I just purchased a new home in the suburbs of Boston with move in date of November 2017. The home has felt unusually drafty and cold and i've had to increase the heat about 4 degrees above what I usually use. Ordinarily i'm comfortable at 68 but in this house the heat needs to be at 72 for equivalent comfort. In addition to feeling a bit uncomfortable, it has been very dry inside the home for the past 2 weeks. I purchased 2 ThermPro Hygrometers and they have consistently read 10-13 RH for the past 2 weeks. Right now it is 5 degrees outside and the RH is measured at 11% both upstairs and downstairs.

We have 2 exhaust fans that appear to run 24/7. They are both located in bathrooms on each floor. I removed the vent cover and there is a switch that reads they are both operating at 110CFM. This means total venting is 220CFM? Curiously, in our home energy audit packet we received it says the mechanical ventilation rate is 145CFM. I wonder why the difference?

I tried turning both exhaust fans to 50CFM for a total of 100CFM and the home seems a little more comfortable but the very low humidity hasn't changed. These fans are running ALL the time. The wall switch will not turn them off.

I'm worried that even at 100CFM the house is being over ventilated causing cold air to be pulled into the house through the building envelope and exhausting warm moist air too quickly causing the house to dry out. I'm also worried about the opposite problem in the summer - too much humid air being pulled into the house.

Here are the specs on the home:

Basic stats of home: - 2700sq feet - unfinished attic + basement - spray foam in attic + basement - fiberglass batt in exterior walls - forced hot air heat. Dual zone w/ furnaces in Basement and Attic - 8 foot ceilings.

Info from energy audit - Infiltration rate: Htg 688 Clg: 688 CFM50 - Method: blower door test

From 2016 IECC r-406 Confirmed Energy Index Report - R402.4 - Envelope air leakage maximum leakage rate: (3 ACH50 for CZ3-8) PASS

Are indoor RH levels of 10-15% in winter months unusual for a cold climate like Boston?

Could my home be over ventilated?

  • 3
    Tempatures are very relative, and measuring devices don't have the accuracy you would expect. I can set up a demo that show 5 different temps from 69 to 74 in my living room using 5 different digital thermometer/sensors. Meaning it may just be the perception that you need it warmer, because the thermostat is calibrated differently.
    – Tyson
    Jan 2, 2018 at 3:23
  • We have 2 exhaust fans that appear to run 24/7 ... why don't you turn them off?
    – jsotola
    Jan 2, 2018 at 5:12
  • 1
    FWIW, I'm in Cambridge, MA and the RH in my house now (69F inside, 3F outside) is 15% according to my ecobee3 thermostat. We run humidifiers in the kids' bedrooms. Hardwired-on bathroom exhaust fans are a widely accepted way to provide the required ventilation in newer, tightly sealed structures without the expense of an HRV/ERV. You may want to consider a whole-house humidifier, that makes a big difference in comfort at a given temp. Jan 2, 2018 at 5:14
  • 2
    When it is this cold outside it is really dry already, forced-air heat just dries it out even more. Boil some water or get a humidifier.
    – ArchonOSX
    Jan 2, 2018 at 10:57
  • @Shimon Rura, yikes, hard-wired, always-on bathroom exhaust fans?! Are builders really doing this? What is the service life of an always-on bathroom fan? Would these be special designs that allowed the motor to be easily replaced? Jan 2, 2018 at 11:45

3 Answers 3


If you have an energy saving, very tight home, then why would you run an exhaust fan of any size 24/7? Bathroom fans should only be run when the occupants are showering or doing the other thing that requires ventilation. I would allow the fans to run when the bathroom is occupied either by the use of a timer, or by a motion detector. If you need that much ventilation just open a window! There are energy recovery systems that move air in and out of the house and maintain a balanced pressure in the home. They have a type of heat exchange mechanism, to transfer heat between the outgoing and incoming air to make the units "more green" or energy friendly. Oh, by the way, contrary to some peoples opinion, forced air heating systems or any heating systems do not "dry out" the humidity in any home. The humidity is relative to the temperature, raising the air temperature lowers the relative humidity and visa/versa. For your house, you will have to add 1 or maybe 2 whole house humidifiers to the heating systems. Raising the homes humidity will yield a more comfortable feeling at a lower temperature.

  • 1
    It's probably not an exhaust fan, but a heat exchanger/heat recover ventilator (HRV). This is a common setup in Minnesota. The heat is recovered and indoor air pollution is reduced.
    – isherwood
    Jan 2, 2018 at 14:27
  • @isherwood, so somewhere in the house there would be a fresh air inlet with a heat exchanger to warm the incomming air? Jan 2, 2018 at 14:34
  • Yeah, usually all bathroom exhaust ducts route to the mechanical room and there's a housing on the ceiling with 3-4 ducts attached. Sometimes it's controlled by switches or timers in the bathrooms, and sometimes it's simply always on.
    – isherwood
    Jan 2, 2018 at 15:05

One fan going I think is more than enough. I would say that 30-minutes every 24-hrs is enough. A humidifier would be a nice addition. Thermostats vary considerably.

  • 1
    I would agree. Turn the fans off entirely unless someone is taking 30 minute showers. The other thing you can do is put a bypass on your dryer vent and vent it inside for additional humidity.
    – ArchonOSX
    Jan 2, 2018 at 11:01
  • It's probably not an exhaust fan, but a heat exchanger/heat recover ventilator (HRV). This is a common setup in Minnesota. The heat is recovered and indoor air pollution is reduced. Advising against running it as intended isn't wise.
    – isherwood
    Jan 2, 2018 at 14:27
  • @isherwood the problem is, the gadget isn't recovering humidity. Also the better way to reduce indoor air pollution is do all your smelting, painting of automobiles, manufacfure of drugs, and cooking fish in an outbuilding. Jan 2, 2018 at 19:45
  • Those are not examples of the primary causes of indoor air pollution. (Maybe you're being funny.) The right approach is to introduce moisture when it's needed, not to try and preserve stale moisture.
    – isherwood
    Jan 2, 2018 at 20:15

I'm worried that even at 100CFM the house is being over ventilated causing cold air to be pulled into the house through the building envelope and exhausting warm moist air too quickly causing the house to dry out.

The warm air being vented outside isn't moist, because it's warm. When you heat air, you reduce its RH (conversely, when you cool air, you increase its RH). Thus, as your furnace warms up the intake air, the RH of the resulting warm air is low. The colder or drier the incoming air, the lower the RH of the warmed air.

This is a very common problem in the winter, especially in the northern climates (and 10% RH is not unusual). Many people use humidifiers to add moisture to the air to keep the RH at a more comfortable level. Some people use humidifiers in individual (bed)rooms, or you can install one on your furnace to humidify the entire house.

Low humidity levels lead to dry skin and chapped lips; more shocks from static electricity; increased condensation on windows, especially in the morning; doors and windows sticking or not closing properly; damage to solid wood furniture.

Air at a given temperature can hold some amount of water vapor. The relative humidity (RH) is a measure of how much water vapor that air is holding relative to the total amount it can hold. Air's moisture capacity increases with temperature - so warm air can hold more water than cold air. The air coming in to your furnace is holding some amount of water. As it's heated by the furnace, it doesn't lose any* of that moisture - but, because it's warmer, the total amount of water it can hold increases and thus the RH drops.

*or only very little

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