Poly vapor barrier under XPS foam board in slab radiant pour
I am building a 28×40 garage in New Hampshire. I am doing an I slab radiant pour. I plan to use 2 inch xps foam board under the slab and as a thermal break to to frost walls. The issue that I have is that this install is for a garage build and will not have a roof over the floor before the slab pour leading to the possibility of rain water ending up being caught between the vapor barrier and foam board before the pour. My question is why would taping the xps foam board seams not act as an effective vapor barrier? If this is not acceptable for some reason would it be reasonable to lay poly vapor barrier down, then foam and tape the foam board seams to keep any rain water from getting in between the vapor barrier and the foam? Thank you for your insights.
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