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?
Best Answer
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.