Some people have asked where we managed to get an airtight dog door. After Blake and Stuart did some searching, Blake found an awesome door at http://www.freedompetpass.com. Our blower-door (air-tightness) tests have included this pet door in the mud room door to the back yard. And it works! Our final test measured an extremely tight 0.20 ACH50 for the entire house (using the Passive House volume calculation).
Here is a video of the door during one of the blower-door tests.