[Greenbuilding] R-value of crushed stone under slab

Alan Abrams alan at abramsdesignbuild.com
Tue Dec 8 04:14:21 CST 2015


23.8  Table 3A, for "Sand and gravel or stone aggregate of 140 pcf density"
it lists its thermal resistivity (ie  R-value per inch thickness) as being:

                  (oven dried)  0.11     and   (not dried)   0.08    in
units of   h*ft^2*degF/Btu

thanks, that same citation is included in the AGS, but methinks that it
refers to a solid, compacted mass of sand AND gravel, whereas the condition
in question is washed, crushed stone, occupying maybe 60-70% of the volume,
and the remainder being air. I would also assume there would be less
conductivity between the stones than in sand and gravel.

the notion of including the mass in the thermal envelope is interesting,
but complicated, considering the complexity of grade beams and pier
footings that were incorporated in the slab, not to mention its air
barrier. the original grade was sloped as well, so the fill was a simple
means of leveling the surface for the slab system.

here's the significance of all this angst. this project was pre certified
by PHIUS over two years ago. Now that the house is complete, and has been
occupied for over a year, I'm finally getting around to documenting the
as-built conditions, for final certification. When I revise PHPP to
de-couple the slab from the earth, and use R-0.11 for the stone, and the
value Norbert provided for specific heat, it bumps up the modeled heat
demand for the entire house by 8-9%.

On the face of it, it sounds dramatic, but then again the heat demand is so
low to begin with, the net difference is miniscule. On the other hand, it
bumps the house over the 4.75 KBtu PH maximum.

If, however, I use the lesser R-value that PHPP prompts for dry sand: R-0.6
to R-1.0, the modeled heat demand plummets, even lower than my original
projection.

I had fiddled with these two values--soil heat capacity, and thermal
transmission--during the initial submission to PHIUS, and was instructed to
keep my hands off the default values that the spreadsheet is loaded with.
If I honor those instructions, the house should pass certification, and
this conversation (insofar as it relates to the project) is moot.

But my preference is to reflect actual conditions. At some point, it begins
to call into question the value of certification processes like this. The
cost of the 9 inches of high density EPS foam was ponderous. I did a
what-if analysis in PHPP of reducing the thickness to 2" of foam, and
concluded that it would require only a few dozen extra therms a year,
representing a payback period measured in many decades.

factor in the chemical impact of the foam itself, and it's questionable
whether this approach will go far to save the planet.

-aa

PS--thanks for noting the missing R

Alan Abrams

*certified professional building designer, AIBDcertified passive house
consultant, PHIUS*
*certified passive house builder, PHIUS*
cell     202-437-8583
alan at abramsdesignbuild.com
HELICON WORKS *Architecture and Education*
<http://www.heliconworks.com/index2.html>


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