I use tin foil all the time for brewing! I use it around my chiller where it exits my kettle so nothing sneaks in. My favorite use is to sanitize small amounts of water in my 1 liter flask - cap it with tin foil, give it a boil on the stove for a few minutes and the heat and steam will kill any bugs. Then after it cools I can use it for re-hydrating dried yeast. Or I can do the same thing with a bit of priming sugar in there before bottling.

Speaking of tin foil and JZ... he BITCHES about not liking having stoppers on carboys because stuff drifts into the crack between the stopper and the carboy... while using a carboy hood fixes this, so does a simple piece of foil wrapped around the base of the airlock.

Ahhhh....the many uses of aluminum foil.

Ahhhh....the many uses of aluminum foil.

You can wrap your head to keep out cosmic rays aka demons but it makes a poor condom.

There should be a constant off gassing (verry slowly) of CO2 so UNLESS you are adgitating the carboy constantly there SHOULD be no worry of O2 getting into solution, or verry little. Now if your going for Ninkasi or your looking to open a brewery maby this is something you should worry about, " Don't worry have a homebrew" Charlie Papazian

If you are cold crashing with a large headspace, that CO2 is going to 'shrink' and pull in ambient air through your airlock/etc. This would be a potential source of oxygen coming in contact with your beer.

Yes but even when your cold crashing there should be a constant off gassing of CO2 to make up for srinkage.... (I said srinkiage)

I disagree. If you are cold crashing, you should be done fermentation, so no new CO2 will be produced. The only CO2 in there will be what is dissolved in solution, and the solubility of CO2 is only going to increase as the temp drops, which to me means negligible CO2 will be emerging.