If my outdoor temp ever fell to -50F,odors would be the last thing on my mind.

It did get down to -19C at night but I'll clarify: the temp went up to 54F. :-) All the snow melted and the stench came alive...

Can you tell if smell is coming from a specific drain, like shower?

Any chance of a dry trap? Do all the drains (esp floor drains) get used regularly?

The smell could be from the shower and floor drains, which are close to each other and 3 months old at the most. I've used a homemade sugar scrub 3 or 4 times but can't imagine this would create such a problem.

Probably dry trap, possibly toilet flushing or other similar pulling (like suctioning) water out of trap. Just to try: keep a bottle of water nearby (since you are getting water shut off at night), when you get the smell, pour some water down one or either of these drains to see if that stops it. Hopefully you can isolate which one (or both) is the issue. Since you did reno recently, perhaps plumber can deal with it.

Thank you--I think you're onto something. Last night I let a combo of baking soda and vinegar sit in the two drains for a while and flushed them with boiling water. It is way less unpleasant today. Next time I will try to just flush with boiling water.

I would first make sure there is a trap and not a straight drop. I believe a lot of corners were cut in building during that time frame.

Thanks, greg_2015, you explained that better than I would have. Lucija, what you did doesn't hurt and may have helped if the problem was a blockage or just something nasty in the drain (you can also use a plunger to clear out), but the fact that it keeps happening suggests that the trap/venting combination is not working to keep water in the trap and block sewer gas - so probably the main advantage of what you did was just adding water. As a short-term fix and to diagnose, putting regular water in just refills the trap and blocks the sewer gas from coming up - a few large glasses should be enough. Then once you know that, you'll be able to discuss with plumber what needs done. Maybe a better trap, maybe it's only caused by your own toilet flush suctioning water out (and some 'local' solution might be possible), or maybe it's an issue through the whole building. It couldn't hurt to ask your neighbours (especially above and below you because it's probably the same vent) if they have this problem too. Or maybe they know a better plumber. ))

I have a shower that has a similar issue and it's obvious that running the shower for a few seconds after flushing the toilet stops the sewer gas issue - the toilet flush suctions water out of the trap. Will get the plumber on a better solution sometime soon.

Lucija, re-reading your original post, I just paid new attention to the point that this only started (reappeared) when temperatures changed. If that's a clue to anything, it may be that the vent pipe somehow got blocked - snow or ice? That would be a question to building maintenance people, and another good reason to ask neighbours. At any rate worth asking to have that checked before undertaking any significant work in your bathroom.

Thanks dchall and amoured.

The reno is already complete except for the mirror cabinet :-) But, we did switch the locations of the major components—sink, toilet, and tub (which is now a shower). According to my husband there has always been some type of "aroma"--so part of the issue is the building itself--but the porta-potty smell was unlike anything I've experienced before the reno. I will ask the person who did the reno (husband's bro-in-law) about the trap. So far this is day two without the "porta-potty" smell.