Which will be multiphase as the insulation starts out as being full of air and later on may contain water. It may also be phase change, doable but not recommended for a beginner (pressing the buttons is easy, understanding why you're pressing them is a little less so).
If you assume a mass of water you need somewhere for the air to escape. But a velocity boundary would sort of suitable, but likely to give the wrong result.
Take a very big step away from the computer. Sketch out the whole domain on paper. What are the physical phenomena that you're modelling? If you did the experiment where, and what flow rates, temperatures etc do/can you control? What do you want to measure in the model? Be specific, and work out controlled values against resulting values (for flow in a pipe I can set a pressure drop or a flowrate; I then measure the other). Then look at where Fluent can help answer the above based on the inputs.
Then come back with what you think you need to model and we'll help more. There's a fine balance on here between helping and teaching: the former is quicker, the latter is much more useful longer term.
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