[Cob] hybrid, bags or straw..

[Scott Van Kirk]

Earth bags are usually filled with UNstabilized earth.  They go up very 
quickly and form a very solid wall.  The earth in the bags compresses and 
becomes quite hard.  Because there is no mixing of the dirt, no straw, and 
no waiting, bags are considerably quicker to build with than cob. Just fill 
the bag, tamp it down and go on to the next one. As you noted, you do need 
to put a plaster on the outside of them, especially if you use some sort of 
plastic bags.  Plastic bags degrade in sunlight, but last indefinitely if 
coated with an earthen plaster.  One of the cool things people have been 
doing with earthbags is making earthbag domes.  This way, you can use 
earthbags for the entire building.  Check around on the web, there are some 
good sites which show this.  Some even have plans you can buy for small 
earthbag homes.

As a side note, some people use bags filled with gravel for the first few 
rows to prevent water from wicking up into the wall.  Many people advocate 
the use of earthbags as a foundation for straw bale buildings.   I would 
talk with an engineer or someone with experience with this before going 
this way though.

The only advantage I can think of for cob is that you wouldn't need to find 
a source for your earth bags (  such as misprinted poly rice bags  ).  I 
guess the purists might object to having plastic in their walls as well.

As for straw bale vs other systems.  Straw bales go up a LOT quicker than 
cob or earth bags.  They have some thermal mass because of the plasters on 
either side but they are mostly insulative. Straw bales do need to be kept 
away from moisture so they would require a foundation with some sort of 
moisture barrier.  You also have to pay more attention to the finishing 
detailing of doors and windows with straw.  Those are the places where most 
bale walls run into trouble.  There are many people out there who could 
explain how to finish windows and doors properly.  Its not hard.

The people I have known who have done hybrid systems typically use a high 
mass wall like cob or packed straw clay on the south and straw bale on the 
north, east and west.  I have also seen people who make cob/straw bale 
hybrids where they use bales on the flat sections of the walls and just cob 
around the bales.  This seems to have promise.

-Scott

On Thu, 6 Nov 2003 21:17:18 -0800, Jilly <JILLPRUETT at peoplepc.com> wrote:

> Can anyone tell me the differences (benefits / drawbacks) between cob and 
> earth bags? I think it is called earth bags. You put "stabilized earth" 
> into bags and then use cob or plaster to cover over that.
>
> In the cob books I have, there is a suggestion to use straw bale or other 
> type of hybrid system, particularly for the north wall but also for the 
> speed / ease of building. IF using a hybrid system, what would you 
> suggest - straw or bags? or...?
>
> Thanks for the gypsum info... I need to read more. I am still confused.
>
> jilly
>
>
>
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-- 
-Scott