[GSBN] R value export straw blocks?

Andrew Thomson ab3ajt at bath.ac.uk
Tue Jan 27 06:31:08 CST 2015

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: 

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).
> 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.
>> Graeme
>> On 27/01/2015, at 9:06 AM, Graeme North <graeme at ecodesign.co.nz 
>> <mailto:graeme at ecodesign.co.nz>> wrote:
>>> Bruce
>>> Interesting
>>> 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
>>> Graeme
>>> 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
>>>> BuildWellLibrary.org
>>>> <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.
>>>>> Best.
>>>>> Martin
>>>>> 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
>>>>>> http://sustainablesources.com/mailman/listinfo.cgi/GSBN
>>>>> <StakBlockFacts.png><StakBlockR-Value.png>_______________________________________________
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> John F Straube
> jfstraube at uwaterloo.ca <mailto:jfstraube at uwaterloo.ca>
> www.JohnStraube.com
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