[Cob] hybrid bales bags

[Scott Van Kirk]

Joe, you bring up some good points that I think anyone building their own 
homes should consider.
Building a house is not like building a tree fort or a table.  There are 
serious ramifications
to building a house that is not structurally sound.  Injury and death can 
come from not understanding
what is necessary.  For that reason, I would always advocate getting 
someone who has been involved
with building for a long time to help a person building a house for the 
first time.  Even better,
get an engineer/architect with experience in these things to approve of 
your design and oversee the construction.  ( I know some people might throw 
stones at me for this, but I think the money invested in engineering is 
money well spent ).  I agree that many of the structures being built today 
might
not last, but I think that is a failure of understanding more than a lack 
in the materials themselves.

I am not sure that you are correct on the strength of earthbags.   It is 
certainly true
that bags of gravel or some sort of aggrigate rely on the strength of the 
bags but
it is my understanding that for an earthen fill with a good clay soil (15- 
30%), the soil becomes
compacted much like rammed earth ( or cob ) and gains a great deal of 
strength. In
this case the bag looses any support function that it might have had.   I 
have been told by
people who build these types of houses that after the wall has set, the 
bags can be just cut off
if desired.

I would be very interested in hearing about the failures you have seen with 
strawbale.
I am working with people who have built dozens of these buildings, both 
loadbearing and
infill.  What they tell is that the failures that they have seen always 
results from poor detailing and subsequent water damage.  A good, 
successfull straw bale design begins from the ground up. First, the straw 
should be 6-8 inches from the ground and
have a good water barrier between it and the foundation.  Next, the doors 
and the windows
need to be well detailed with drip sills and flashing to keep water from 
the straw. Finally
It needs a solid, leak free roof which provides for a large overhang to 
protect the walls from driving rain.  If you have these features, a straw 
bale house will last as long as any
stick framed house.  There are example straw bale houses which have been 
around over 100 years.  Like I said before, get someone who knows 
construction to help.  Proper detailing
for windows, doors and roofs is well known.  All the techniques developed 
over time for
conventional houses apply just as well to straw bales.  Also, all natural 
building techniques
that I have heard of require good maintenance.  Getting out yearly and 
filling cracks
in your plaster is important when building with materials that can degrade 
with water.
I'm not trying to be complete in my list here, there are other things to be 
aware of, but
those are covered in many books and newgroups.

I agree that the walls gain a lot of strength from the completion of the 
box, but I don't necessarily agree that a straw bale wall does not count in 
completing the box.  Straw is
very strong and has some nice properties that cob doesn't have.  Straw 
walls that have
been kept dry are very resiliant.  They will not tend to fail like a more 
brittle material.
The tests that I have read about show that a straw bale wall is very much 
stronger than
a stick frame wall.  In the general, hand waving sense, they tend to resist 
earthquakes better than cob because they are flexible and elastic whereas 
cob is brittle and can crack.
Straw bales themselves are perfectly adequate for supporting a roof all by 
themselves, but
when you coat them in a plaster, they become even stronger.  If you make a 
straw bale wall
to complete the box, you will be continuing the cob onto that wall in the 
form of the
plaster.  With this I would think that you would get good wall integrity 
and strength.

On top of this, bales provide excellent insulation.  Some people claim that 
all you need is thermal
mass to provide comfort but this is not always the case. In places with 
long cold, cloudy
winters, uninsulated cob will perform very poorly.  I have talked to many 
people who verify
that this is so.  Therefore, I think that it is worth exploring ( 
cautiously ) how these
materials can complement each other.

I have to admit that for me, most of this is a mental exercise.  I have not 
gone out and
built a hybrid house myself.  I have not seen one last 100 years.  But I 
have seen very
old straw bale houses which are essentially straw bales coated with cob.  I 
am familiar with the
properties of both of these materials and it seems like they complement 
each other very well.


-Scott

On Sat, 8 Nov 2003 13:34:10 -0000, Joe Skeesick <joe at skeesick.com> wrote:

> Personally, (and I'm sure many will disagree) I don't like any of the 
> hybrid
> systems. I think you're asking for trouble with this sort of design. No
> matter how organic you shape you're walls it is the completion of the 
> "box"
> (or circle) form of the house that gives cob its strength. The omission 
> of
> one wall from this system by creating it out of another source seriously
> degrades the over all strength of the structure. The other systems 
> mentioned
> also suffer for drawbacks that bring into question the integrity of those
> structures over the long haul.
>
> Contrary to a previous post about the only advantage to cob is you don't
> have to buy bags there is significant strength advantages to cob over 
> "super
> adobe" which has built in lateral shear lines engineered into the 
> structure
> itself. The strength of super adobe is completely dependant on the bags
> used, deterioration of the bags results in structural failure of the wall 
> in
> time. It will be interesting to see what the longevity of these 
> structures
> are.
> Along those same lines I have issues with strawbale after viewing several
> failed structures using this system.  I do not believe that most of the
> strwabale structures build today will have a life span of over 30 years,
> most much less than that. There are some good strawbale systems out there
> but most of them involve similar skills and resources as any modern stick
> built house and just aren't accessible systems for a solely owner built
> home. Now some people don't mind if their house comes down in 10-20 
> years,
> in fact some on this board might see that as a benefit and if so that's
> fine. My goal in building however is in creating a homestead to pass to 
> my
> children. Hybridizing cob would seriously degrade the possibility that 
> there
> would be anything to hand down to them, where as a proper historically 
> build
> cob home will provide not only myself, but generations of my family with 
> a
> home... makes all the work worth it.
> J
>
>
> -----Original Message-----
> From: coblist-bounces at deatech.com [mailto:coblist-bounces at deatech.com]On
> Behalf Of Jilly
> Sent: Saturday, November 08, 2003 3:29 AM
> To: coblist at deatech.com
> Subject: [Cob] hybrid bales bags
>
> Oh I DO have the book, The Hand-Sculpted House.
> That is where I first was given the impression to use straw-bale or 
> hybrid
> system.
> I understand how to use straw bales for a small area. But to create the
> entire north wall or large span, I was not sure. Also, was not sure if it
> was superior to earth bags.
> If I make the north wall all straw bale, load bearing (is that really 
> safe?)
> won't it compress when I add the roof? So how much higher should the top 
> of
> the bales be in compared to the cob? Precompresed bales?
>
> Also I was questioning using all straw bales, then covering with earthen
> plaster / cob.. or earthen bags and doing the same. Wondering about
> strength, ease of building, insulation, cost, time in building, ect.
>
> jilly
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-- 
-Scott