[GSBN] R value export straw blocks?

John Straube jfstraube at uwaterloo.ca
Mon Jan 26 18:25:49 CST 2015

I am not sure if I can send images but I am trying
If you are at high density (soil, wood etc), then reducing density increases R-value (that is decreases thermal conductivity). If you are at very low density, then decreasing density decreases R-value
The plot below is made up of many many materials.
The vertical axis is conductivity and the horizontal axis is density.
You can see the lowest conductivity / highest R-value per inch, occurs around 30 to about 125 kg/m3.  (2 to 8 pcf). The obvious deviation around 30-40 kg/m3 (2-2.5 pcf) is due to foams filled with gases other than air (Freon etc).

[cid:94635A9B-5E89-4C25-9492-DF86AF4F7D50 at gateway.2wire.net]
On Jan 26, 2015, at 3:35 PM, Graeme North <graeme at ecodesign.co.nz<mailto:graeme at ecodesign.co.nz>> wrote:

thanks John - I knew that there were at least some research answers out there.
Reminds me once again how lucky we are to have such wonderful researchers and scientists contributing so willingly to this group.


On 27/01/2015, at 9:06 AM, Graeme North <graeme at ecodesign.co.nz<mailto:graeme at ecodesign.co.nz>> wrote:



My observation of R values of different materials over many years , (esp with earthen materials  mixed with  aggregates of different densities ranging from stone to straw), generally points to an almost linear and direct inverse correlation between density and R value, whereby R value increases as density decreases - so am a bit puzzled by this discussion
Unless the  k value for straw is significantly better than that of still air?
And is there really any significant convective flow of air within a straw bale, plastered or not?
Has anyone got any figures?

More questions than answers


On 25/01/2015, at 12:43 PM, Bruce EBNet <bruce at ecobuildnetwork.org> wrote:

Let me jump right in with a couple of things to add to Martin’s post:

1)  I have been a board member and partial owner of Stak Block for ten years, so speak with a bit of certainty when I basically agree with Martin;  the thermal tests were a bit funky (an undergrad engineering student doing his first unguarded hot plate test at Cal Polytechnic University in California).  I emphatically agree that we don’t know, and would like to know, the optimal density of a straw bale (or block) for insulating purposes.  My gut sense is that it will be denser than conventional bales, maybe even as dense as Peter’s super-compressed bales.

2)  The widely-accepted R-values Martin quotes are averaged values taken across a plastered bale section, including the thickness of plaster.  A plastered straw bale wall is an intricate composite assembly that achieves far better structural, fire and thermal properties than the sum of its constituent parts.  That is, a straw bale by itself probably has much less than R1.3 or 2 as described because there is no plaster to arrest convective air flow across the assembly.

3)  When we ran the straw bale research program 14 years ago we did look at super compressed bales, but only glancingly.  If someone contemplates using them in a building, they should consider not only R-value, but also bond of plaster to the face of straw (is it better?  worse?).

cheers everyone,

Bruce King

(415) 987-7271

<BWL logo for email.jpg>

On Jan 24, 2015, at 3:12 PM, martin hammer <mfhammer at pacbell.net> wrote:

Hello Lance,

A delayed reply on this.

A company in California called Oryzatech (http://www.oryzatech.com/) has for years been in the development of manufacturing a compressed straw block called Stak Block (see attached fact sheet). They have made claims of an R-value of 3.89/inch (see 2nd attachment). I like this product in many ways and think it has tremendous potential. However I’m skeptical of the R-value claim because I haven’t seen a bona fide testing report, and it’s hard to believe the R-value of a compressed straw block would double compared to a typical straw bale.

The R-value for a straw bale, from the most trusted test in the US (the 1998 guarded hot-box test at Oak Ridge National Laboratory) is R 1.3/inch laid flat and R-2/inch on-edge. This is still a matter of debate, but this is what the testing showed. The difference in R-value per inch is explained by the predominant orientation of the straw in a bale (or at least in the bales tested).

Though counterintuitive, it’s possible a compressed bale would have a higher unit R-value than a normal bale, if by being compressed it confines more air spaces. Thermal resistance is all about maximizing the number of confined air spaces and reducing thermal bridging. Regarding the latter, I would expect the thermal bridging across a bale would increase when it is compressed. There is likely an optimum density for straw that will yield the highest unit R-value, but this has yet to be researched and demonstrated.

Another point of thermal resistance comparison is polyiscocyanurate, which has the highest unit R-value of any foam plastic insulation at R 5.6/inch. For years polyiso claimed an R-value of 6.0/inch (or higher), but it was adjusted downward a year ago under new testing protocol. (Sorry to bring a distasteful petrochemical insulation into the discussion of natural insulation! It does have quite an ability to insulate however.) Fiberglass insulation is said to be R3.1 to R4/inch (material only, not including thermal bridging of framing).

Regarding density, from the Stak Block fact sheet, the 1’x1’x2’ blocks weigh 30 pounds. So they are 15 pcf or 240 kg/m3. Peter’s compressed bales are 468 kg/m3. Those are quite dense, almost twice as dense as the Stak Blocks, and 4 times as dense as a typical straw bale. Even if you trust the R-values I’m stating for a typical straw bale and for a Stak Block, I don’t know how you would reliably extrapolate them to a denser block. The obvious answer is to subject Peter’s blocks to a reliable test.

You or Peter Torok might contact the co-founder of the company Stak Block to better understand nature of their blocks and their tested thermal resistance.  Ben Korman: d2bdesign at gmail.com

Speaking of Peter, was he ever seconded and brought into GSBN?

I hope this is helpful.



Martin Hammer, Architect
1348 Hopkins St.
Berkeley, CA  94702

On 1/1/15 7:41 PM, "Lance Kairl" <sabale at bigpond.com> wrote:

Any one have  an idea on R value for super compressed export Bales.

Any info will be passed on ,
Although I should nominate Peter  to join the list.
Is there a seconder out there, and then I will fill you in on his good works.

Regards lance kairl
Hosue of Bales.

From: Peter Torok [mailto:torokenterprise at me.com]
Sent: Thursday, 11 December 2014 1:13 PM
To: House of Bales
Subject: R value

G'day lance,

As discussed these bales are very well compacted, the dimensions are 400x500x480 45kg or 400x500x240 22kg baled at less than 12% moisture and compressed at 5000 psi. If the bales were sitting on the 400 side, the straw runs horizontal. I inquired about lowering the pressure and he felt the integrity of the bale would be jeopardized, but more pressure can still be applied. I hope that is enough information to calculate a rough R value for both thickness', I look forward to hearing what you come up with. Thanks for helping me out with this, it's very much appreciated.

I have found old studies from around 2003 that calculate between R1.4-2.4 US measure / inch

This R1.4 – 2.4 relates to standard housing bales,
Export ones may equate to the R value  for Timber??

Regards Pete Torok

Earth Wood & Straw

0411 304 794 <tel:0411%20304%20794>

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John F Straube
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