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Community and Q&A

High humidity and mold

mosesdraper | Posted in General Questions on


This is my first summer in my new house. Most of the summer, we’ve been experiencing high humidity inside the house. Our small, inexpensive thermostat/moisture meter, is usually at 99%. We have started to notice a couple isolated items with mold on them and I’m wondering what to do about it.

I live in southern Vermont. The house is about 1200 sq ft open floor plan space (about 14000 cubic ft because of vaulted ceiling). It’s double stud walls with cellulose (12″ thick) and about 18″ + in the roof. The roof is vented. The blower door came in at 0.39 ACH50. We have 2 pairs of lunos which we run on high all the time, except at night when all windows are open to cool down the house.

We have a range hood and bath fan that we are religious about using.

Do we need to increase our whole house ventilation, or is this just endemic to summers in a humid climate? Is something like a dehumidifier the best response?

Any help much appreciated


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  1. user-2310254 | | #1

    New construction often contains quite a bit of moisture. Posters on this site are often advised to run a dehumidifier for several months to accelerate the drying process.

    That said, I wonder if there is something about the way you are venting your house that is exasperating the problem. I will be curious to see how the experts respond.

  2. walta100 | | #2

    First I do not understand what you are measuring at 99%. Is this the reading from a pin type moisture meter or is this the relative humidity in the room. They are very different things.

    I do not think you will get mold in your home unless, you have a surface in your home that is colder than the dew point of the air in the room. This can happen with basement walls in the spring. Or poorly insulated spots in the winter.

    I think a dehumidifier can be a good idea for a few situations. 1 For a few months during and after new construction to dry things out. 2 An old damp basement that lacks damp proofing and working drainage around the footing. 3 In miserably humid climates.


  3. brp_nh | | #3

    What is your heat source, do you have a mini-split? How many people live in the house? What are these isolated items?

    When you say inexpensive meter, are you referring to one of those ~ $10 AcuRite (or other brand) ones? If so, I would get at least 2-3 of them so you can get multiple readings across the house: one in a bedroom, one in bathroom, one in main area.

  4. GBA Editor
    Martin Holladay | | #4

    First of all, neither your indoor relative humidity, nor the moisture content of any wood in your house, is likely to be at 99%.

    Summertime indoor relative humidity in a house without air conditioning can easily be at 60%. If the indoor RH is 70%, that would be high.

    Wood moisture content in the 20% to 28% range are high. Above 28% would be very worrisome.

    So the first order of business is to get one or more hygrometers to determine the actual indoor relative humidity.

    If you are seeing mold on indoor surfaces, you should tell us the locations and describe the mold.

    Operating ventilation fans usually helps reduce indoor relative humidity levels, especially in Vermont, except when the weather turns hot and humid.

    When the weather turns hot and humid, the best way to lower your indoor relative humidity is to (a) close the windows, (b) reduce the amount of time that your ventilation fans are operating, and (c) to operate an air conditioner, operate a dehumidifier, or both.

    Possible moisture sources are (a) construction moisture -- the most likely culprit, or (b) a damp crawlspace or a damp basement. If you provide more information, we might be able to tell you what to do if your foundation is damp.

    It can easily take a year for construction moisture to dry out.

  5. mosesdraper | | #5

    Hey All,

    Thanks for the help so far.

    The house is a slab on grade, so no basement. The air barrier is on the inside (intello) and is connected to the sub-slab vapor barrier, so that slab is floating and isn't connected to ground moisture. The foundation/slab and shell have been in place for 3 years now in terms of drying time of new construction, however we did put an earthen floor in last summer, which could still be drying.

    The mold is isolated pockets of it. Examples include shoes in entrance closest, magnet on fridge, kids wooden toys lost in corners. We used to live in a fabric yurt and mold was endemic, so I'm not sure if some of the things we brought in from there are more prone, but it has us more alert for it. The mold is white or blue surface mold (dusty, comes off with cleaning). No black mold. Just on objects inside the house and spread out throughout the house (i.e. not just in the bathroom or something).

    The 99% reading comes from a $10 AcuRite temperature/moisture reader, so clearly that's one step to get the hygrometer.

    We have no air conditioner. The heat source in the winter is wood stove that is direct connected to it's on air source (it's a Stuv, which is gasketed to itself and the air source).

    We are a family of 4, 2 adults and 2 kids.

    Our moisture readings on the AcuRite were pretty low and consistent over the winter, it's just been since summer that this is happening. It has definitely been a hot and humid summer here in Vermont.

    We'll be getting a dehumidifier for sure, I just want to make sure there isn't something in our design that is problematic.

    Martin, to your suggestions about when it gets hot and humid, if we turn the lunos down and run the dehumidifier, so you think it's still okay to have windows open at night? It's the only way to keep the house cool, without getting AC, which we'd prefer not to.

    Thanks so much for all the help

  6. GBA Editor
    Martin Holladay | | #6

    Q. "Do you think it's still okay to have windows open at night? It's the only way to keep the house cool, without getting AC, which we'd prefer not to."

    A. If you want to lower the indoor relative humidity during hot, humid weather, it's essential that you keep your windows closed. Once you close your windows, you often feel hot -- so most people use an air conditioner under these circumstances.

    If you like to keep your windows open during hot, humid weather, your salt shakers won't work, and your envelope flaps will start to stick together before you lick them. That's the way I live, but it's not for everybody. If your indoor relative humidity is high under these circumstances, don't be surprised.

  7. Expert Member
    Peter Engle | | #7

    Martin's got it right, and he's local. This summer is particularly hot/wet and your house is going to be humid. That said, 99% with mold growing in the corners is not what most people expect from a new home.

    If you are willing to pay the energy penalty of running a dehumidifier, you should think twice about running A/C instead. Dehumidifiers are pretty much the same thing, but they reject their heat back into the room instead of outside. if you close the windows and run a dehumidifier during the day, it's going to get pretty warm in there.

    OTOH, it would only take a small minisplit to heat, cool and dehumidify your house and it would probably use about as much energy as a portable dehumidifier. When the outdoor dewpoints go below 65 degrees or so, go ahead and open the windows. When the dewpoints come up as they have this summer, use the minisplit night and day.

  8. Expert Member
    Dana Dorsett | | #8

    A $10 Acu-Rite is pretty good in the middle ranges but not at the humidity extremes, and seem to suffer permanent damage/mis-calibration if used in consistently high to very high humidity environments. If it's been reading 99% for awhile, it's probably toast. The nighttime ventilation periods should show readings MUCH lower than that, except during the most humid of days. Buy a fresh $10 Acu-Rite and compare the reading side-by-side, in some room other than a bathroom or kitchen (where humidity spikes are common.)

    Running the Lunos on HIGH adds more humidity to the house than it purges in most houses. With a lot of moisture stored in the cellulose to purge your house might be the exception, at least for now. In winter running it on high can purge a reasonable amount of moisture. On sticky summer on days when outdoor dew points are higher than your indoor temps it's only adding moisture.

    If your indoor humidity was anywhere near that high the occupants would all be suffering from chronic fungal infections of the skin & lungs and you would have mushrooms sprouting from wood surfaces, not just a few patches of mold in the corners.

    A dehumidifier &/or a window-shaker AC unit would be able to bring the indoor humidity down to something reasonable, at well under a 100% duty cycle on a house that size.

  9. walta100 | | #9

    I to do not believe you have 99% humidity in your home. Try another measuring interment. I do not think a dehumidifier will make you happy in the summer as yes they will remove water from the air but they also heat the air. Warming the air alone will lower the relative humidity. You will not be comfortable. Are the lunos HRVs or just fans? I think the energy spent running a dehumidifier would be far better spent running an air conditioner. With only .39 ACH your fans are almost unless you also open a window. Walta

  10. mosesdraper | | #10

    Hey Everyone,

    I really appreciate the help and time figuring this out.

    We ended up finding more mold this morning, so we're going to find some way (humidifier/air conditioner/Mini-split) to keep windows closed and keep the space lower humidity. I've got some new hygrometers on the way and will keep a much closer eye on this.

    It's an eye opener to keep a closer eye on the situation and now I've got some ways to work with it.


  11. breaton | | #11

    We are in a very similar situation: new 1200sf house, moved in April 2018, ach50=0.3, HRV draw from baths and kitchen and feeds the bedrooms and living room, same accurite thermometer/humidity sensor, two stories on insulated slab inside the air barrier, 8” cellulose+6” EPS walls. We are running a Mitsubishi Dual zone mini split with a 6000 BTU head downstairs(open floor) and another one in the stairway upstairs with three bedrooms and a bath. We live in MA north of Boston.

    I’ve been watching the humidity as well and it’s usually pretty high, 60-70%. You can see it in our dining table top un-cupping from the low humidity this spring and in the slab floor which seems to absorb moisture and is darker than it was when we moved in.

    The downstairs is comfortable, but it can get quite cool if running on the dehumidify setting, 66-68F, although the humidity will drop to the mid 50’s usually. If I turn the HRV to the lowest “away” setting then it drops quicker. If the mini split is run on AC and is set at say 72-74, it doesn’t seem to need to work much and the humidity quickly rises again. It tracks with the outside humidity quite quickly. Humidity seems lowest in late afternoon when the AC works hardest. I’ve fooled with the HRV settings, but I’m hesitant to turn it to “away” when we aren’t and with the windows closed. We aren’t seeing any mold growth yet (similar to you, we moved from an old 900sf cottage that was very damp and lots of mildew and I’ve been worried about the stuff we moved from there) and I was starting to think the humidity shouldn’t be much of an issue unless the moisture keeps up when it cools off in the fall. If the temperature falls into the mid 60’s, I’ve been opening the windows and it cools quicker than with the AC. Then close them in the morning.

    Our Minisplit head in the upstairs stairwell unfortunately appears to be in the wrong spot which was why I was searching the site. While the upstairs is cooler than outside, we’re seeing around a 5F temperature difference between the bedrooms and stairway and the downstairs (follow the dog). We’re using fans to help cool the bedrooms. The HRV seems to circulate some of the cooled air, but not enough to eliminate temperature differences from room to room. I was thinking of moving the upstairs unit to the end up the upstairs hall just outside two bedrooms or changing the head to a ducted unit with feeds to the the three bedrooms. Just moving the head seems like it would be fairly simple, but I’m still not sure it would cool the bedrooms without fans. I was also considering moving it to our bedroom and then let the kid suffer. I was also wondering about reducing HRV return air to the open living room/kitchen/dining and increasing it to the bedrooms.

  12. Expert Member
    Dana Dorsett | | #12

    Bruce: If you haven't figured this out already, in normal cooling mode set the fan to it's lowest or next lowest possible setting. At lower fan cfm the temperature of coil is cooler, removing more moisture than it does at higher fan settings. This won't be a total solution, but it helps.

    The dew point of 75F / 60%RH indoor air is 60F. When the outdoor dew points are north of 60F (which has been the case most of the time for the past 4-5 weeks in y0ur area, despite a relatively dry June) minimize the amount of outdoor air ventilation, since the outdoor are would then raise rather than lower the humidity in the house.

    1. breaton | | #13

      Thanks Dana. I think that’s basically what I’ve been doing. I have been setting the fan to auto and it is typically running at low speed since the temperature isn’t fluctuating much. I’ve also been running the HRV at the slowest speed that’s not the “away” setting.

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