Heating Help

Suspended slab cracking? Portland and sand

I have a question about the suspended slab. I am using a 1-1/2" dry mix 1:5 portland/sand on the sub-floor with a vapor barrier and sleepers. From my days as a tile setter we used lath on the floor. I really don't want to use lath unless I have to, so I was considering Nycon fiber reinforcement, nylon fibers. I have heard some people using latex. I wonder with the sleepers as relief points at 2-4' apart what would be necessary, if anything? The main concern is to keep the slab from cracking over the pipes I guess, although in warm board and under-the-floor applications the pipes are exposed to the floor. Do you have any suggestions? A radiant heat installer I spoke with says that with a floating engineered wood floor over it, it would be fine functionally and wouldn't need any reinforcement really and cracking would not be a problem.