We will have a radiant concrete slab over plywood sheathing on 9 1/2″ LVL joists over an unheated garage in a climate zone 4 (Bay Area).

In a normal (non-radiant) floor situation, one would fill the joist space with insulation and put rigid underneath, then sheetrock.

Is this still necessary with the radiant slab, or would 2 layers of semi-rigid mineral wool insulation be enough to keep the radiant slab from losing heat downwards?

Is mineral wool (Roxul) the wrong way to go?

How about an air or vapor barrier, considering it’s a poured concrete slab?