[GSBN] R value export straw blocks?

Pete Walker P.Walker at bath.ac.uk
Tue Jan 27 06:57:56 CST 2015

Thanks Andy,

I would add that the thermal conductivity of compressed straw board 
(STRAMIT) is quoted at 0.102 W/mK at a dry density of 380 kg/m3. This is 
not so far from what a linear extrapolation of our data between 63 and 
125 kg/m3 density would predict it should be.


On 27/01/2015 12:31, Andrew Thomson 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).
>> 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>_______________________________________________
>>>>>> GSBN mailing list
>>>>>> GSBN at sustainablesources.com
>>>>>> http://sustainablesources.com/mailman/listinfo.cgi/GSBN
>>>>> _______________________________________________
>>>>> GSBN mailing list
>>>>> GSBN at sustainablesources.com
>>>>> http://sustainablesources.com/mailman/listinfo.cgi/GSBN
>>>> _______________________________________________
>>>> GSBN mailing list
>>>> GSBN at sustainablesources.com
>>>> http://sustainablesources.com/mailman/listinfo.cgi/GSBN
>>> _______________________________________________
>>> GSBN mailing list
>>> GSBN at sustainablesources.com
>>> http://sustainablesources.com/mailman/listinfo.cgi/GSBN
>> John F Straube
>> jfstraube at uwaterloo.ca <mailto:jfstraube at uwaterloo.ca>
>> www.JohnStraube.com
>> _______________________________________________
>> GSBN mailing list
>> GSBN at sustainablesources.com
>> http://sustainablesources.com/mailman/listinfo.cgi/GSBN
> _______________________________________________
> GSBN mailing list
> GSBN at sustainablesources.com
> http://sustainablesources.com/mailman/listinfo.cgi/GSBN

Pete Walker
BRE Trust Professor of Innovative Construction Materials
Head of Department
Director BRE Centre for Innovative Construction Materials
Dept. Architecture & Civil Engineering
University of Bath
Claverton Down, Bath, BA2 7AY
Room 6E2.18
Tel: 01225 386646

Follow Architecture & Civil Engineering at the University of Bath:
Facebook & Twitter

More information about the Gsbn mailing list