[GSBN] R value export straw blocks?

John Straube jfstraube at uwaterloo.ca
Sun Jan 25 11:00:54 CST 2015


Hi all
Learned people have already given some great background on what we know about thermal performance
Scientifically, given the much we know about heat flow, R-value of denser bales could be higher than fluffy bales.
Cellulose, rockwool, fiberglass and expanded polystyrene all increase their R-value as density increases from very low levels  All have an optimal point, which varies slightly, and then the R-value drops again.
We now have fiberglass batts that R-4.2/inch— they do that by increasing density and decreasing fiber diameter.
Stonewool semi-rigid board has a peak R-value/inch at a density of around 4 pcf.  
I expect (based on fiber diameter) that the density for optimal R-value will be higher than fiberglass or rock wool, but based on other materials it would likely be in the 8 to 12 pcf range (125-250 kg/m3).  But it sure is worth checking. If the density gets really high, then it is certain that R-value will drop again.
Finally, thermal resistance, unlike the fire resistance say, of a SB wall is the sum of its layers, assuming that we are talking only about thermal resistance and not air leakage or other construction/design flaws.  The R-value will be the R-value of SB like the R-value of fiberglass is that of fiberglass.  If we install either product incorrectly, leaving air gaps around any of the six sides of the bale / batt, the thermal resistance of the ASSEMBLY (not that of the straw bale or batt) will be degraded.  If we install either product in a badly designed assembly (that is, one without a good air barrier in contact with one side, and a convection suppressant layer on the other), then the R-value of the ASSEMBLY (not the batt or bale) will be degraded.
The purpose of using hot plates / heat flux (ASTM C177 / ASTM C518) methods is expressly to avoid confusion of assembly performance (overall R-value) with that of material performance (R-value/inch).
That said, the unassailable physics of steady-state flow tells us that you can’t get an assembly of overall resistance R3/inch out of materials with R-values of less than 3/inch. A straw bale wall that is plastered will have excellent air control, one with tightly butted joints or joints filled with light straw clay, will allow for full rated material performance.  The plaster, if normal earth, lime and cement plaster will have very little thermal resistance, perhaps R0.5/inch in best case, light clay with straw.
Hence, I think it still would be useful to measure the R-value of straws of different density and orientation just to gain a little better accuracy. I feel (and others may not) that we have a pretty good idea of R-value of compressed straw material: R1.5 to a touch over R2/inch.  But we should have more data points from numerous researchers. And then we need to much better define the assembly affects of quality of installation and quality of design: this must be done at near full scale, likely in made for purpose hot boxes (aka ASTM C1363).
One of the challenges of  straw bale material R-value is the fact that bales tend to come in 14-18” widths, and almost NONE of the material test equipment (177/518) can test that thickness.  
I have a student who is doing his Masters degree building a new large scale guarded hot plate that will allow for 4’x4’ chunks of insulation to be tested with thicknesses of 16-24” (also the thickness of cold climate country attic insulations these days).  Maybe in a year I will be able to get some SB and finally test the R-value myself…..
For those interested in more of the nitty gritty of this stuff as applied to the whole world of products, not just SB, check out 
http://www.buildingsciencelabs.com/new-hot-box-apparatus/
and download the many detailed reports at
http://www.buildingscience.com/documents/special/thermal-metric?topic=specialprojects


John Straube
www.JohnStraube.comOn Jan 24, 2015, at 6:12 PM, martin hammer <mfhammer at pacbell.net> wrote:

John F Straube
jfstraube at uwaterloo.ca
www.JohnStraube.com






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