Vapor-open heated slate-on-basement-slab mats?

I hope this is the right category to post this in. We are having a slate floor installed over our basement slab, about 170 ft.² in two rooms. We are already wired for 240V heating circuits, but we are now at a loss for how to install the heating cable itself without creating a mold-conducive microclimate between the slate and the slab. It seems to me (and I’m willing to be wrong) that either an installation mat or a polystyrene board on top of the slab is asking for moisture trapping. Is using cable strap the only viable option to keep it vapor open? And if so, are there any other downsides besides the not-insignificant heat loss to the slab? Any other solutions? Thanks for sharing your experiences regarding this.