Vacuum sealed bags "ballooning"

This is being caused by steam pressure from the water in the bag, not the air.

This will basically just happen at higher sous vide temperatures. You won't see much of it at 131 degree F cooking of med rare meat, but if you're cooking at higher temps, then vapor is going to form and puff up the bag. The higher the temp, the more vapor pressure.

There's really no solution for this (if you're cooking at the temp you want) other than to cover the bag somehow and push it back into the pot. I use a small metal rack with a bag of (clean) rocks on it.

Thank you so much ! Vegies, 185 degrees ! Brilliant !

If you don't have a suitable rack / weights, the only other method I've found which works reasonably well if you have a deep water bath is to have a section of 'extra' bag above the food - if you position it correctly (and it usually takes some adjustment about 5 minutes after adding to the bath) you can get the 'extra' bit to fill up with the water vapour and float on the top, whilst the part of the bag with the food in remains under water and with sufficient water pressure to push the sides of the bag against the food.

Some vegetables such as cauliflower or onion will release gas when cooked.
The other option is that you can use Ziploc bags and open and reseal them after the gas has been released by the vegetables.

All very helpful, and as I think about it, the potatoes I did in duck fat, did not ballon at all. Must have been the fat.
Carrots have not behaved badly either, I add butter.

As I have found, there is a definite learning curve !

Like everybody else who has cooked SV veggies, I too have experienced the irritation of ballooned veggie bags.

As Dave mentioned, carrots are usually well behaved, as are asparagus. Broccoli is a little ornery, but my favorite, corn on the cob, balloons like crazy every time! When I subsequently chill and freeze them, the bags go right back to the tight vacuum they had before I cooked them.

Dealing with the floating is another issue. In my SVS, I lay down two large wrenches across the top of the main rack. The wrenches are just long enough that they reach just below the two bars that protrude out of the water. This way, the bags try to float and lift the rack, but then the rack gets locked against the wrenches and keeps the entire surface of the bags under water. This technique works best once the bags have already inflated a bit and started to float. I'm sure there is a more elegant object to use here besides a wrench, but it works for me.

I like LeeW's idea, and have tried that, but I usually can't get the veggies to stand up that way for very long, especially corn on the cob. Corn bags seems to produce so much gas that even standing up, they overpower Archimedes and don't allow much surface contact. However, I believe water vapor is still a much better conductor of heat than air (isn't it?), and at cooking times of an hour or more, I haven't had any problems with undercooked veggies.

I love corn on the cob, so I often cook-chill-freeze in batches. Nothing is more annoying than trying to cook 16 cobs at 185. For extreme floating situations like that, I arrange the corn in the SVS cooking rack, then put some small plates IN the top level of the rack, facing down. This helps weigh down the rack, and the plates extend to cover most of the area of the bath, holding down the evil bags that like to sneak out of the rack and make a break for the surface.