[Cob] cob/straw bale sandwich wall

[Robert Alcock]

Thanks, everyone, for your responses to my query about how to build a 
straw bale/cob sandwich wall. There seems to be a lot of interest in the 
topic!

You can see photos of our house going up at 
http://abrazohouse.org/gallery2/main.php?g2_itemId=323

There is a diagram of the wall structure at
http://abrazohouse.org/gallery2/main.php?g2_view=core.DownloadItem&g2_itemId=441

It seems that the way to go is as follows:
- build the straw bale wall first, with sufficient lateral stiffening to 
prevent distortion. Make sure the bales are protected from the weather 
while building. Preload it to ensure that the bales are compressed, then 
build the cob walls on the inside and outside
- apply slip to ensure a good adhesion between cob and straw, and making 
sure all gaps/joints in the bales are filled with cob.
- build the cob in layers from the straw bales outwards, not upwards in 
a monolithic wall, because this would pull away from the straw, leaving 
a gap in between the two.


"Unless your bales are extremely loose they should hold up the roof just 
fine - assuming that everything is well braced against lateral 
movement." ...
"The general theory is that it's the plaster that's carrying most of the 
weight in plastered strawbale construction."

These two statements seem to contradict each other. I guess the idea of 
pre-loading the bales is to ensure that the load is spread between the 
bales and the cob.


"Another method I've seen is to build a temporary frame first and put up 
the roof, then 
build the walls underneath. When the walls are about done, remove the 
frame and set the roof down on the walls, then just fill in the gaps. 
Voil?!"

I'd considered this, but building a temporary roof over a 2-storey, 
100-square-meter building on a windy site is not something to be taken 
lightly.

"The classic answer from cavity wall construction is indeed that the 
inner leaf should be the bearing one."

I think this is not applicable, because in cavity wall construction the 
insulation in the cavity has no structural strength.

"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."

Surely it would be the same amount of cob in the end? Also, I reckon 
that the time you spend waiting for the cob to dry so you can move the 
shuttering would be just as well spent applying cob directly to the 
walls. Also you've still got the problem of the cob separating from the 
wall when it shrinks.

"Just wanting to understand your reasoning: why are you doing both 
types? Did
you find that in your first building you didn't have enough insulation with
just cob?  What are your average (cold) temperatures?

Maritime climate; winter avg min T is about 6C. The cob cabin we built 
already needs quite considerable heating in the winter. We do get quite 
a bit of sun even in the winter (average sun hours in December=75, i.e. 
2.5 hours a day) so the combination of straw bale envelope and thermal 
mass should ensure very modest heating needs. (We're also planning a 
pond located to the south of the house to reflect more of that lovely 
winter sun into the house.)

Climate graphs for our locality are at 
http://www.lesspress.com/twiki/bin/view/English/Climate

"Another issue for us is the gap between the earthen walls and bales.  No
matter how hard you push those bales against the dry cob its not a tight
fit, a perfect little network of passages for mice etc."

I reckon that's a very good reason to build the cob against the bales, 
not vice versa.

"I would connect strawbales and fill the bales joints with cob (not 
throw the balewall, only edges - beacause of coldbridges). Then I put 
inside ca 15 cm cob (6 inches) and outside clay and lime plaster 
(directly to the strawbale). This method will insure that the whole wall 
is monolithic and load bearing. When you put strawbale wall and cob wall 
- then you have two walls, that play very differently. But cob in bale 
joints give strength and the wall seems to me like cob wall. ...
I studied this cob/strawbale hybrid method from Ianto Evans. I have used 
it many times, it works."

That's more or less what I had worked out, thanks for confirming it with 
your experience!

All the best

Robert