Heat Pumps and Electric Heating - Porch with Cement Floor Radiant Heat

I have a porch with a cement floor that needs to be leveled. I'm going to use presure treated 2 x 4 cut to the angel of the slope and use 3/4 tonge and groove plywood to get it level. From there I would like to add radiant electic floor mat under ceramic tile. I will need a vapor barrier. Does vapor barrier go under ply wood or on the cement floor before the wood? Can the radiant mat go right on the ply wood? Are there any points that you could give?

I would put the radiant heat down then add inch of self leveling concrete.

When you say a porch, I picture a typical seasonal space that isn't really well insulated. What does your picture look like? :)

Bud