[Cob] Cob R Value

[otherfish]

Steve,
Here is a reply I received some time ago from Robin Clarke, a
Senior Experimental Scientist regarding R value of cob.  It seems pretty
through, although I'm not a scientist.  It starts out in metric, but finally
gets to US units towards the end.  The upshot is a cob wall 2' thick would
have an R value of 4.3.  Reading his reply is seems that this is not a
linear value tho because there are exterior & interior surface coefficients
that I belive are constant regardless of the wall thickness.  If you play
with the numbers you should be able to figure out thr R value of whatever
wall you are considering.

I've attached also my origonal question to Mr. Clarke to give you a bit of
context.  Hope this helps.
john fordice
........................
Robin Clarke wrote:
John

I am not familiar with cob technique, except for a quick Google browse that
I have just done.  Presumably walls made in this way have a density that is
around rammed earth and mud bricks, possible somewhere between the two.  I
suspect that the fraction of straw is relatively small and that there is
some "ramming" of each layer.

Speaking metrically (SI units), all the work that we have done confirms the
fact that mud bricks have a thermal conductivity of about 0.8 to 1.2 W/m.K
typically and that rammed earth is more-typically 1.0 to 1.5 W/m.K.

The press release that you may have seen on the web was for 200 mm but
things I said were for a proposed 300 mm thickness.  You also need to be
careful whether you are talking about overall R value or just the R value of
the wall.  Uninsulated earth walls have such a low R that the surface
coefficients are a big part of the overall R.

It is easier to ignore this press release and just go back to the basic
numbers.  Looking at the values I proposed above, you could consider that
the thermal conductivity of the cob construction might be about 1.0 W/m.K
typically.  I defer to anyone who has better knowledge that this, but I
would expect that this value would certainly be within the normal range at
least, perhaps a bit on the optimistic side.

Such a conductivity makes maths easy.  R = t/k, for t being thickness in
metres.  So if k is 1 then R = t.  Thus a 2 foot (0.6 m) wall has an R of
0.6.  It is usual to presume surface coefficients of about 0.11 for inside
and 0.04 for outside, giving a total of 0.15.  Therefore the hypothetical
wall has an overall R of 0.75.

The conversion factor to imperial R is 5.678.  Thus in imperial speak, this
2 ft wall has an overall R of 4.3.  This is not all that great, especially
when you note that the presumed conductivity was on the optimistic side.

As the article suggests, I am not a fan of uninsulated high mass
construction for climates that are not mild.  Southern California is of
course rather mild.  In mild climates, low R is not so important and high
mass can have a good stabilising effect.  I would not expect this type of
construction to lead to low energy consumption for houses in the Seattle
area.

The significance or otherwise of R and mass is easily investigated by one of
the many building thermal modelling programs that are available.  These
demonstrate the value of mass and the value of R for any proposed
hypothetical construction in any climate.  There are some Australian
programs, but many American programs as well.

What has been debated here is the possibility that these modelling programs
are somehow incorrect and that there exists some mysterious effect that
makes the energy consumption of low R, high mass houses less than the
programs predict.  I suspect that there are other reasons to explain the
anecdotal "evidence", as there is for the anecdotal evidence of "insulating
paints" and other such things.

"Standard Design Tables" is a euphemism for the content (and including
tables) of the ASHRAE handbooks, plus the Australian (AIRAH) and British
(CIBSE) equivalents, containing significant differences in content and
emphasis.  We use hard copies of all 3.  I am not aware of how much similar
material is available on the web but the handbooks are part of the IP of
these organizations and I would expect that they are not given away for
free.

Regards

Robin



***************************************************************
Robin Clarke
Senior Experimental Scientist - Thermal Engineering Project
CSIRO Manufacturing & Infrastructure Technology
PO Box 56   Highett  Victoria  3190
Tel:  +61 3 9252 6044    Fax:  +61 3 9252 6251
Email:  Robin.Clarke at csiro.au
Web:  http://www.cmit.csiro.au
***************************************************************

 -----Original Message-----
From:     otherfish [mailto:otherfish at comcast.net]
Sent:    Wednesday, 3 March 2004 3:28 AM
To:    Robin.Clarke at dbce.csiro.au
Subject:    rammed earth R value / relevance to cob construction

Dear Mr, Clark
I'm an architect in Berkeley, California.  I am interested in the potential
R value of earthen walls built with the Cob technique.

I've read your CSIRO  Press Release: 27 Apr 2000 , Ref: 2000/110  concerning
the thermal performance of rammed earth and mud bricks.

Here is are two excerpts from that article
............. 

"CSIRO¹s tests confirmed that the overall thermal resistance (R-value) of a
typical rammed earth wall is less than R 0.4, the same as the disputed
values provided for by standard design tables.

This is low compared to everyday insulating materials such as glass fibre
batts which typically come with ratings of R 2 or R 3," he says.

"The tests used sample panels 200 mm thick, about one metre-square and
weighing over 400 kg. These were tested in CSIRO¹s one metre square heat
flow meter apparatus - a new record for the size and weight of panels tested
on our rig"
...............

Mud brick is closely related to rammed earth but slightly less dense and
slightly better insulating. Based on the rammed earth measurements, the data
for mud brick must also be regarded as reliable, suggesting an overall
R-value for a 300-mm mud brick wall of less than 0.6. It would require only
20 mm or so of most insulation materials to achieve this same performance.

There is a current exemption in some regulations for walls thicker than 180
mm, not because of the acceptable thermal performance of these walls but
because established constructions such as double brick and rammed earth or
mud brick are difficult to insulate."
............. 

Can you please confirm the wall or brick thickness upon which the R values
given are based:

Is the R 0.4 for rammed earth based on a 200mm thickness?

Is the R 0.6 for mud brick based on a 300mm thickness?

The article makes reference to "standard design tables", can you tell me
whom has created / published these design tables and if this information is
available online?

Also, can you comment on the applicability of your earth wall thermal
performance research to walls constructed in the cob technique?

There is a growing interest in earth building and Cob in particular here in
the US.  Design professionals such as my self and owner-builders scattered
across the countyr are struggeling with gaining official acceptance of these
materials and any infor mation you can point me towqards will be greatly
appreciated.

Thank you very much,
john fordice - architect
on 6/20/05 7:25 PM, Anna Young at avjyoung at shaw.ca wrote:

> Hi Cobbers,
> 
> We have an home energy consultant reviewing our cob house plans. She wants to
> know the R value of cob. I heard around 0.7/inch, but only anecdotally. Does
> anyone know of any tests or work done to come up with an accurate range for R?
> To contrast the consultant had heard the value was closer to R1 for a 24"
> wall.
> 
> Thanks,
> 
> Steve
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