Replies

1. GBA Editor
   Martin Holladay | Sep 22, 2016 05:16pm | #1

   Clay,
   A bathroom exhaust duct should be sloped so that it drains to the exterior termination. If any condensation occurs on the interior of the duct, the moisture will drain. There is no reason to expect any condensation on the exterior of the duct.

2. 
   whitenack | Sep 22, 2016 05:31pm | #2

   Oops. Sorry Martin. I was meaning the HVAC ducts.

3. GBA Editor
   Martin Holladay | Sep 23, 2016 04:38am | #3

   Clay,
   Sorry. I misunderstood. It wouldn't hurt to use insulated ductwork (or to wrap the duct in duct insulation) in this situation. It's an inexpensive step.

4. 
   whitenack | Sep 23, 2016 11:51am | #4

   Thanks Martin. How much is enough? The R6 duct sleeves you find at the big box stores?

5. GBA Editor
   Martin Holladay | Sep 23, 2016 12:56pm | #5

   Clay,
   That's plenty.

6. 
   whitenack | Sep 29, 2016 03:45pm | #6

   Hey Martin, on a related question, the HVAC sub is suggesting that I put insulation on all the ducts, thinking I will have risk of condensation not only in the bathroom but everywhere. Thoughts? How cold is the supply air during cooling mode? We have pretty humid summers here in central KY.

7. GBA Editor
   Martin Holladay | Sep 30, 2016 07:45am | #7

   Clay,
   In general, what you want to avoid is the chance that humid outdoor air will contact a cold duct. Your indoor air should be relatively dry, so there isn't much worry about condensation if all you are talking about is indoor air (rather than air in a vented attic).

   That said, I'm happy to defer to GBA readers who have experience building in hot humid climates, since my building experience is in Vermont.

8. 
   whitenack | Sep 30, 2016 07:57am | #8

   Thanks Martin. The HVAC sub used the analogy of a cold glass of water sitting on your kitchen counter. The cup is inside the conditioned space, but the glass still drips with condensation. He said the ducts will do the same thing. This makes sense to me, but I guess it depends on how cold the surface of the ducts will get compared to how cold it would need to be to develop condensation. A cup of water is quite a bit colder than the ducts, I would imagine, I just don't know how much.

9. 
   user-626934 | Sep 30, 2016 08:41am | #9

   The main risk factors with uninsulated sheet metal ducts inside conditioned space in a humid climate are:

   1) Inconsistent or strange occupant behavior - such as open windows on a mild, very humid day, then AC set at 68F the following day.

   and

   2) Low airflow due to poor design/install and/or loaded filters: low airflow = colder supply air = colder duct surface = increased potential for condensation.

   Room air at 75F and 50% RH has a dewpoint of 55F. Typical temperature drop across a heat pump heat exchange coil in cooling mode is in the range of 15F to 20F (but much less than this if you're dealing with variable speed compressor running less than 100%). If you're on the high end of that temperature drop range with return air at 75F, then supply air would be at 55F....definitely in the risky zone.

10. 
    whitenack | Sep 30, 2016 10:43am | #10

    Thanks John, that is good information.

    Regarding factor #1, I could imagine being a contributor to inconsistent and strange occupant behavior you describe.

    Regarding factor #2, I have had the ducts professionally designed. I assume they are correct, but I am trying to double check. Do the same rules of thumb for air velocities apply to ducted mini splits as they do traditional systems? Keep your trunks and branches in the 600-700 FPM range and the supply registers something less than that?