Replies

1. GBA Editor
   Martin Holladay | May 31, 2011 05:09pm | #1

   Michael,
   I vote for R-8 duct insulation. Forget the bubble wrap.

2. 
   user-976050 | May 31, 2011 05:19pm | #2

   Michael,

   To stop condensation the first step is to stop the duct leaks. Make sure your ductwork is properly connnected and cinched. Next, seal all seams with a mastic or pressure sensitive tape. "hardcast Iron-grip mastic" or "Hardcast aluma-grip tape". Duct leaks could cause bigger problems that the insulation. It is a must to minimize duct leakage.

   For insulation, 1 1/2" (R-5) Fiberglass ductwrap with a FSK vapor retardent jacket would work well. Seal all seams with an aluminum tape. The key is to ensure that the jacket is continuous. Any exposed ends or gaps will break the vapor seal and cause problems.

   A closed cell foam insulation insulation like Armacell works great and doesn't have the mold issues that fiberglass might, but is expensive and has flame/smoke issues.

   As for Foil-bubble wrap, run away from anyone suggesting to use bubble wrap as duct insulation. I would not trust anything from a foil/bubble wrap manufacturer.

3. 
   user-976050 | May 31, 2011 05:26pm | #3

   Michael,

   3" of fiberglass insulation with a K factor of .27 at 75 deg f would equal R-8. It's a bit of an overkill in a conditioned basement, but it wouldn't hurt.

4. 
   Kgaog6rDhE | Jun 01, 2011 07:31am | #4

   Thanks for the responses. The house is only 5 years old and the tape looks like new. I'll check for leaks before I wrap it. Any tips for sealiing around the multiple supply outlets?

5. 
   user-659915 | Jun 01, 2011 08:02am | #5

   I'm wondering why you're getting condensation on ducts running through a conditioned basement. Especially when you're running a dehumidifier. Sounds like you need to do some serious basement air-sealing and/or waterproofing.

6. 
   Kgaog6rDhE | Jun 01, 2011 08:42am | #6

   Please explain James. The basement is at 70F right now (Basement ac set to turn on at 76F). Relative humidity is 40%. Upstairs ac set at 74F. Upstairs ac supply ducts in ceiling of basement is where the condensation is dripping. Both heat pump inside units are in basement. The basement is unfinished. The basement has three sliding glass doors on two sides (House built into a hill, so its more like a bottom floor.) Any insight into possible problems/solutions would be greatly appreciated.

7. 
   user-976050 | Jun 01, 2011 01:09pm | #7

   I doubt your at 70 deg F and 40% RH. The dewpoint at that temperature and humidity is 44 deg F. Discharge temperature coming off of a air conditioner is typically 55-60 deg F. If it's 55 deg F and you are getting condensation on your ductwork, your RH is likely 60% or higher. I doubt that your dehumidifier can maintain 40% RH at 70 deg F.

8. 
   user-659915 | Jun 01, 2011 11:55pm | #8

   Michael, you say the basement is unfinished but conditioned. If that means you have fiber insulation in the walls but no sheetrock or trim you probably have a pretty air-permeable assembly. If the house above is also leaky at the floor plate and the ceiling and has a conventional vented attic then you'll have a pretty strong chimney effect pulling humid outside air into and through the basement, more than the dehumidifier is capable of dealing with, hence the condensation on your ducts. If this is the case the condensation is just the canary in the coalmine, indicating excessive air leakage through the entire home, in which case you have a larger problem than will be cured by just wrapping the ducts.
   Of course this analysis is just surmise based on the limited available information. I'd recommend getting an energy audit of the whole home to clarify the situation if you haven't done so already.

9. 
   Kgaog6rDhE | Jun 02, 2011 09:25am | #9

   Thanks James, that would explain alot. I wanted to make sure the condensation was fixed before I finished the basement, but that sounds like part of the solution. With that in mind, I plan to:
   1. Seal leaks in ducts, 2. Seal leaks in house, 3. Wrap ducts, 4. Drywall on the walls, 5. Drywall ceiling.
   Any suggestions on what I'm missing? Thanks again James.

10. 
    user-659915 | Jun 02, 2011 09:59am | #10

    Sounds like a plan. Sealing air leaks in the overall enclosure is probably your most critical step in controlling basement humidity issues.