I intend to finish a part of my basement following this guideline from Fine Homebuilding and Building Science. In this configuration, the foam board completely contacts the concrete. My conern is that should a small water leakage event occur, such as a washing machine or a water tank leak, the water will have nowhere to go and gradually seep through the foam and eventually lead to the entire floor needing to be replaced.
I've noted some products on the market make a point in having air space in their floor contact area, for example AmDry, DriCore RPlus and ThermalDry. See sample photo below.
I'm thinking it would not be too hard to use a table saw and dado blade to cut channels in the back of the panels. I was thinking I would cut half inch deep, quarter inch wide, on a 4 inch grid.
The down side is that this will decrease the PSI capacity of the foam. I was thinking I might have to increase the thickness of the plywood over the foam. I'm not sure if this effort would even be worth it without the water-proof layer that the commercial products offer – however, I assume anything is better than nothing. I'm not sure if this will weaken the insulation and cause it to crack along all these lines.
Best Answer
I did almost what you propose, but used a commercial dimpled tile mat between the foam and the slab floor. It worked out great, no issues after 6 years.
That said, the go to place for detailed energy questions like this is Green Building Advisor http://www.greenbuildingadvisor.com/ (I have no affiliation with them).