No room to bring them inside. Thinking that leaving extra room for expansion, like 3/4 full carboys might be safe. Also considering wrapping in a tarp for insulation, and hoping the heat from fermentation will help... but then I'd have to catch just when that stops. Maybe someone who has freeze-distilled outside in a glass carboy might have a tip on survival chances of my carboy.
It is a cider/mead (cysor). Expected ABV 10%, already got 5%.
I'm not intending to freeze the cider, but I don't need to avoid it as far as I know -- except for the glass breaking potential.
Also have concerns about a 3/4 full carboy (for expansion room) and the associated surface area/oxygen access (expecially when fermentation slows/stops). But probably if it's too cold for booze production, vinegarization won't happen either.
Update: Made through our mild winter (it appears). Full Carboys had just the slightest ice on top, no risk of full freeze and therefore broken glass. I don't think it got colder than about -7C though. A half-full carboy of vinegar did get more freezing, and shook them to keep the ice moving and it was fine. Fermentation has resumed. Alls well.