I have a few slab-related questions I’m hoping to resolve before the concrete is poured, possibly in a few weeks.

My building, a workshop with in-floor heating, is in western Montana, IECC climate zone 6B.  The foundation is a frost-protected shallow foundation for an unheated building (from bottom up):  subgrade of sandy clay with lots of rocks,  6″ grave subbase, 3″ layer of XPS insulation, 16″ of fill on the interior (no fill under the perimeter footing), vapor barrier, monolithic slab.  Drainage on the site is generally very good except periods of fast snow melt.

1.  Subbase material:  The local cement plant suggested 3/4″ crushed gravel because it is easy to place, compact, and level, and tends to stay where you put it.  It is dusty and not washed but I presume the fines have been screened out.  Is that a good choice or should I look for washed gravel?  I was going to use the same material for the fill as I used for the subbase.

2.  Footer drain:  I was going to lay perforated 4″ schedule 40 PVC on the subgrade 24″ out from the slab footprint.  It will drain to daylight.  It seems like overkill in this climate but it is inexpensive (15 sections of pipe + fittings).  Is it worth it?  Actually the drain is simple, having cleanouts is a bit of a PITA.

3.  Footer drain filter:  Should I lay down geotextile fabric to keep fines from infiltrating the subbase?  A roll of material runs $400 although I need only 1/3 to 1/2 of that.

4.  Insulation at doorways:  If you imagine a typical detached garage the slab is exposed in front of any garage door and is in direct contact with the threshold of any people door.  How are these spots typically insulated?  I am thinking of adding blocks of XPS insulation to the inside of the concrete forms at these locations.  So at the garage door the concrete slab will be in the plane of the exterior surface of the door and vehicles will drive over a 5-1/2″ wide “gap” of XPS foam.  Suggestions?