Terrible Smell When it is Freezing Outside

[GP151001]

Every winter, when it gets it's coldest, -12c or lower, our lower level bathroom emits a serious odor. It appears to be coming from the interior wall. This, I imagine, is where the vent ("stink") pipe runs.

The only things I can think of that may cause this, is a blockage of the PVC vent pipe, or loose joint in the same pipe. We have known to have small squirrels climbing the roof, too. The toilet bowl wax ring and seal have been checked and replaced, but no help.

Any suggestions?

Thanks.

[pawnhead]

Possibly water or snow is getting in to your vent pipe and being compressed solid. The purpose of a vent is not to release gases, but to relieve suction in the line. Without it, the water seal in the trap under your basin/bath/floor waste, could be getting sucked dry every time you flush your toilet. Especially an upstairs toilet.
Are you sure that the smell is coming from the wall, and not from your bath, floor waste, or basin?