I live in Montreal and every year I leave for 6 weeks in the winter to get a little break. Is it ok for my bike to be on my balcony (kind of covered) during these 6 weeks where the temperature gets from around -20 to -5. Never above 0? Or should I put it inside my work bike storage area which is room temp? Cheers!
I assume you're in Celsius in Canada.
Although the air temp doesn't get above freezing the bike might in the sun. This would melt any snow that had blown onto it. There are places where the resulting wet would cause problems if it refroze.
So I'd store the bike dry and well oiled/greased, then properly covered, not just relying on the balcony above. In particular I'd work plenty of grease into all the cable ends. In general there are arguments for and against this with modern cables, but IMO keeping out water (that might freeze) in winter trumps the others. Of course all the moving parts should be lubricated as normal, a bit on the generous side.
Better would be to put in indoors, probably on a mat of some sort as it's like to drip and not cleanly. But of course this may not be possible if you live with others and they're present while you're not
Shimano mineral oil should be good down to -35C and oils tend to get denser at lower temps. DOT5.1 freezes below -50C other DOTs at -40C so hydraulics should be fine at -20C as long as it is water free.
Greases might separate/degrade slightly at low temperatures. Some calcium/lithium greases I checked out have min operating temps around -15/-12C. But who knows what grease they put in your bearings and to what extent this is an actual problem. If you are riding it at those temps I'd prefer to store it inside to have the grease warmer initially to keep the lubrication properties ok.
If it's clean, dry and protected from snow/water my guess is outside is ok. Where I live everything gets stolen so I keep mine inside all year round.
Very low temperatures may affect rubber and other elastomeres, making them brittle and develop cracks. This may affect things like bearing seals, brake pads, handle bar grips, the saddle and the like. But -20°C is probably not much below the design temperature range, and I suppose you ride your bike when you are home, so it has been exposed to such temperatures before with no ill effects.
I would not worry too much. Water is much worse, but you said you cover it, and given the permanent frost there won't be any liquid water around to begin with, so I'd risk it.
I would not worry about rust if the bike will be covered. I would worry about some fluid or grease getting frozen, but these temperatures are still not extreme and you may even want to use your bike in these temperatures.
I leave my older bikes in a shed all year long and it is not heated either and nothing bad happens to them. They both use cables, no hydraulics. But it is not Canada, just Czechia around 400 m. a.s.l. so temperatures are less extreme.
If the flat is going to be empty, you could also perhaps leave the bike inside the room?