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  1. #1

    yet another thread about condensation...

    I have a 2 story brick house in St Louis (and it gets hot and humid here in the summer). It's a 100 year old house with a flat roof, and has a 4 ton AC unit. The AC unit and Heat exchanger and everything else in the basement is about ten years old. I have my HVAC guy come every year and service it.

    In addition to the AC, I run a dehumidifier in my basement in the summer. I figure that helps keep things dry and takes some strain off the AC (feel free to correct me on this).

    The second floor was rehabbed about 15 years ago and the ductwork was run along the ceiling and has a drywall bulkhead built around it.

    For the last 2 or 3 years the drywall bulkhead upstairs has been dripping and has just gotten worse. Last summer I ripped out some of the drywall around the ductwork hoping it would stop the condensation. However, the condensation never went away. I discovered the ductwork is totally uninsulated.

    So I am ready to do something about it before summer comes again. I have read as many threads as I could find here regarding condensation and I didn't see my exact situation dealt with (lots of problems with crawlspaces though).

    The first 3 or 4 years in this house there were no signs of condensation. Would something cause this condensation to start happening? What is a good way to remedy this problem?

    I guess one more thing, the roof was leaking last spring. I had a new roof put on and no more leaks. And the condensation kept happening. So I am pretty certain there is no source of moisture other than what's in the air.

    Sorry for the long post, just trying to give as much info as possible. I appreciate any advice I can get.

  2. #2
    Join Date
    Sep 2009
    Location
    Arnold mo
    Posts
    3,967
    Was the bulkhead built with new lumber? It may have been air tight when built, but over time the lumber dried out and pulled away from the drywall. There was a pro here some time ago that mentions that his company insulates ductwork in attics with spray foam. If you are willing to spend a few bucks more, I would look into installing exterior insulation on that roof and air seal & close off that small attic area under it off. This would stop the condensation and likely reduce your cooling cost and make the upstairs more comfortable.
    An answer without a question is meaningless.
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  3. #3
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,447
    The moisture level or dew point of the air coming contact with a cold surface is above the surface temperature, condensation occurs. You have a certain amount of fresh air infiltration containing moisture entering the home. Inside the home, the occupants are adding moisture. Your a/c and dehumidifier are reducing the dew point of the air.
    Most occupants like <50%RH inside the for comfort. At 75^F, 50%RH, you have a 55^F dew point. Most a/cs produce <55^F air which cools the ducts <55^F which may cause condensation.
    You can insulate your ducts with an airtight insulation or reduce the dew point of the surrounding the duct. Warming the air inside the duct is not recommended because you depend on the a/c to remove most of the moisture in the home.
    I would track the temp/%rh during cooling to assure <50%RH. During wet cool weather, the a/c may not run enough to remove the mositue in the home. Getting a whole house dehumidifier may also reduce the dew point in the home. There adhesive backed insulations that can be applied to the metal duct surface that will reduce condensation.
    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
    Quote Originally Posted by tipsrfine View Post
    Was the bulkhead built with new lumber? It may have been air tight when built, but over time the lumber dried out and pulled away from the drywall. There was a pro here some time ago that mentions that his company insulates ductwork in attics with spray foam. If you are willing to spend a few bucks more, I would look into installing exterior insulation on that roof and air seal & close off that small attic area under it off. This would stop the condensation and likely reduce your cooling cost and make the upstairs more comfortable.
    Thanks for the reply. I do not know if the bulkheads were built with new lumber. It seems likely though. The attic area isn't really accessible and it's probably about a foot or two from the top of the ceiling to the roof decking. I do know that these flat roofs are designed to have an airspace between the ceiling and the roof decking. There are vents in either side of the house to allow for air flow.

    The ductwork isn't installed in this space, sorry if I wasn't quite clear. It is mounted to the ceiling and wall, and then the bulkhead is built around the ductwork. Obviously, it's not a perfectly sealed area and some hot air from the air space ago the ceiling can get in. It sounds like you are recommending that I insulate the ductwork from the untreated air space, correct? Spray foam sounds like a good option. I'll look into that. Thanks!

  5. #5
    Quote Originally Posted by teddy bear View Post
    The moisture level or dew point of the air coming contact with a cold surface is above the surface temperature, condensation occurs. You have a certain amount of fresh air infiltration containing moisture entering the home. Inside the home, the occupants are adding moisture. Your a/c and dehumidifier are reducing the dew point of the air.
    Most occupants like <50%RH inside the for comfort. At 75^F, 50%RH, you have a 55^F dew point. Most a/cs produce <55^F air which cools the ducts <55^F which may cause condensation.
    You can insulate your ducts with an airtight insulation or reduce the dew point of the surrounding the duct. Warming the air inside the duct is not recommended because you depend on the a/c to remove most of the moisture in the home.
    I would track the temp/%rh during cooling to assure <50%RH. During wet cool weather, the a/c may not run enough to remove the mositue in the home. Getting a whole house dehumidifier may also reduce the dew point in the home. There adhesive backed insulations that can be applied to the metal duct surface that will reduce condensation.
    Regards TB
    Good to know. So with the insulation, do you think some type of spray foam could prevent me from having to have the bulkheads torn down? Or do you recommend just biting the bullet and having all of this redone?

    Why would the guys who rehabbed this house not insulate the ductwork? It seems like a fairly obvious thing to do. Oh never mind. I think I can answer that question

  6. #6
    Join Date
    Mar 2005
    Location
    Suppy NC
    Posts
    4,513
    did they run any water lines in this space with the ductwork they may possibly be leaking from electrolysis

  7. #7
    Quote Originally Posted by tinknocker service tech View Post
    did they run any water lines in this space with the ductwork they may possibly be leaking from electrolysis
    nah, there are no water lines there.

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