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.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part