I had an energy audit done recently and one of the recommendations was to air seal and reinsulate the attic which was expected. What I did not expect was the recommendation to install a bathroom fan which would provide the equivalent of constant 20 CFM air exchange. In effect the fan would counteract some of the air sealing efforts which feels very counterintuitive, as in the winter it will suck more cold air in.

The house is 1600 sq ft and the blower door test was 2810 CFM50. The estimate after air sealing is 2248 CFM50. I understand that houses need to get fresh air but 2248 still seems high.

edit: Thanks for the feedback. To provide a little more detail, no I don't currently have a bathroom fan. The fan to be installed would be 110CFM but it would be controlled by a SmartExhaust switch which would be set to run a 20 minutes per hour, as well as come on when the light is on. It claims to be ASHRAE 62.2 compliant which I believe is the standard they are trying to meet.

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    That sounds crazy, we usually install a small duct on the intake side to pull fresh air in and the excess is pushed out through kitchen & bath vents and any other areas that have leaks this is the standard method if your home is tight after the sealing where is the fresh air going to come from? Backseat dampers are usually in the kitchen vent. Just sounds strange to me.
    – Ed Beal
    Sep 27, 2018 at 22:05
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    Do you have any fan in the bathroom now? Maybe this is a recommendation to add ventilation of moisture that will have nowhere to go post attic sealing.
    – auujay
    Sep 27, 2018 at 23:04
  • Do you have any combustion appliances in the house, and if so, are they natural draft or sealed combustion? Sep 28, 2018 at 0:09
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    You want a bathroom fan to exhaust at about 110CFM, not 20CFM. You want the moisture exhausted immediately, not slowly over the course of a day or so.
    – Lee Sam
    Sep 28, 2018 at 1:01

1 Answer 1


I agree with Ed, bathroom fans are turned on intermittently to exhaust MOISTURE not to INTRODUCE fresh air into your house, especially on a continual non-controlled (no thermostat or no humidistat) manner. Even outside air intakes on furnaces only work when the thermostat is calling for heat or air conditioning and it usually has a manual damper that you can adjust for the amount of air desired.

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