I am converting my garage to living space.  We live in climate zone 6 (Montreal).  The garage slab is at-grade and the room next to it has a slab that is 12″ above-grade.  The house is built in the 1950’s and there is no insulation under the slabs.  The garage has concrete block walls and the other room has wood walls.  Both areas are being finished with a plywood+hardwood floor that is at the same height as the rest of the first floor (30″ above grade).  We are essentially creating a small space under the floor (~18″ high “crawl space” on one side and ~7″ high on the other).  The ducts will go through the space so it will get some heat indirectly and the return air will vent through the crawl space (via an open return duct under the floor), so it should generally be conditioned.  I will be able to access the 18″ space via a trap door, but won’t be able to access the 7″ space after the floor is installed.  I plan to use the 18″ space for storage.  Prior to this renovation, the floor in the non-garage area was quite cold in winter and was insulated with 1″ of EPS with a floating floor on top.

I am currently planning to lay down 2 inches of Durospan GPS directly on the slab for some basic insulation and then cover that with a 6 mil or 10 mil vapour barrier (VB) to block out moisture and soil gas (running the VB up the short walls).  I will also apply 2″ of GPS to the concrete walls of the garage area.  The heating ducts will rest directly on the VB/insulation.

I am unsure of a few things:

1) Should I put the VB first directly on the slab?  All advice I can find online focuses on below-grade and under-slab insulation, which isn’t my case here.

2) Will this be ok in terms of fire safety in the 7″ section?  Foam needs to be covered with something that is fire resistant, but I’m not sure whether that applies here since there is a floor above it (but 7″ above it).  Also, there is ductwork, electrical and plumbing in that space too.

3) In the 18″ section, will I need to lay down plywood and cover the walls for fire safety since I am storing stuff there?

4) Should I put a thin layer of wood between the ducts and the VB/insulation?  For instance 1/8″ plywood.

5) Should I just forgo the insulation and stick with just a VB?  I am worried about cold floors, but perhaps the return air under the floor will suffice?

I appreciate answers to any of these questions, as well as any other suggestions.  Note: I do not plan to use spray-foam due to health issues (allergies/asthma) in our family.  My wife is also allergic to mold, so I need to make sure the space will stay completely mold-free.

Thanks!