So now I have a plumbing "emergency". When I was showering last night, I heard the toilet going "glug, glug" like something was trying to escape the bowl. I immediately knew what was going on and shut off the water in my shower.
When I originally framed out the walls for the bathroom and ran the vents up to the attic, I went with an AAV (air admittance valve) instead of penetrating my roof on the front of the house or running a pipe to the other side of the attic and tying into the main stack.
I also did blown-in insulation in the attic. I had taped a bag over the vent when I blew in the insulation to keep the fibers out of the fans of the vent and took the bag off after the dust settled.
I still think some dust may have gotten in there and the weather has gotten colder. I think the water vapor that goes up the stacks is freezing in the vent and preventing it from easily pulling open when water goes down the drain.
To get it to open, I flushed the toilet in the original bathroom which is downstream of my bathroom and I heard the water whoosh out of the toilet and shower drain in my bathroom. Back in business. For now.
I plan to cut into the axillary stack (that runs through the little closet in the bathroom) that is capped by the current AAV, install a T with a 90° elbow attached then install a second AAV on top of the elbow.