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[idahohay]

I'm about to insulate under 1/2" pex , 8" o.c. held up by aluminum plates (3/4" subfloor).  The joists are 12", and I am using kraft face R-19 fiberglass.  The space below is also heated (in slab- over 2" rigid insul.)  The insulation is already on hand  and am thinking of putting the faced side up towards the pex. If I leave the insulation toward the bottom of the joists, there will be about 4" empty space under the tubes. Should I push it up to the pex? Should I put the kraft face down and put the fiberglass all the way up?  Any suggestions?

[juddspaintballs]

That's a tough one.  If you didn't have the plates I'd say leave the void at the top, but since you do I'm more inclined to tell you to put the void at the bottom so all of the heat goes into the plates or stays in the pex to heat the next plate.  No sense in heating a dead space that won't radiate into a living space I guess. 

[idahohay]

That makes some sense.  If foil faced was available, I would have installed the foil side up under the pex but since this is kraft face, it probably doesn't matter but would be easier to leave it down.

[renoman]

I put dubble bubble under the plates (foil up ) then pushed insulation tight up to that then plastic netting to hold it in .
seems to work very well thru 5/8 ply  with hardwood floor on top. Actually thought all the wood might insulate from the heat coming thru but still ok.