Re: [GBlist] Re: Big Green: Radiant Floor Heating

["Mike Rogers" <rogers.mike@verizon.net>]

Clark wrote:
> Ok, this may not be a valid question but would the weight of concrete
> compress the two inches of foam and then adding another two inches of foam
> just cause it to settle enough to crack the slab?

Good question.

If you have a foam with a high compressive strength, you won't get much
compression in a residential slab, and what little compression there is
should happen when the concrete is poured.  25 PSI =3,600 pounds per square
foot!  (I apologize to the metric users for the units.)

All foams are not equal.  I'm simplifying a bit with the following
information, but it can give you a rough idea of some of the differences.
You would need to check with specific products for individual specs.

  Expanded Polystyrene -- EPS
    Typical compressive strength around 15 PSI, but available higher
    R-value per inch about 4.0
    Water absorption < 3% (of volume)
    ~2.5+ perm-in

  Extruded Polystyrene -- XPS
    compressive strength around 25-40 PSI .
    R-value per inch about 5.0
    Water absorption < 0.1% (of volume)
    ~1.0 perm-in

I like the higher compressive strength of XPS (caveat, you need to check,
and I believe the highest compressive strength EPS exceeds the lowest XPS).
I also like the fact the XPS doesn't absord as much water.  And although the
EPS has a higher perm rating, XPS is still "semi-permeable".  For
comparison, aluminum foil has a perm on the order of 0.001 and polyethylene
on the order of 0.005  And I like the higher R-value of XPS, especially with
a radiant slab.  However, EPS is typically less expensive than XPS, and I'd
find it hard to argue with Joe's and David's recommendation for EPS.

Hope this helps,
Mike


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