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Community and Q&A

How can I improve indoor humidity?

lgr123 | Posted in General Questions on

My home has high indoor humidity in the summer (~60% to 65%) and low indoor humidity in the winter (~15% to 25%). I live in climate zone 5a, and the indoor humidity more or less follows the outdoor humidity. It’s humid in the summer here, and dry in the winter. I would like to keep the indoor humidity between 30% and 50% year-round if possible.

Is this range of indoor humidity (~20% to 60%) typical and unavoidable in a conventional home in this type of climate? The home is a relatively new (just a few years old) standard stick built home. It’s about 1500 sq ft, one level, and has a heat pump and electric furnace. I added a whole-house dehumidifier (Honeywell DR90) and an ERV a few years ago. The dehumidifier reduced the humidity in the summer somewhat, but not enough. I realize that the ERV brings in outdoor air, but I have shut it off for weeks at a time to see if it makes a difference in humidity. It makes only a small difference as far as I can tell (maybe a few percent, not much). I am beginning to wonder if my house is leaky enough that I don’t even need the ERV.

What is the best approach to try to improve humidity levels? I’ve read that air-sealing gaps in the floor of the attic (between the drywall and framing, just above the indoor ceiling) might help. Of course, it’s a lot of hard work and it’s time-consuming. Would I be better off having an energy/building consultant come in for an inspection and measurements first to try to better identify the problem areas? Any advice would be appreciated.

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  1. Expert Member
    Peter Engle | | #1

    How much do you run your A/C in the summer? If you mostly open windows for temperature control, you're pretty much stuck with outdoor conditions. If you mostly condition the air in summer, then there is a good chance that the A/C system is oversized. Oversized A/C units cool the house quickly, but don't have enough time to really process the air in the house and fail to remove the humidity effectively. A properly done Manual J calculation will let you know if the system is properly sized. If you have more than one zone, try shutting one zone down most of the time and/or running one zone cooler than the others. This forces just a single unit to cool the whole house, making it seem like you've got a smaller system.

    High RH in summer and low RH in winter also suggests air leakage from outside, as you suspected. There is plenty of information on this site to help you locate and correct air leakage, if you want to take the time to learn it and do the job properly. If not, a home performance contractor can help with air sealing and insulation. Both may help.

  2. GBA Editor
    Martin Holladay | | #2

    In general, 30% indoor RH is a good goal in winter, while 50% indoor RH is a good goal in summer.

    Running your ERV during the summer, in most climates, will raise rather than lower your indoor RH. To lower your indoor air during the summer:

    1. Seal leaks in your home's thermal envelope. The best way to do this is to follow a technique called "blower-door-directed air sealing."

    2. Close all windows and doors, and turn on your air conditioner.

    3. If this doesn't work, purchase and use a portable dehumidifier.

    For more information, see these two articles:

    All About Dehumidifiers

    Preventing Water Entry Into a Home

  3. Expert Member
    Dana Dorsett | | #3

    Whole house dehumidifiers are usually extreme overkill for managing the latent loads. If the DR90 isn't keeping up it's an indication that you may be running excessive ventilation rates on the ERV, have a very leaky house, very low (or no) air conditioning use, or have other unstated/unknown sources of indoor moisture.

    Blower door & infra-red imaging guided air sealing is usually worth it just on heating energy use savings alone in zone 5A. Fixing just the big leaks would usually be enough to deal with the infiltration moisture, but since you already have an ERV in place taking the place to ultra-tight levels would not be a problem, and would only help.

    If you have central air ducts that are unbalanced, unusually high infiltration can occur whenever the blower is running. If the ERV is using an unbalanced AC duct system to distribute the ventilation air that could be a large part of the problem.

    An unsealed uninsulated basement in damp soil or high water table conditions can sometimes drive humidity levels up a bit for the whole house.

  4. lgr123 | | #4

    Thanks all for the answers, they are very helpful. It looks like the first step might be to get an energy audit, with blower door testing and infrared inspection, assuming I can find someone in my area to do it. Would this type of energy audit also check for unbalanced central air ducts? Or should I have an HVAC technician look for that?

    Either way, how (or what) does one actually measure to see if the air ducts are unbalanced? I previously had an HVAC technician spend the better part of a day trying to troubleshoot this problem, at great expense. He verified the AC and dehumidifier were working properly, but he never mentioned unbalanced air ducts as a possible cause of the humidity problems. He also never mentioned a Manual J calculation.

    To answer the other questions, I run the AC all summer long, and I always keep the windows closed. I remember the AC cycling on and off during the day in the summer, so maybe it's oversized.

    I don't think the ERV is overventilating. It's programmed for my home size, and even when I've turned it off for several weeks in a row during the summer, it's still humid in the house. I also don't think there are any other unusual sources of moisture in the home. The foundation is a crawlspace, but it's unvented and has clear plastic sheeting laid over the gravel and dirt (the plastic sheeting seams are not taped though). It's a dry crawlspace, I've never noticed any water or excess moisture in it.

  5. Expert Member
    Dana Dorsett | | #5

    Hand held dual-input manometers are useful for measuring room to room pressure differences. There are cheaper versions for under $100 with 0.01" resolution that can find the worst offenders. An Energy Star duct system has to have room-to-room pressure differences no greater than 3 pascals (0.012" water column) under all operating conditions, room doors open/closed, so you won't quite be able to measure to that resolution with a cheap manometer, but you can get close. When measuring near the zero mark with a cheap manometer you need to measure both ways swapping which tube is on which side of the door/wall.

    Poorly balanced duct systems will usually have a few rooms with 0.2-0.3" or higher room pressure differences and those rooms are usually the major infiltration drivers when the air handler is running.

    A right sized AC system will have hours-long duty cycles during the hottest hours of the hottest days that hit temps near or above your 1% outside design temp. If yours is cycling on for 8-10 minutes at at time a couple of times an hour during hot weather it's probably way-oversized. It's possible to measure the oversize factor by tracking it's duty cycle against outdoor temperature on a few hot days:

    ERVs are usually specified to deliver at least ASHRAE 62.2 levels of ventilation or more. By many people's reckoning ASHRAE 62.2 is already overventilating for most homes (maybe not if your smoking or using art paints with volatile solvents, etc.)

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