAC guy said he thought that was unusual, that attics normally have vents but he doesn't think there should be vents between the floors. He covered these vents with duct tape as experiment to see if that stopped it, but it hasn't stopped the condensation problem after a day and a half.

    -there is an air return duct that runs down from the 2nd floor though the inter-floor space, right next to the hatch that lets us look down in there. A general contractor noticed that there is an opening in this return, covered by a grating. He thought this was unusual. This vent is sucking air from between the floors, into the return duct and the AC system. If it's sucking air out of this space, air must be coming into this space somewhere else. So, it seems likely that the negative pressure from this return vent is sucking air from the outside into the space between floors. The contractor suggesting having this vent covered up.

    For a little history:
    Last year we had big mold problems on the ceiling of the first floor. We had mold remediators and other come to the house and they believed it was caused by a leak in the roof around a bathroom exhaust vent. They re-did some bad flashing around that vent and fixed other roof issues that they thought were causing the mold. The first floor is slightly bigger than the 2nd, so it has a roof area above part of it and that's where they thought water was leaking in. It appears that they were wrong about the source of the moisture, or that there may have been a roof leak but it wasn't the only source of moisture. We spent a bunch of money on mold remediation.

    We need to definitively identify and fix the source of this moisture so don't end up repeatedly spending thousands of dollars on repairs every summer. Nobody seems to be an expert at finding the causes of moisture that lead to mold. The mold remediator does repairs but doesn't seem to have much knowledge of how to determine the cause of the moisture other than obvious liquid water leaks. He told as to consult an HVAC company. The HVAC guy and a general contractor were basically scratching their heads and making guesses.

    I've read some stuff about insulating AC ducts, but to do that would require ripping apart much of the house. This condensation is happening in several places over a large area, and it may be happening in other areas that we can't see. It seems like a better approach would be to fix whatever is causing the humid air in this space.

    Does anyone have any ideas? Should we close up that return vent between the floors, like the contractor suggested, or do something else?
    Second story overhangs are notorious for air leakage plus you have vents. I would inject foam in the ends of the joist cavities to form air dams to stop any infiltration all around the home.
    What is the temp/%RH of the return air and supply air? During high cooling loads, the a/c should maintain <50%RH in the indoor space. If the indoor %RH is much lower during high cooling loads, the a/c coil maybe too cold, <45^F. This makes the ducts sweat. Increasing the air flow of the a/c will make the ducts warmer. The house will have a high %RH but less potiential for duct sweating. What is happening in the basement, temp/%RH?
    If the home has large negative pressure, damp air is sucked from any leaks. attic exhaust, kitchen hoods, and clothes driers are all negative pressure generators.
    Feed back helps all of us.
    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"

  11. #11
    Join Date
    May 2004
    Location
    south louisiana
    Posts
    3,790
    Post Likes
    it will take someone operating the blower door that has experience with
    open joist bays on exterior of house. otherwise it will just be percieved
    as excessive leakage of interior walls.

    take a look at the article I posted.

    best of luck.
    The cure of the part should not be attempted without the cure of the whole. ~Plato

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •