[Cob] Mix

[Henry Raduazo]

> Janet: Note the R-value below for bales placed on their edge as  
> Ianto has done. Recent calculations have lowered this rating  
> because of air infiltration between the bales, but air infiltration  
> through 9 inches of cob on the inside and two inches of earth  
> plaster on the outside is zero.

Ed



R-Value of Straw Bales Lower Than Previously Reported

Much of the published information on the energy performance of straw- 
bale buildings is based on measurements done in 1993 by Joseph McCabe  
at the University of Arizona as part of his masterÕs thesis. McCabe  
used a Òguarded-hot-plateÓ apparatus (procedure ASTM C-177-85) to  
measure heat flow through a single bale that was 23Ó (580 mm) wide by  
16 1Ú2Ó (420 mm) tall. He reported R-values of 48.8 (RSI-8.6) for  
bale on edge and R-54.8 (RSI-9.7) for the bale laid flat. Thus he  
concluded that the insulating value is R-2.68 per inch (0.054 W/m¡C)  
when heat flow is perpendicular to the orientation of the straws  
(bales stacked on edge) and R-2.38 per inch (0.061 W/m¡C) when the  
heat flow is parallel to the straw orientation. These values were  
reported in EBNÕs feature article on straw as a building material  
( EBN Vol. 4, No. 3) and in many other publications.

Follow-up studies conducted since 1993 have given widely differing  
results. In 1994 a thermal probe was used by R. U. Acton at Sandia  
National Laboratory to deduce the R-value of a 16 1Ú2Ó-wide (420 mm)  
bale as R-44 (RSI-7.7), which seemed to support McCabeÕs findings,  
but this is considered a fairly primitive testing procedure. In 1996,  
Oak Ridge National Laboratory (ORNL) constructed a bale wall that was  
stuccoed on the cold side and covered with gypsum drywall on the warm  
side. This test found the R-value to be only R-17 (RSI-3.0). On a per- 
thickness basis, this is just R-0.94 per inch (0.15 W/m¡C). The  
explanation for this very low R-value, suggested researchers, was  
that an air gap resulted from the way the drywall was attached to the  
bale wall; this could have created convection currents in the wall,  
depressing the R-value.

On Jul 18, 2010, at 7:19 PM, Janet Standeford wrote:

> Understood. But I'm getting this permitted and they want to be sure  
> the insulation value is there and short of putting regular  
> insulation into the wall I don't think I can convince them to let  
> me do it with narrower walls. If anyone can get me true r and/or u  
> value for cob, let me know.
>
> Like the idea of the front tine tiller or cultivator. There should  
> be quite a few people here working on it. I'm going to get the high  
> risk youth out here with OIT so the kids can learn another way to  
> build as they are already doing stick built.
>
>
> Janet Standeford OR
> www.buildingnaturally.info (Owned by you)
> A resource for healthy homes.
>
>
>
> On 7/18/2010 3:26 PM, Henry Raduazo wrote:
>>     "three feet thick"!!!!? Wow that is very ambitious and a lot  
>> of mixing. With good cob, you could support a 5 story building  
>> with that wall. Even with fair quality cob that seems excessive.
>>     I have a picture of a slightly larger building that Ianto  
>> built. The walls are about 2 foot thick, but at least one foot of  
>> that thickness is a wall of straw bales. It has many times the  
>> insulating value of a 3 foot thick wall of cob and plenty of  
>> strength to support the roof, and the 8-12 inches of cob has more  
>> than enough bio-mass to store a day's worth of solar heat. You  
>> have to buy a lot of straw, but I think you will get your money  
>> back in lowering the amount of wood you need to cut to heat your  
>> building.
>>     Also, consider a front tine tiller or cultivator if you are  
>> going to mix that much cob. I doubt that two people can foot mix  
>> that much cob in one season and still have time to do the roof and  
>> all the other stuff you need.
>> Ed
>>