Originally Posted by Bobby_M
Conceptually, the best solution is to have a mylar balloon attached to that tee. When active fermenting, it will inflate it with CO2, then when full it will outgas through the airlock. When you drop the temp to lager or cold crash, the vacuum created in the carboy will just pull in the CO2 and deflate the mylar bag. The last thing you want is fridge air being drawn in.
If you're really uptight about this, why not take it a step further and put the balloon (or condom, for AnOldUR) on the top leg of the tee, and drop the tube on the bottom into several inches of StarSan. If you use a long cylinder, your could get close to 1 psi of CO2 pressure in your fermenter (need 15 inches for that, IIRC). You are not very likely to create a pressure drop then that is more that -2 psi, which would pull the StarSan up the tube and into the fermenter.
For my conicals, I have an elbow on the lid, and a 1/2 inch tube down to a small plastic bucket with StarSan. I didn't do this because I was worried about the "suck-back". I just don't have the head space in my fermentation fridges for an air lock. I usually only have enough StarSan in the buckets to make sure the hoses stay covered, perhaps 3 inches.