[Greenbuilding] passive solar in southern NH

[Alan Abrams]

<I'm a bit puzzled about slab on grade and radon.   Wouldn't the first 
thing placed under the slab be a sheet of plastic as a water barrier 
which would also be a radon barrier?>

Good point--the vapor barrier (implicit in my previous comments) is a
necessary part of the radon barrier--indeed it turns out to be MORE
important than the concrete--rereading Appendix F of the IRC 2003 on radon
protection, it IS permissible to place "a uniform [4"] layer of
sand...overlain by a layer...of geotextile drainage matting designed to
allow the lateral flow of soil gases" or "other materials, systems or floor
designs" that can provide depressurization of the subfloor.  Because it is
such common practice, I had earlier been assuming that the slab was a
requirement...

(I am baffled by the logic of allowing geotectile material, however--isn't
this stuff vapor permeable?)

This goes back to George Nesbitt's earlier suggestion of placing a layer of
rigid foam directly over a gravel bed, overlain with a floating finish
floor--a very elegant solution.  If there were a vent with intake embedded
in the gravel, and a vapor barrier beneath the foam, then this would
arguably meet the "other materials, systems, or floor designs" clause.

It should be noted--at least for basement floor applications--that a
conventional slab typically functions to resist lateral soil pressure
against the bottom of the basement walls.  If the slab were omitted, lateral
forces would have to be handled by some other means.

-AA