[Cob] Coblist Digest, sandwich walls

[Dean Sherwin]

The classic answer from cavity wall construction is indeed that the 
inner leaf should be the bearing one.  the best way I think would be 
to have a cob wall for the interior built first, then straw bales and 
the exterior plaster wythe  (at 5cm it is not really wall) applied 
later.  They are non-loadbearing and the exterior layer can go on 
directly to protect the straw.
However surely this would need an interior  wall of stand-alone 
thickness (at least 10 -12", 25- 30 cm)  ?
Understandably you want to keep the total width managebale (haha nice 
typo slip there);  have you considered poured cob for the inner 
layer?  Form the one side and use the straw bales as permanent 
forming/shuttering for the other.  Yes extra expense of shuttering ( 
can be moved in sections) but less cob and quicker.  Still has to dry 
out, to keep mixture not too wet best to use a vibrator when 
placing  (in maybe 4' high lifts).  Also will work it well into 
straw.  the inner leaf, the cob wall, will be slightly higher than 
the straw bales and will still be the structural support, with 
flexural rigidity from the straw etc  (wall ties placed in or between 
the bales)
I met a guy some years back in UK who was doing research into poured 
cob.  If necessary could possibly resurrect contact info.

Dean Sherwin
Devonshire native



At 03:00 PM 7/21/2008, you wrote:
>We are looking at using a load-bearing hybrid straw-bale/cob wall
>consisting of straw bales laid on edge (35cm thick) with cob either side
>(10cm interior and 5cm exterior). We plan to pour a concrete bond beam
>at first floor and roof level of this two storey, 100m2 per floor, house
>and study centre.
>
>Does anyone on the group have experience with this "sandwich" type of
>wall construction?
>
>Is it better to put up the straw bale wall first, put in the bond beam
>and roof/floor beams, and then do the cob? The advantage of getting the
>roof on quickly is obvious, but then I'm concerned that the straw bales
>alone won't be strong enough to hold the roof.
>
>If the cob and straw bale go up together in parallel,  the bales will be
>exposed to the weather for a lot longer (at least, the top of the straw
>bale wall will be) and we get rain regularly here, even in summer.
>
>In this case, on the other hand, we need to think about what will happen
>to the dry cob and straw bales when the load of the (green) roof and
>soil are applied - since dry cob is virtually incompressible it will be
>taking the whole load of the roof on its own, without the straw bales
>helping.



Dean Sherwin CPE
Certified Professional Estimator
LEED Accredited Professional
CONSTRUCTION COST MANAGEMENT
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