Replies

1. 
   Jon_R | Nov 23, 2018 10:12am | #1

   Fiberglass is a bad choice since it keeps it cold while allowing moist air to get to it. Use two part spray foam, covering both the boot and any remaining outside air leaks in the rim joist.

   Also measure the duct air temperature. Low air flow rates will cause this to be excessively low, causing more condensation. Should be about 57F (which would cause no condensation at your reported 50% RH).

   Also look into having some more air sealing done (this will reduce your need to run a dehumidifier all the time). Or possibly the problem is other moisture sources.

   Why does this boot condense more than other ducts exposed to similar conditions? Verify that there isn't a water leak in the boot area with you having cause and effect reversed.

   On a speculative note, it's not clear to me why centrally ducted AC systems don't have an A-coil air bypass such that the coil can be optimally cold (good for latent removal) and yet the supply ducts can be warmer (say 62F, where you won't get condensation even at 60% RH). Eg, 520 CFM/ton in the ducts but only 400 CFM/ton of that passing through the A-coil.

2. Expert Member
   Dana Dorsett | Nov 23, 2018 01:22pm | #2

   If you air seal and insulate the basement walls (with rigid foam against the foundation, not just fiberglass batts in a studwall) the air conditioning will do most of the drying of the basement. In most basements in the "-A" climate zones the primary summertime humidity source is outdoor air infiltration.

   The sensible cooling loads of basements are low, possibly even negative most of the time in zone 3A. Ventilating your insulated-sealed basement with conditioned space air from the rest of the space rather than a coolling duct register would allow the central AC remove the latent load without excessive sensible cooling. If the basement is well air sealed it doesn't have to be much, a continuous 20-40 cfm of air exchange from the fully conditioned upstairs to the basement would probably be enough to suppress the duct condensation even without running the dehumidifier.

1. 
   BrianPontolilo | Nov 23, 2018 06:53pm | #3

   Hi Dana,

   Jamie said that the rim and walls are already insulated and air seal. Assuming that the work is done well, I'm curious how you would set up the air exchange between the basement and the rest of the house that you are proposing. And also, do you think his plan to condition the basement with a mini-split will help the situation?

1. Expert Member
   Dana Dorsett | Nov 24, 2018 07:09pm | #5

   A small efficient bath fan such as the Panasonic Whispergreen series bathroom drawing from one corner of the basement ducted into the return duct of the central HVAC and a floor grille on the first floor to the basement in an out of the way location on the far corner of the house to supply conditioned air into the basement would work. When the AC is running it will be pulling additional humid moisture from the basement into the AC coils, which will dry that air, and the floor grille will send dry but not super-cooled air into the basement. When the AC isn't running, output of the Whispergreen will be distributed to the better-conditioned upstairs primarily via the return registers at very low cfm per register, with a very small amount distributed out the HVAC supply registers.

   Even the smallest mini-splits are likely to be oversized for the cooling and heating loads of a basement. The cooling load is predominantly latent and would be likely be totally covered by the central air system drawing air into a return register in one corner of the basement, supplying only conditioned air, not HVAC supply air to the basement from a floor grille, with or without the Whispergreen whenever there's a real cooling load, but it may still need the Whispergreen during the low-load shoulder seasons.

   To get a handle on the heating load would require calculating the load using Manual-J or I=B=R methods, but with insulated basement walls and not much window area the heating load will be "not much". A hydronic baseboard loop running off the water heater, with the baseboard sized correctly for the tiny heating load wouldn't have much negative impact on domestic hot water performance, and could get it done pretty cheaply.

3. 
   JRoyal | Nov 24, 2018 12:46pm | #4

   Hey guys,

   Thanks for the replies. I only touched briefly on it in my original post, but as Brian mentioned, I've air sealed and insulated the rim joists and walls with rigid foam and one-part spray foam (you can see the rim joist part in the corner of the picture I attached). As any good GBA'er :), I've air sealed penetrations from piping and electrical through the basement ceiling. The basement is a walkout, with 3 poured concrete walls and one framed. The assembly for the concrete walls will be Concrete wall>1" rigid foam>1/2 air gap>2x4 framed wall>R13 or R15 fiberglass batts. Everything but the batts are in place right now.

   I think I've seen Martin comment there isn't a concern about fibrous insulation as long as the air sealing is done prior to installation. I plan to use batt insulation in the ceiling and partition walls of the basement for sound proofing purposes (mineral wool is also an option, but for this conversation I think they are the same).

   I guess I'm asking if I'm sweating (pun intended) this for nothing since the 1st floor ducts are going to be sandwiched between two conditioned spaces. Or if this is likely to keep happening, then I need to figure out a solution. Either spray foam the boot, which I have a strong preference against, or insulate it somehow.

   I'll also check the forced air supply temperature next time I have a chance (its pretty cold right now). I would also like to check the air volume, as I've read low volume can cause excessively cold temperatures, which would contribute to condensation problems. However, I haven't found a cheap DIY way to do that yet.

   Thanks for the input and help!
   --Jamie