[GSBN] R value export straw blocks?

John Straube jfstraube at uwaterloo.ca
Tue Jan 27 07:46:22 CST 2015

Andy thats a fantastic piece of work.  Thx for sharing. Large samples not yet bale sized but bloody close and a real nice density range to study.  Very useful data to add to the pot.
The flat bottom of the density vs conductivity curve is actually close to what we would expect if you go back to that scatter plot I sent.  The conductivity is just a little higher than what one might be able to achieve (I put a circle between k=0.04 and 0.063 and density of 60-120 kg/m3). The 0.06 is a on the high side for what we would expect at low density for a normal material, but most materials on the plot I have shown are insulation materials— they have the best insulation value for their density, a density they need to provide other functions.
The effect of RH is known, but very hard to quantify as far as I have seen, and you show a large effect. Important to track down.  Of course 80%RH is not where want straw to operate….
The effect of temperature is highly predictable— it is almost entirely due to radiation transfer and occurs in all insulations.  The effect is larger for highly open pore materials (usually low density) and smaller if the fibers are small. I attach  couple of plots, the colourful one is from our labs and is based on about 8 different mean temperature. (The polyisocyanurate PIC has a blowing agent that condenses and so has a very different signature)

I am beginning to think that the fiber diameter of SB varies sufficiently between types to explain results that vary almost 50% from 0.040 to 0.060. A value of 0.04 is hard to imagine achieving in practise as it basically what most stone wool products achieve, an they have a lot more control of fiber orientation and diameter.  Running some fibers through a light hammer mill to split and open the fibers, remove dust, then rebale until we find a good way to make the fibers fine long but smaller diameter would be a thought Worth it? Doubt it, but then we could perhaps identify which straw in the field is best for thermal performance, and which is not..

The square modernistic box style of the BaleHouse worries me more than a little: what has been done to change the bale walls to accept the much higher rain load they will now receive given that architecture and how are the water injection sites of the plaster to wood joints managed?

[cid:D4CBC5E1-688E-4DA2-98B4-990D5C6DFA58 at gateway.2wire.net][cid:BF8F8BBE-F2B0-4DF0-ACA7-C2B57BFA3CA4 at gateway.2wire.net][cid:CE174D05-193B-46D1-ABF9-7EDBCCB9A4CA at gateway.2wire.net][cid:FD6F46E0-BBC1-46F0-A4EA-5BCA7B992320 at gateway.2wire.net]

On Jan 27, 2015, at 7:31 AM, Andrew Thomson <ab3ajt at bath.ac.uk<mailto:ab3ajt at bath.ac.uk>> wrote:

Hi All,

I have recently joined as a posting member to GSBN so thought I'd add to the conversation on thermal conductivity of straw at different densities. Colleagues of mine at the University of Bath have done some testing in this area and published the findings - details here: http://opus.bath.ac.uk/30137/

The paper includes a graph (copied below) of tests done at Bath Uni and others, including FASBA. The results suggest that below a density of 120kg/m3 thermal conductivity is less sensitive to deceasing density than might be expected. It appears that the thermal conductivity of straw is actually more sensitive to the temperature of the straw and it's moisture content. Hope this is of interest.

Cheers, Andy


Dr Andrew Thomson MEng PhD
BRE Centre for Innovative Construction Materials
Dept of Architecture and Civil Engineering
University of Bath

01225 386621

On 27/01/2015 00:25, John Straube wrote:
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).

<Mail Attachment.jpeg>
On Jan 26, 2015, at 3:35 PM, Graeme North <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> 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>

GSBN mailing list
GSBN at sustainablesources.com
GSBN mailing list
GSBN at sustainablesources.com

GSBN mailing list
GSBN at sustainablesources.com

GSBN mailing list
GSBN at sustainablesources.com

GSBN mailing list
GSBN at sustainablesources.com

John F Straube
jfstraube at uwaterloo.ca

GSBN mailing list

GSBN at sustainablesources.com
GSBN mailing list
GSBN at sustainablesources.com

John F Straube
jfstraube at uwaterloo.ca<mailto:jfstraube at uwaterloo.ca>

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://sustainablesources.com/pipermail/gsbn/attachments/20150127/8fa9ddab/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: F1 Conductivity temperature insulation.jpg
Type: image/jpg
Size: 1509816 bytes
Desc: F1 Conductivity temperature insulation.jpg
URL: <http://sustainablesources.com/pipermail/gsbn/attachments/20150127/8fa9ddab/attachment.jpg>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: insulation conductivity density graph.jpg
Type: image/jpg
Size: 194904 bytes
Desc: insulation conductivity density graph.jpg
URL: <http://sustainablesources.com/pipermail/gsbn/attachments/20150127/8fa9ddab/attachment-0001.jpg>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Thermal conductivity temperature.jpg
Type: image/jpg
Size: 142071 bytes
Desc: Thermal conductivity temperature.jpg
URL: <http://sustainablesources.com/pipermail/gsbn/attachments/20150127/8fa9ddab/attachment-0002.jpg>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: PastedGraphic-2.tiff
Type: image/tiff
Size: 592302 bytes
Desc: PastedGraphic-2.tiff
URL: <http://sustainablesources.com/pipermail/gsbn/attachments/20150127/8fa9ddab/attachment.tiff>

More information about the Gsbn mailing list