Cob in really cold climates

[Shannon Dealy]

>         Thought I would start off asking my first (and most important)
> question about earthen construction, how does it do for insulation?  The Cob
> Cottage Companies web page says in the "What is Cob" section "Cob's
> resistance to rain and cold makes it ideally suited to cold climates like
> the Pacific Northwest, ...."  Sorry, I've live in the Pacific Northwest and
> it *does not* have a cold climate.  Cool and damp for sure but not cold.
> Minnesota has a cold climate.  So my question is, what is the R-value of
> cob?  Does anybody know? When it gets down to -30 F every year for a week or
> so, you care about the R-value of a building system.   I have heard that
> rammed earth does not work in cold climates because earth is great for
> thermal mass but poor for insulation.  I don't see why cob is much
> different.  In the little I read it seems like it works best in cool,
> maritime climates (like England and the Pacific Northwest).  Just want to
> get all the issues out on the table so we don't think that one building
> system is the solution for all climates.  It seems to me that each climate
> requires a different type of building system that utilizes local resources.  
> 
Cold is a relative term, when I was in Cairo, the locals put on heavy jackets
when the temperature got down to about 60 degrees.  You are correct however,
the temperature in the Willamette Valley where Cob Cottage Company is located
rarely goes below 10 degrees, though areas of Eastern Oregon and Washington 
get well below zero every year.

So far I haven't given you an answer, because I don't have one.  It's a
question I would like an answer to myself.  Does anyone have an R-value
for Adobe?  Cob is just one form of adobe, so Adobe R-values should give
a good general idea.  The R-value for cob may vary quite a bit depending
on the amount of straw in the mix (I think 10% by volume is considered 
typical).  It will also depend on the methods used to build the cob, since
some techniques could result in lots of little air pockets in the cob.

As a starting point, some best/worst case boundaries could be estimated
based on the materials.  For a worst case: exterior cob walls are typically
18" - 24" thick, assuming no straw in the mix this would give an R-Value
for the wall of about 4 (pretty lousy).  On the other hand, since straw 
is about 10% of the mix, we could apply strawbale R-Values and earth 
R-Values to get about 9 (still not particularly good).  Obviously there is
no substitute for actual measurements, but this should be reasonably close,
as a starting point for discussion.

The above all assume that you build a standard cob wall.  There is nothing
to prevent building a wall with some form of insulation embedded in the 
middle of it, though I am not aware of anyone having done it.  I am 
planning to build an experimental Dog house (we have 4 dogs) with straw 
sandwiched in the middle of the walls to see how well it works.

Cob Cottage Company's approach is to make use of the thermal mass of the
walls and store heat during the day to keep the house warm at night, though
in really cold climates or during prolonged periods without direct sun
this probably won't work to well.  In addition, their houses are usually
quite small, so they are much easier to heat (which probably makes up for 
the lack of insulation).

Shannon Dealy
dealy at deatech.com