I am about to begin building a floor for a living space inside an at-grade garage. I live 500 yards from the beach in northern California where it is not that cold but very wet. The following is my plan; I'd appreciate it if a more knowledgeable builder would tell me if it's any good. I have already laser measured the slab and rebuilt the doors so that they will swing over a finish floor as high as 5 1/4 inches above the highest point on the slab. I'm going to seal the slab with dryloc and a sheet of plastic, the edges of the plastic going up the inside of the walls a few inches. Then I'm going to lay down 3 inch-thick panels of high-compression foam insulation. Then I'm going to cover that with a radiant barrier facing up. Then I'm going to lay 1x3 furring strips every 16 inches o.c. over the foam panels. Then I'm going to adjust the furring strips to level across the whole space with shims (the slab is pretty uneven). Then I'm going to put a loop of 3/4 inch PEX down the middle of each pair of furring strips. I'm going to have all the PEX come out where I want my future boiler. Then I'm going to cover the PEX and the furring strips with inch-thick plywood. Then I'm going to build my non-load bearing interior walls on top of the plywood subfloor, hopefully nailing only into the furring strips. Then I'm going to build my finish floor with cement board, mortar and tiles.

Does this plan sound good? Is there something I'm forgetting? Will I need to fasten the plywood to the slab through the foam?

 

I think there are several possible mistakes in your plan but I am going to defer to others for the time being.