[Cob] cob/straw bale sandwich wall

[Robert Alcock]

Hi group,

As you may recall, we have built a small cob cabin (Snail Cabin) here in 
northern Spain and we are now starting work on a larger building.

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.

A compromise would be to build one storey of straw bale, pour the 
concrete bond beam, then apply the cob up to that height, relying on the 
weather protection offered by the concrete, and also the preloading that 
the concrete (+possibly roof/floor beams) provide.

Any other guidance would be most welcome.

Thanks a lot,

Robert
www.lesspress.com