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Long-term solution for a humidity issue?

rshuman | Posted in Energy Efficiency and Durability on

I am trying to identify a long-term solution to address high humidities in a recently purchased house. The circa 2015 house is almost 1900 ft2, including finished first and second stories and a walkout basement of which ~40% is finished. The basement was poured using ICFs (2.5” foam insulation on either side of the concrete walls) and has 2” of foam under the slab. The framed portion of the walkout basement was built using 2x8s and insulated with R-30 roxul between the studs. The walls on the first and second floors use 2×6 framing and include R-23 roxul between the studs. Currently, somewhere between R30 and R40 worth of blown in fiberglass exists in the attic, increasing this by topping things off with blown in cellulose is in the plan. The house is heated with a direct-vent propane heater, hot water is supplied using an electric water heater. I recently had a blower door test conducted and found an ACH of ~1.4. The house is located along the coast in Lubec Maine, zone 6.

RH readings in the house are oftentimes 20 points higher than outdoor readings, I don’t recall indoor readings less than 60% since last June or so (when I started taking some measurements using a $10 Acurite meter). Sources of moisture include two adults, showers taken with exhaust fans on, a couple plants, a gas stove/range (with no ventilation), and the great outdoors.

Three strategies have been identified/discussed to help reduce moisture levels including (1) run exhaust fans in one or both bathrooms 24/7, (2) run a dehumidifier 24/7, or (3) install a mini-split as the primary source of heat and rely on its capabilities to reduce the humidity. The bathroom fan approach relies on air being sucked in from leaks in the house and exhausts heated air. Given that it would be difficult to retrofit a full ERV/HRV, options like the Lunos products might be a useful refinement on this idea. I have seen mention on GBA of the Nexxt unit that ventilates at rates as great as ~50 cfm. Given the moderate to good tightness of the house the ventilation aspect would probably be a good thing. Therefore, perhaps 1-2 units of this type make sense.

Running a dehumidifier 24/7 doesn’t appeal to me and does nothing in terms of ventilation.

Heating the house with a mini split on the first floor would result in lower heating bills. Things I have read at GBA and elsewhere (some possibly outdated) discuss a dry mode that would help me deal with humidity in the summer (the mini split is likely to see little or (more likely) no cooling duties). I’ve seen mention (in a ~2015 post) of a Daikin unit that can help from a dehumidification standpoint during heating and cooling seasons, I don’t know if other units are on the market now. This approach doesn’t do anything in terms of ventilation.

So, that is my understanding of the pros and cons, please correct me where I am misinformed. Input about the best path forward would be greatly appreciated.

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  1. STEPHEN SHEEHY | | #1

    Rob: It sounds like you have identified the only sources of moisture. Have you tried not using the range for a few days to see if the propane is the primary culprit?
    The outdoors in Lubec is probably wetter than where we are in Whitefield, between Augusta and the coast. For what it's worth, right now our indoor humidity is 39%, while outside is 60% at 25 °. Our house has no sources of combustion. We never use the dry mode on our minisplits.

  2. GBA Editor
    Martin Holladay | | #2

    Hygrometers are often inaccurate. So Step One is to buy a couple more hygrometers, and scatter them around the house, to verify whether your RH readings are accurate.

    Are your windows running with condensation that pools on the window stools? If so, you have a problem. If not, you may just have an inaccurate hygrometer.

    You don't mention whether or not the house has a mechanical ventilation system. If it doesn't, it should.

    Now that winter has arrived in Maine, start out by leaving your bathroom exhaust fan running for 24 hours a day. Tell us whether that helps.

    If that works, you have just installed an exhaust-only ventilation system. If you like it, you're all set. If you want something better, install a real ventilation system.

  3. rshuman | | #3

    Steve - I sort of ran this test the last few days. We were gone for three days around the holiday (i.e., ,no range use) and the RH when we got home was in the mid-60s.

    Martin - I get quite a lot of condensation on the windows on not-so-cold mornings. It has yet to flow down to the stools (in part because I wipe it up) but it is pretty heavy. I think the problem is real, and not just the result of an inaccurate hygrometer.

    There is no mechanical ventilation. One vendor thought it would be a challenge to install a full-fledged ERV/HRV because of, among other things, the open floor plan. That led me to the comments/questions above about the Nexxt unit. I did run the bathroom fans for ~10 hours one day and reduced the RH from the mid 60s to about 59% (outdoor humidity was in the 40s if I remember correctly).

  4. chrisjri | | #4

    I have the same issues in my 1920 sq' ranch w/walkout basement (3800sq'). I run a portable dehumidifier in the spring and fall to keep the humidity in the 40% range.

    I also run my 2 bath fans for hours at a time (not 24/7). Also the over the range microwaves vent fan.

    Do you air conditioning in the summer?

    A range ventilation fan would not be as difficult as an HRV or ERV, maybe?

    I know it's a little waste full, but to get the moisture down to acceptable levels, crack a few windows open. Cold air has very low RH.

    I run my ducted heat pump on fan only to circulate/mix the dry and humid air.

    I leave my blinds up off the sill a little so some air can circulate behind them and that reduces the amount of condensation on the glass.

  5. Expert Member
    Dana Dorsett | | #5

    "RH readings in the house are oftentimes 20 points higher than outdoor readings..."

    The delta between the indoor and outdoor RH is completely irrelevant, since RH statement of the humidity saturation relative to a temperature. Tracking indoor and outdoor DEW POINT is more useful, since dew point is a measure of the absolute, not relative humidity, and would tell you when ventilation would be adding rather than removing moisture from the house.

    Stephen's 39% RH @ 70F would be a dew point of 44F. His outdoor reading of 60% @ 25F is a dew point +14F. So the outdoor air is a LOT drier than the indoor air at his house, despite having a relative humidity 21% higher than the indoor RH.

    60% RH @ 70F is a dew point of about 56F, and could indeed create copious window condensation in your location, even during cooler shoulder season days, since the interior glass surface of a clear glass double pane hits 56F when it's in the mid-20sF outside. But with a U0.32-ish low-E double pane that won't happen until outdoor temps are nearing negative double digits, so window condensation isn't necessarily the best indicator if you have decent windows.

    At 1.4ACH/50 (less than half IRC code-max) you clearly need active ventilation, and is a primary reason humidity levels remain high. If the interior RH has been tracking north of 60% RH forever there is a lot of stored moisture in the materials inside your house. Running bath fans 24/7 for awhile will probably do it, but it may still take awhile to dry the place out. When it stabilizes to under 40%RH at some duty cycle, the duty cycle would give you an indication of how much HRV it would take to keep it there.

    The Daikin Quaternity series would be an expensive and not necessarily complete solution to this problem, and is less than an ideal candidate for a downeast Maine climate.

  6. walta100 | | #6

    I think you need to look for a large moisture source.

    How do you dry your laundry?

    Try taping 2 foot squares of clear plastic exposed concrete walls and floors, after 3 or 4 days look for moisture collecting under the plastic.


  7. SwitchgrassFarmer | | #7

    Perhaps this is a situation that, to some degree, might "cure" itself.

    My own home, in roughly the same climate zone, with walkout ICF foundation, and an HRV, had indoor humidity levels that were high the first year or so. Rob, your home, "circa 2015" with "ACH of ~1.4" and no HRV, may be about to turn the corner.

    (I said "roughly the same climate zone" as our county shows as zone 5, but where I live is geographically closer to the adjacent county that is in zone 6.)

  8. rshuman | | #8

    Thanks for the replies.

    Chris - We do not air condition. IMO, there is generally no need for it (where we live), everyone should suffer a little ;-).

    Walta - An electric dryer is used for laundry. The appliance appears to be properly vented. It is generally used one day a week but the humidities remain high pretty much nonstop. I will finally get around to taping poly to the floor to look for moisture. I don't have that opportunity in terms of the walls, as they are insulated with foam (ICFs were used as forms for the concrete foundation).

    Andrew - Yes, perhaps it is moisture from costruction materials. Although it is two years since the house was built it has been lived in fulltime only since last June. Given its tightness and lack of consistent ventilation perhaps what seems like a long time to me really isn't.

    Dana - Point taken re RHs vs dew points. Some measurements I took in the early part of last July showed dew points in the basement that were actually slightly lower than outside dew points (like low to mid 50s vs mid to upper 50s). I think those days are gone, however, with the coming of winter and lower outdoor humifdities. On one or two occasions I have noticed very different RHs (indoor >> outdoor) at temps that were nearly identical. But I will nail this down more completely with further measurements.

    I am going to conduct longer-term experiments with the bathroom fans to see what change I can effect. Depending upon the responsiveness of indoor moisture levels to those fans I will use them to help define my HRV needs as you suggest. In the meantime I will explore further mechanical venilation options that accomodate the constraints imposed by the design of the house.

    Do you have any 'experience' with the Lunos units (especially the Nexxt models)? Also, why is the Daikin unit a less than ideal alternative for my location? I recognize that it (or any mini-split) does not address the ventilation issue which is probably my biggest problem.

    Again, thanks all.

  9. Jon_R | | #9

    For a temporary issue, I wouldn't hesitate to run a dehumidifier. It will be more than 100% efficient (kwh/kwh) at providing heat.

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