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Community and Q&A

High Humidity

djhaus18 | Posted in Mechanicals on

I’m having a problem with high relative humidity on the upper floors of my home just outside of Boston, MA. The home built in 2014 has about 6000sqft of conditioned space. It’s insulated with a mix of closed cell foam (basement walls, band joists), open cell foam (exterior walls, rim joists, and attic rafters) and fiberglass batts (exterior walls).

The relative humidity¬† in the basement has been hovering around 45% with a pretty steady temperature around 68 degrees. The first floor ranges from about 45-50% RH with a temperature around 71. The second floor ranges from 55-70% with a temperature around 70. The third floor is similar to the second floor, but occasionally has a lower RH than the second floor which seems odd to me. This is also part of the reason why I’m going to sanity check the thermostat RH values (see below). For reference the square footage of each floor is roughly 1500, 1500, 2100 and 800.

I’m measuring humidity via my Ecobee thermostats. I’m questioning how accurate these are, so I have ordered a hygrometer and calibration device from Amazon to sanity check that the thermostats are accurate. I’ve read some reports on forums where many people complain about the accuracy of the Ecobees RH reading.

The house is tight. According to the HERS report I received when I purchased the home it is rated at 1.7 ACH50. Unfortunately the builder choose to not install a whole house ventilation system. Instead he choose to use bathroom exhaust fans on the second floor (at opposite sides of the house) to run continuously to ventilate the house. One of the fans runs at 50cfm and the other runs at 40cfm. These values are the ones configured on the fan, not an actual measurement so I suspect the actual cfm rating is less due to duct runs, etc.

I suspect the root cause of my humidity problem is that the exhaust fans are drawing in humid air in from outside and the AC doesn’t remove enough moisture from the air.

My current thinking is that a proper whole house ventilation (an ERV) system will solve this problem and probably improve the indoor air quality of the home. However, I’m looking for differential diagnosis from the experts here. Assuming the RH measurements are accurate, is there something else that could be causing this problem? I’ve inspected the second floor for leaks and can’t seem to find anything. I’ve also checked the dryer vent pipe to ensure it’s secure and not clogged.

Assuming my theory is accurate, is the best temporary course of action (until I get some contractors to inspect and estimate for an ERV) to run a dehumidifier on the second floor and reduce the CFM settings of the continuously running exhaust fans to help limit the amount of moisture drawn into the house? Would it make more sense to only run the exhaust fans in the basement? Operating under the assumption that drawing air into the house from the lower level would draw in (hopefully?) cooler, less moist air? As compared to the second floor fans which perhaps are drawing in warmer, moister air.



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  1. GBA Editor
    Martin Holladay | | #1

    Installing an ERV may be a good idea, but I doubt that it will significantly lower your indoor RH.

    Your indoor RH levels aren't too unusual for summertime readings. It's always possible to achieve lower indoor RH levels if you want them, of course, by simultaneously operating a dehumidifier (or dehumidifiers) and your air conditioner. The only downside is the energy penalty.

    For more information on this issue, see "All About Dehumidifiers."

  2. Expert Member
    Dana Dorsett | | #2

    With a house that tight and no control over where the ventilation air is leaking in from it's likely that the higher RH areas are where the highest leakage levels are (barring identifiable local humidity sources in those areas.)

    A balanced ventilation system would likely even-out the humidity levels from floor to floor, room to room, but at the same cfm won't reduce the average humidity levels in the house. Lowering the ventilation rate whenever outdoor dew points are north of 60F would give the AC a better shot at keeping up with the latent loads.

    The dew point of 55% RH 75F air is 58F.

    The dew point of 55%RH 70F air is 53F.

    In eastern MA in mid-summer the average outdoor dew points are above 55F about 2/3 of the time, and above 60F a bit more than 1/3 the time. So 2/3 of the time the ventilation air is adding to the moisture load assuming the goal is to keep it under 55% @ 70F. But it is only a significant load about a third of the time during the muggiest 4-5 weeks of the year.

    For now it's not crazy to just turn off one of the bath fans (except when the bathroom is in use) when it's muggy like this. The AC may be able to keep up with the indoor humidity just fine if the ventilation rate is cut by roughly half.

  3. Yupster | | #3

    You could also consider checking the cfm flowing over the coil in your A/C. Builders usually install @ 400 cfm per ton. If you drop that to 350 cfm per ton, the total capacity of your A/C will drop a little bit but the latent heat removal (humidity) will go up by a few percent. If you have multiple pieces of equipment, this could improve your situation significantly. You can calculate CFM over the coil using the methods found here:

  4. Jon_R | | #4

    Short-term, I would do nothing - your humidity isn't that high. That being said, negative building pressure is a bad idea in hot weather - it creates localized humidity problems. Positive building pressure would be better.

    If you have a ducted heating/AC system, then checking balance in every closed door room would be wise.

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