Every passive house takes a different approach on slab Insulation. We like the description provided in “Superinsulated Slab” from FineHomebuilding Energy Smart Homes 2015 page 109. Also available as a video. We also learned from Hunt technology, and Carly Coulson's approach with Newenhousem and the "Build it like This" article from Fine Homebuilding.
We read about some considerations on installing poly under a slab:
We would like a slab that has minimal cracking - and a nice finish - as it will be our final flooring surface. Concrete specs from the article: “The water/cement ratio should not exceed 0.45,” said Kanare. “You should specify a 5-sack mix, and the largest possible sized aggregate. For a 4-inch slab, you should be using 1-inch aggregate.”