I have a vacuum insulated wall and it keeps turning into carbon dioxide. It's right above a triple airlock. It seems to happen randomly whenever dupes go through it.
I thought there was some weird interaction between the doors and the airlock so I added in the extra layer of insulation tile between the two. It obviously didn't make any difference.
Make triple airlock. Make vacuum above it. Have dupes go through it and hope you get (un)lucky.