Last weekend after noticing my bath fan wasn’t working. I went into my attic to see what was going on. I found the vinyl flex duct that was snaking across the attic floor full of water, so I swapped the fan for a high-quality model and did a nice job running 4-in. aluminum pipe to the outside. The fan is rated at 70 cfm and now my bath doesn’t fill with steam and my windows are no longer fogged up in the morning.

While I was connecting the duct and wiring to my new fan, I had the R38 fiberglass batts pulled away from the drywall ceiling and found a 3” x 10” thermal bypass. I could see all the way to the basement! Feeling smug, I got a piece of foam and my foam gun and plugged the hole. I even filled some electrical holes while I was at it. I replaced the batts carefully and went to get a shower. The fan worked great.

While I was showering with my new fan, I was running the clothes dryer and we had the range hood running full blast (You can see where this is going?) I come out of the shower and I smell that distinctive smell of an unvented kerosene space heater. I say to myself “XXXX!”. I immediately know exactly what I did. I turn off all the fans and the smell dissipates and the oil forced-air furnace starts drafting again.

I’ve been experimenting and now I’m confident it’s only when I’ve got the bath fan, dryer, and range hood running that there’s a problem. Even when two of the three are running, the furnace seems to draft properly, although I don’t have real way to measure the amount of draft. My house is tiny (700 sf ) and on one level, although I wouldn’t describe it as especially air tight. I haven’t done a blower door test, but I will this weekend. My guess is that the leaks will amount to a few hundred CFM.

The question is, what do I do? I thought I’d install a makeup air vent to the outside in the basement with an electric damper that’s controlled by the thermostat. When the thermostat calls for heat it will open the damper and close when the thermostat is satisfied. I explained the situation to a smart coworker and he suggested I could skip the electric damper and install a cape damper instead.

I know I should also air seal the basement ceiling to keep my fans from pulling air down the chimney. Am I on the right track? I’d really appreciate your thoughts on my situation.

PS: I do have two good hardwired CO detectors with battery backup. Thanks for the help guys.