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First steps in reducing marginal summer humidity

andy719 | Posted in Energy Efficiency and Durability on

I’m looking to do a little summer humidity control and wondering the first steps to try, if anything is needed at all.  It’s a code built house in CZ4, Maryland, built in 2021, under the 2018 IRC and IECC. Blower door tested at 2.5ach50. I have just under 5000sf (two stories combined) above grade, with a full basement. One heat pump for the top story, and one for the middle and basement combined. All floors are open to each other though due to the open stair.

I have temp/humidity monitors around the house, and the basement stays about 5% higher humidity than the upper floors all year. Levels last year were winter (35% upstairs/40%bsmt), shoulders (45/50%), and summer (65/70%). I have a supply-only fresh air intake attached to the lower level hvac duct return system that opens its damper every time the blower fan runs. 

I have not seen any evidence of condensation, and I can deal with comfort level in the summer with ceiling fans.  I don’t know enough to know if I should be worried about having 65-70% humidity for 3 months a year though.  If so, what options should I start with:

1.  If I’m comfortable enough, don’t worry about it.
2. The company that makes the damper for the fresh air intake also makes an outdoor mounted temp/humidity controller to only allow it to open in a certain range, but with a minimum daily run time safety control. Get that installed.
3. Try a portable dehumidifier in the basement.
4. Assume no portable will be big enough, and go straight to whole house dehumidifier.

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  1. walta100 | | #1

    My first move would be to close the fresh air input as it is likely adding moisture in the cooling season and I think it is unnecessary in a large fairly leaky house unless you have 15 people living in the home.

    I would reactivate the system if you started seeing winter humidity upstairs above 50%.

    If you think the basement is starting to smell musty, I would get a portable dehumidifier for the basement with the understanding that 3 years is a long life for the stuff sold today.


    1. andy719 | | #2

      Thanks Walta. We have 7 people in the house (4 adults and 3 kids). It only feels like 15 some days.

      The fresh air damper is at the air handler, and has about 30 feet of insulated flex duct connecting it to the exterior wall cap. I assume the damper is normally closed as the indicator light only powers when the fan is running. If that's the case, should I just disconnect it and leave the damper closed, or do I need to cap it at the building envelope?

  2. walta100 | | #3

    Sounds like unplugging the wires should do the trick.

    I would not do anything permanent until you are happy with how things are working.


  3. Expert Member
    DCcontrarian | | #4

    What are the temperatures like? Those humidity numbers would be what you would expect if the dew point of the entire house was the same and the basement was a few degrees cooler, which is pretty typical. In that case, if you want the basement to be less humid you need to either warm the basement more (tricky in summer) or dehumidify the whole house. A standalone dehumidifier in the basement will do that.

    1. andy719 | | #5

      Yeah, the top 2 floors stay about the same temperature and the basement is pretty consistently 2-3 degrees lower. So around 73 vs 70 in the summer and 68 vs 65 in the winter.

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