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Community and Q&A

Tight House, High Humidity

foxtrace | Posted in General Questions on

We recently put an addition on our home and installed a mini split system…we have ducts in the floor.  However, the humidity has always been above 65 and as high as 78.  The installer says the problem is that our house is built “too well or too tight.”  They have increased the run time and now say we need to install a dehumidifier.  He’s had us drop the temperature so low that I need a sweater in while in the house.  Originally, we were told we would not need one because it was built to take care of humidity.  The thermostat is a Honeywell and I don’t see a “dry” mode.  Can our house be the cause of the humidity problem, or are we being mislead?  Very confused.

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  1. Expert Member
    PETER Engle | | #1


    It would help us all if you tell us where you live. Outdoor conditions can make a huge difference in how your HVAC system performs. That said, your contractor may be right, or at least part right. People with very tight houses and low A/C needs do report problems with excess humidity. Stand-alone dehumidifiers are sometimes required in these houses. However, I am somewhat dubious that an addition could cause these issues unless the existing house was already super-tight. It is more likely that you have excess indoor humidity sources and/or that you have excess outdoor air infiltration that's bringing warm humid air from outside.

  2. Jon_R | | #2

    Poor part load dehumidification is a common problem in mini-splits. Try to reduce CFM/ton. And install a dehumidifier - any system needs it when it's 75F and 80% outside.

  3. gstan | | #3

    Your contractor, apparently, doesn't have the logic quite right - humidity usually
    refers to relative humidity - the amount of water in the air divided by the total
    amount the air can hold at that particular temperature. Colder air can not hold
    as much as higher temperature air - if you lower the temperature the humidity
    will increase not lower. You are probably going to have to install a dehumidifier.

  4. walta100 | | #4

    In my opinion connecting mini splits to a conventional thermostat is a poor decision.

    It cripples the unit’s ability to modulate its speed forcing it to cycle on or off. You also lose the different modes like the dry mode is seems you desperately need.

    Cold and clammy is the classic symptom of an over sized system! It cools the house below the set point without enough run time needed to remove moisture from the air. When the unit starts does it run for at least 20 minutes?

    Most thermostats do have an adjustment for the number of cycles per hour if you get that set to 3 but the temperature will deviate from the set point with fewer cycles per hour.


  5. foxtrace | | #5

    We have a bosch system. We live in nw VA between warrenton and leesburg

  6. charlie_sullivan | | #6

    The construction process can sometimes leave a structure with significant moisture in it. That may require running a dehumidifier until equilibrium is reached, even if you won't need one in the future. I would run a portable dehumidifier for now, and consider a permanent installation if you end up needing to run it regularly.

    Additional factors:
    1. This time of year can be a challenge for humidity: it's no longer so hot that you need a lot of cooling, so you don't get the dehumidication provided by the cooling process, and it's not yet cold enough for the outside air to be very dry.

    2. What Walta said about a conventional thermostat controlling a minisplit. It's best to use its own control system. If you say more about the minisplit brand and model we might be able to provide more specific advice.

  7. Expert Member
    PETER Engle | | #7

    Also a more detailed description of the overall HVAC layout. Is the minisplit serving just the addition? How large is the addition and what capacity is the minisplit? How is the rest of the house conditioned, and how/when was it built?

    Picking up on Post #3, if the rest of the house is "standard" construction, it is probably very leaky and lots of warm, humid outdoor air can get inside. If you run the addition cooler than the main house, then the relative humidity of the air in the addition will be higher than in the main house. Since it seems that you have some sort of humidity sensor, it could be helpful to measure temperature and humidity in both the main house and the addition.

  8. qofmiwok | | #8

    Tight houses need ventilation to start with, if you have an ERV that will take care of some of the humidity problem. But tight houses almost always need some amount of dehumidification. Some people say "always" but I personally live in 6B and it's a super dry heating dominated climate; my current leaky house is lucky to get to 20% RH. I am building now and will know in a couple years whether a dehumidifier is even needed in my climate if you have a very tight house and good ventilation. (Note I am fully prepared to dehumidify the first year due to construction moisture but I suspect that may be it.)

  9. jackofalltrades777 | | #9

    I have a tight ICF house and have to run a dehumidifier for a few hours each day during peak summer but otherwise an ERV or opening a window on a cool dry night helps a lot. I try and keep indoor humidity to around 45-50% during spring - fall. It's just the nature of the beast with tight houses. A tight house is still an energy win/win as letting a house "leak" via leaky walls, windows and ceilings, is a huge energy penalty and can cause problems within the wall assemblies.

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