Hmm. If you run your exhaust-only bath fan even when the outdoor dewpoint is high, would it be a huge problem? The infiltration caused by depressurization would most likely occur in many locations in the envelope and there would be mixing throughout a presumably larger volume. If the bath fan is on a timer and doesn’t run all day, is the mixing of infiltrated humidity with cooler, drier air a big problem?
On the other hand, even if the outdoor dewpoint is lower than the bath dewpoint, infiltration of humid air can still be condensation/mold risk in a dark, wood frame stud cavity.
It points to the benefit of balanced ventilation with humidity recovery.

Still you make good point for having the fan in auto. I would especially say to be in auto if you have humidity issues.

Thank you. Appreciate the response.
I now remember rooting around in the installer setup menu a few years ago to make the fan run all the time in auto, because I read that either here or at GBA, so that is why and how mine are always on. I’m not the client you worked for yesterday, but have a similar setup and also keep my t-stats at 78.
Hopefully you will do a future article on how your client’s situation resulted in overcooling! Very curious.

We just built a new home. It’s fairly tight at ACH50 =1.7 and insulated. We are in central VA where humidity is routinely high. We have a variable capacity HP (Carrier Infinity/ Greenspeed) that periodically runs in a “dehumidifier “ mode that is low-level fan + an older 70-pint/ day stand alone dehumidifier from the old house. Result: 40-43 percent humidity even in the worst summer days. And, my AC only drops the temp about 1/2 degree F to supplement on those really humid days…so no overly-cold rooms. So, variable speed AC and a small dehumidifier (and tight house) really works well. As far as the CO2 levels…we are struggling to manage them without near 100% running of our ERV. Hmm 🤔

Re-heat is unbelievable expensive for dehum. For the cost, I’d put money into the inverter. ERVs are all too often turned off at some point, particularly in high humidity areas and I am not a huge fan personally, although they have their places. While house dehums are cheaper options (albeit not cheap) and with proper design can work extremely well.

Yes to Inverter systems and from experience it’s not just a claim. That said, there’s “combo” designs integrating ductless into ducted designs. These will remove humidity better than traditional ductless but not as good as inverter technology I am referencing (Ex. Carrier Infinity 24VNA9/6 or 25VNA8/4). With proper installation and duct design, these will remove a great deal of moisture therefore I am fan of spending money the HVAC system that will suit both needs. 24VNA936/58CVA070 and 24VNA924/59TN6B060 in Atlanta for reference I stay steady 71 and 55RH.

If you install the exhaust fan directly over the shower stall and the shower stall is well enclosed, than you might be exhausting higher temperature saturated air that is higher dewpoint than outdoors. Most bath exhaust fans that I have seen are installed in the middle of the bathroom, not over the shower, thus the shower air is diluted by the air in the rest of the bathroom before it gets to the exhaust fan. It would be good to do some measurements to see if this is true.