Cob more straw in cob=more insulation?; sun tubes

[Will Firstbrook WCB of BC]

Hi Vernon,

I have no direct experience but it seems to make sense that additional
straw would increase it's insulation value. I have heard that the R
value for cob is approx. R1 per inch, whereas straw is over double that.
The other factor is the thermal mass of cob. If the cob R value is
increased the thermal mass will also be decreased however slight this
effect may be. 

The concern is if too much straw is used the cob will not be strong.
Obviously the thicker the wall the greater its insulation value and
thermal mass will be. The insulation value I expect is mostly due to the
air space in the cob. While building the cob wall you could also poke
lots of holes to increase the air space within the wall. You could
increase the insulation value of the cob on the outside of the house by
using straw rich cob on the outside side of the center of the wall. Then
using good strong cob to hold the straw rich cob in-place. One could
also increase it's mass on the inside of the house by imbedding heavy
rocks on the inside side of the wall interior. 

If the wall is thick on the North Walls you could also corbel pockets
near the outside of the wall that could be stuffed with straw then
enclosed as you are building the wall.

Another option is increase the passive solar incorporated in the design
to make up for the slight loss in insulation value.

The solar tubes do look good but are really expensive. If you do find a
good alternative please post the details. Two earthships I have seen in
Kamloops & Vernon both had an extra set of windows in the roof to get
the sunlight to hit the back walls for additional light and for
capturing more solar heat.

Good luck with your cobbing.

Regards,
Will

PS: How close is very close to Canada, as I am very close (20min) to US
(Blaine).


>-----Original Message-----
>From:	Vernon B. Johnston [SMTP:vajohnston at nas.com]
>Sent:	Monday, August 25, 1997 12:54 PM
>To:	coblist at deatech.com
>Subject:	Cob more straw in cob=more insulation?; sun tubes
>
> I certainly have been enjoying this website.  Since I am in the process of
>building a cob house the information that passes back and forth has been
>relevant and exciting.  The rock foundation is just about finished, and I am
>about ready to apply the cob.  I am in the Pacific Northwest, very close to
>Canada.
>
>My questions are: 
>
> In leiu of using straw bales for the north side of the house, I am thinking
>of adding more straw to the cob mixture, and making the wall thicker in that
>area.  I would hope that I would get more insulation value with the added
>straw.  Does this make sense?
>
>I met with Eric Hoel, who is building with cob in Oregon.  He is adding more
>straw to his walls and making them thicker in hopes of getting more
>insulation value.  Unfortunately, I do not know if the extra work and
>material is significant enough to warrant incorporating this procedure in my
>project.  Does anyone have any experience and/or data with regard to extra
>straw?  
>
>Also, would the extra straw diminish the strength of the wall?  One of the
>reasons I am thinking of using this method would be for its load bearing
>capabilities as well as cob's proven longevity.
>
>Does anyone have any information or plans on building Solar Tubes?  I am
>thinking of incorporating one these for more light.  
>
>Many thanks,  Vernon vajohnston at nas.com
> << File: ATT00472.html >>