[dkfick]

Ok, you've got a carboy with negative pressure. At some point, somewhere in the process you're going to have to pull that plug to get the beer out and unless you are working in an O2 free environment you're gonna get air sucked in. It may be cleaner than the fridge air but pretty much all air is gonna have some bacteria floating around in it.

If you want to cold crash without any possible issue, here is what I suggest. Put a blow off tube into a jar of vodka or neutral grain alcohol and let it suck up a little of that as it drops in pressure. Won't hurt your beer, will prevent air from getting in, wont create a negative pressure environment and is sterile enough for our needs. I use vodka in my air locks, when I use air locks, so that if it sucks anything in, it won't be a problem.

I have been meaning to spray down the inside of my beer cooler with a mild bleach solution but haven't done it yet. For me, no problem cold crashing with foil but as they say in Internet speak, YMMV. Either way good luck.

What I generally do is use one of those breathable silicone bungs.  It lets out pressure but doesn't allow air to enter (one way).  So when I cold crash it creates negative pressure in the carboy.  Then I take the carboy out of the cool chamber and let it warm back up.  This generally equalizes the pressure (not always exactly but pretty close).  Of course I user the vacuum pump for almost all my transfers so I typically just open my chest freezer and turn it off to let it warm up before transferring vs moving it..  I pretty much never move a carboy full of anything.  I put the carboy where I want it when I transfer into it.  The pump allows me to transfer to something that is at a higher location or a lower location or even equal height.

[breweite]

I'd be worried enough to pickup a solid stopper next order or trip to the LHBS. For now, do what you can.

Just don't put the stopper on before crashing. Carboys can't handle any negative pressure.

Interesting...didn't think too much about this one. The difference in temp will prob be 20 degrees, not sure if that's too much to cause issues, but sounds like I don't want to risk it.  Good idea.