I am a retired HERS rater and BPA multi-family rater so my practices and knowledge are at least 10 years out of date. I believe I have uncovered a missing practice that could help some contractors and raters. I put a whole house dehumidifier with fresh air intake into my HERS rated house in North Carolina last fall. The wall mounted controller for the dehumidifier is on the same wall as my thermostat. I was seeing a huge difference in humidity readings on my controller and on my calibrated hygrometer 4 feet away from the controller. The controller was reading 58% (setpoint was 45%) and the hygrometer was reading 41%. I called tech support at the manufacturer and was told to check the wiring hole behind the controller. Well, it was large and when I turned on my kitchen fan the air rushed out of that hole. The air in the wall was coming in and creating a micro-climate in the controller. The sad truth is that in a HERS rated house there was a leak in that wall. It is an interior wall but I think it communicates to the attic. I did not build or rate this house.
I foamed the hole and used a piece of shrink tubing to allow the wires to slide in and out. Now the controller and the calibrated hygrometer are within 1% of each other.
The question is why is this not part of the rating schedule? I would think that thermostats would have the same problem and I mean to check that next. Should it be part of the rating process? I would say that the wall should not communicate with the attic but this check would expose that problem as well as the hole in the wall.