Air Conditioning Ducts Leaking Water: Prevent Condensation

jwilson asks,

I have a leak from my ceiling in my hallway. Last fall I had two plumbers come out to diagnose the problem and neither could find a leak. The last plumber suggested that I call an air duct contractor as there was a drip from the air duct pipe leading from my dryer to the outside. My home was built in the 50’s. The washer/dryer sits behind bifold doors in the hallway. The pipe from the dryer runs up the back wall of the dryer and across the ceiling to the back. The a /c unit sits next to the dryer. The air duct contractor removed and replaced the rusted out air duct pipe. Then just a couple months ago the drip started again just as it did before. I called the air duct contractor but the air duct pipe was fine. He suggested that the air duct pipe and the a/c pipe were too close together and the heat and cold were causing condensation to build in the air duct which then dripped. To help resolve it he wrapped insulation around the a/c pipe. All was well until one day I heard this swish-sound. I went into my hallway and there was a puddle of water on the floor and the insulation was sticking out of the hole that I originally cut in my ceiling when I was trying to figure out what the problem was. Do you have any suggestions. Thank you.

Our Answer

Thanks for all the details. From your description, this does seem to fit the pattern of a condensation leak caused by warm, humid air striking the cold air conditioning ducts. The fact that your dryer exhaust runs so close to this could also be related, especially if any of that very warm and humid exhaust is leaking out along the way.

The solution is simple and complicated at the same time. If you have access to this attic space, you can insulate the ducts, which I know you tried. However, you need to use a duct insulation that has a built-in vapor retarder, such as this Johns Manville Duct Insulation product. Plus, you need to insulate ALL the ducts because condensation can form anywhere and run to the lowest spot to leak out. Lastly, its important that you seal all the seams in the insulation with silver foil tape (NOT “duct” tape! It will dry out and fall off)

In addition, it may help if you improved the attic ventilation. While the relative humidity would go unchanged, more air movement in the attic might increase evaporation a result in less accumulation of moisture on the ducts.