GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

High humidity problems

User38 | Posted in General Questions on

Hi. I would appreciate any assistance anyone has to offer. My home, which is supposedly very well insulated, often has humidity levels in the low 60s% and feels wet.  

I have two Carrier HVACs: one in basement and one in attic above second floor. I don’t have a dehumidifier. The system only dehumidifies by cooling. The home cools well. Downstairs kept around 73 and upstairs kept around 72. Due to covid, we have been spending more time at time, so the house is almost always at these temps.  

We have set the HVAC fans to always run. I have tinkered with the speeds in which they run, but that hasn’t seemed to solve the issue. We do have the overcool-to-dehumidify option on.  

Also, perhaps related, the attic has been experiencing very sweaty ducts, often noticed when the system is working hard to cool in order to dehumidify. The attic is insulated. We conditioned the attic several months ago to try to stop the sweating ducts, but that has not worked and ducts still occasionally sweat  

I would appreciate any advice on how to lower the humidity in the time and how to prevent leaky attic ducts. I am happy to provide more info. as needed.


GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    NICK KEENAN | | #1

    What climate are you in and what time of year does this happen?

    1. User38 | | #3

      I am in DC. It happens most of the year. It’s 63 degrees outside and 64% humidity inside the house this morning.

      1. Expert Member
        NICK KEENAN | | #4

        You need to run a dehumidifier. I'm in DC, I'm running a dehumidifier in my basement right now. I leave it on year-round but it doesn't run much in the winter.

        Since your house is well insulated the HVAC doesn't run enough to dehumidify. The dehumidifier will produce heat in addition to removing humidity which should cause the AC to kick on more often which will help with comfort too. With a dehumidifier you should be able to raise the thermostat to 76F and have the same comfort level so you'll end up using less energy.

  2. Expert Member
    NICK KEENAN | | #2

    Running the fan continuously probably makes the problem worse. When the AC finishes a cooling cycle there will be some condensate on the coil. Running the fan will cause that to evaporate and return to the air rather than drain away.

    If you're experiencing the problem when it's humid outside then air infiltration is probably part of the problem. Running the fan creates areas of high and low pressure within the house that increase infiltration.

  3. Expert Member
    BILL WICHERS | | #5

    +1 for not running the fan all the time in most cases. The fan alone won't do anything for the humidity levels, but it will make any air leaks worse if the ductwork is leaky and traverses through different areas of the structure (such as pulling air in from basements or attics due to leaky air returns).

    DC is entirely correct about the fan evaporating any water off the A/C coil too. For dehumidification purposes, you don't want the fan to run after the compressor in the A/C shuts off, because the fan will re-evaporate a lot of the water that condensed out on the cold A/C coil. You want that condensate to run out of the unit as liquid, not be evaporated back into the air to bring the humidity levels back up.

    If this persistant high humidity is an ongoing problem, you're probably a good candidate for a whole-home dehumidifier unit.


  4. walta100 | | #6

    You may want to set the upstairs unit to a lower temp than the down stairs unit and keep your upstairs doors open. The cold air from upstairs will tend to fall down the stairs with the doors open the upstairs unit will run almost constantly allowing it to remove as much moisture as possible.

    Duct work in an attic vented attic is the worst possible choice. The best option is to get the equipment and ducts into the current conditioned space if at all possible put the unit in a central closet and keep the ducts below the ceiling or go ductless.

    A somewhat better choice is to condition the attic by moving the insulation to the roof line. This option is only marginally better because you must condition so many more square feet and getting more than code minimum R values is almost never happened because it is so expensive.

    If that is not in the budget now sealing the joints on the old ductwork could make a huge improvement for a few hours of work. Clean the joints with denatured alcohol and cover the joint with foil tape. Note regardless of its name cloth duct tape is not good for use on ductwork.

    Blower door testing before and after 20-40 man hours of air sealing your house would likely help a lot.


    1. Expert Member
      NICK KEENAN | | #8

      In the original post he wrote: "We conditioned the attic several months ago to try to stop the sweating ducts, but that has not worked and ducts still occasionally sweat."

  5. Expert Member
    NICK KEENAN | | #7

    What you are describing are the classic signs of an oversized AC. The reason we're recommending dehumidifying is that it's hard to downsize an AC short of taking the whole thing out and installing a smaller one. Plus in the DC environment you often need supplemental dehumidification anyway.

    There are a couple of ways to make an AC work as if it was smaller, essentially reducing the capacity. Lowering the fan speed is the biggest. If you have separate zones upstairs and downstairs you could turn off the downstairs. But the effectiveness varies and rarely solves the problem.

    1. User38 | | #9

      Thanks. This very helpful and much appreciated. It does sound like my next best option is to consider a dehumidifier. Since you are local, any chance you have any good recommendations for someone who may be able to help troubleshoot?

    2. Expert Member
      BILL WICHERS | | #12

      You need to be very careful lowering fan speeds. Too little airflow and the coil will freeze up, and that's a Bad Thing.


  6. walta100 | | #10

    I did miss that the attic is now conditioned.

    Since the ducts are still sweating in the conditioned attic. It seems like it needs more supply and or returns vents the attic. The attic should be more or less the same temp and humidity as the rest of the conditioned space.

    Was the old insulation removed from the attic floor?

    What is the R value of the roof insulation?


    1. User38 | | #11

      The attic space is quite cool now that we conditioned it. It seems to be close to the second floor temp.

      I unfortunately don’t know the R value.

  7. walta100 | | #13

    Was the old insulation removed from the attic floor?

    How thick is the new insulation?


    1. User38 | | #14

      I don’t believe there is insulation under the attic floor. The insulation at roofline above attic is spray foam.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |