Am planning to install Sterling Vikrel Ensemble on ground floor slab. Haven't torn out the old 1 pc fiberglass so don't know how level the floor is yet. Have recommended spun bonded underlayment as recommended by Sterling and may use if slab is ideal for that material. The install instructions don't suggest how much or if you multifold the stuff. They do say place the felt pad if provided. Many pros suggest throw it away and mortar. I also have mortar on hand and plan to use it as an option as Sterling suggests. You posted using visqueen poly for a cover over the mortar in another place but not so here. I was wondering if it depends on other conditions, type of tub, etc., that dictates use or not. Not to get crazy but could you quantify how much you would use in creating those 'piles' which spread when tub is set in. Also how many mil thick would you go on the poly? Like some others who have commented the waffle like molded tub bottom can affect how the mortar spreads, I wonder if this influences the decision for using poly either under, over or both sides of the mortar base. For a lot of us DIY'ers this seems to be a never ending dilemma. Having search extensibly for a consensus, I get the impression the pros go with the mortar. Appreciate all the good scoop on this forum. I don't think I would have tackled this project if not for the freely shared expertise passed along here. Thanks.