In our upstairs bedroom we are experiencing a foul smell just as the temperatures became extremely cold. We experienced something similar when our power when out during a cold spell last year. The smell disappeared once the heat came back on and the temperatures warmed up.

Could this be the waste pipe freezing in the walls? Or something else? We are connected to a sewer system.

Is your house old and all brick? Is you vent pipe cast or PVC? To state the obvious to smell is crack. Or the vent is not right. Is your vent through the roof? Do you have a crawl space to see where the vent pipe comes through the roog. If you see the pipe does it look like its draining down towards the basement?