"Ross Hebeisen" wrote in message
...
i sell outdoor wood furnaces here in minnesota
www.centralboiler.com i have had experience with differant
installations. myself i put the pex in the sand under the concrete about
2" deep with foam only around the outside of the slab about 20" deep.
once the sand is heeted up it's a big thermal storage basin. i recommend
not more than 250' of pex on a loop or zone. keep your beginning hot run
on the outside run in about 20" from edge of slab with 12-16 " spacing
on the loops. keep th hottest part of your runs to the outter working
inwards for the return to manifold. i know of many places here set up
like this and it works well. of course it only gets to around 30 below
here at times.

My research suggests that the need for insulation under the slab is
dependent on the presence of water in the soil under the slab and the level
of any water table. Water will conduct heat rapidly. Dry clay will act as
a thermal mass and while heat will leak out, like you say, heat prefers to
rise. The normal temperature of ground below the atmosphere affected zone
is ~53F. not insulating under the slab will mean that the slab in a covered
building is unlikely to go below ~40F even after an extended period of no
heat, but use caution YMMV. If the heat source is costly and used
constantly then insulation may be indicated, but I would submit that 1" or
1.5" would be sufficient. If the heat source is DIY wood then cost may not
be a major factor and the added thermal mass of the under slab clay may be
more worthwhile, since with wood heat, storage is a bigger consideration
since the fire will not likely be continuous. Not insulating the slab will
also tend to keep the floor cool in the summer.

In any case, heat is far more likely to leak out of the edge of the slab and
I would suggest that is where insulation should/must be placed. As you
suggest 2" vertical rigid board 24" deep and extending to the top of the
slab. Allow the (2x6 frame) wall to hang 2" over the foundation to cover
the top of the foam board. I think that the effectiveness of this board can
be doubled by the addition of another 1" foam board 24" wide buried at 45
degrees which will prevent the frost from penetrating next to the slab.

I have no comment on burying the piping in sand under the slab beyond that
it is an interesting idea. I am inclined to think that the heat conduction
will be better with the tubes in the slab but YMMV.

IMPORTANT, If the tubes are poured in the slab they should be filled with
water before the concrete is poured in order to stop them from floating to
the surface of the concrete.

Good luck, YMMV