After fiddling around trying to make a steam boiler with batteries, I managed to make an open chamber that holds free-standing near-void (<70g carbon dioxide). No other gas is being pulled in.
Interestingly, if I continue from this save file and run the game at max speed, the pressure in the battery room continues to drop to around ~5g carbon dioxide per tile. There will be a slight increase in pressure as the remaining water on the batteries evaporates, but the steam seems to move around the room and never escape.
Before this started, the room was filled with carbon dioxide. I had tried to build an insulated tile in the gap in the battery / hydrogen generator room, but mistakenly built a mesh tile. The problem started after I had the mesh tile tile deconstructed, but I am not sure if this is the root cause.
Built and deconstructed a single mesh tile as a wall to a sealed room