[Greenbuilding] R-value of crushed stone under slab

Tue Dec 8 10:06:54 CST 2015

Decoupling the model from the slab removes it from an assumed heat source. Leave it coupled but add the 24” of gravel as a low value underslab insulation below the foam.

Insulation curves start to flatten after 4” – in Ontario during the 90’s they actually were looking at reinstating r12 as an acceptable wall standard for that reason. Flattens more dramatically after 6”. I think one of the benefits of PHIUS is that it is making large amounts of insulation socially acceptable in the building community. The reality is that there is a great deal of conflict between size of house, insulation levels, ventilation requirements and heat recovery efficiency. 

I think if you accept the concept that there is no such thing as free energy – any energy savings is going to result in a transfer of energy somewhere else – utilization of some equivalent material at some point in the process.  E=mc2 type of thing. Even solar energy is not free as it is simply being redirected.

From: Greenbuilding [mailto:greenbuilding-bounces at lists.bioenergylists.org] On Behalf Of Alan Abrams
Sent: December-08-15 2:14 AM
To: Green Building
Subject: Re: [Greenbuilding] R-value of crushed stone under slab

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, AIBD
certified passive house consultant, PHIUS

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

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Rob Tom T6015O
Kanata, Ontario, Canada

< A r c h i L o g i c at Y a h o o dot C a >

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