Cob verify?

[Speireag Alden]

Sgrìobh Mike Carter:

> Almost there but not quite.  (Mike #3, who has spent his share of 5 degree
> nights in the Sierra Nevada sleeping on snow)

    Augh!  They're everywhere!  :)

>>    My point in reply was that cob doesn't insulate any better than any dry
>>earth (rammed earth, adobe, and whatnot).

> This just isn't true.  Rammed earth is much more dense then cob.  Cob does
> not equal rock.  We use about 1/3 straw by volume in our basic cob mix.
> Sometimes even 85% for special applications.  You can adjust the density and
> to some degree the insulative properties, which are directly related to the
> density.

    Thank you for bringing personal experience to the discussion.  I'll 
defer to your experience as far as straw content and whatnot.  Correct me 
if I'm wrong, but something with 85% straw would be closer to what they 
call "light clay" and such a mixture could not be used to support a roof 
load all by itself, or could it?

>>Cob isn't insulation; it's thermal mass.

> Sorry, it has insulative properties that, although not spectacular, are
> variable and cannot be ignored especially in some of the wall thicknesses
> that have been used historically.  Even R = .25/inch (rammed earth) x 4' =
> R12 for the wall.

    Certainly I agree that there is an insulative property.  Strictly 
speaking, *anything* has *some* insulative property.  However, my 
understanding, from much discussion with the folks on the StrawBale and 
GreenBuilding lists, and from some technical reading, is that insulation 
and thermal mass are fish and fowl.  They behave differently and should be 
understood and analyzed differently.

    The R-value of a substance is determined by putting one temperature on 
one side of the surface, another temperature on the other side, waiting 
until the material reaches a "steady state" of transmission of heat, and 
then seeing how much heat travels through the material, per unit time and 
surface area.

    But that situation is *specifically designed* to avoid almost entirely 
the beneficial effects of thermal mass, because it reaches a steady state 
before the measurement is taken.  The lovely thing about thermal mass is 
the storage of heat or coolness, and the oscillation around a mean 
temperature; that's when thermal mass really performs.

    And, yes, not all thermal mass is created equal; stone conducts heat 
better than dry earth, and as you pointed out, dry compacted earth conducts 
heat better than dry compacted earth with straw added.  All three, however, 
are distinctly at one end of the spectrum between thermal mass and 
insulation.

    I never meant to get into nitpicking about differences between .25/inch 
and .5/inch or what-have-you.  I was responding to Mike, who wrote, "The 
clay and straw have certain insulative qualities that surpass alot of 
modern-day building materials."

    I contest that statement.  While it might be technically true (after 
all, steel is a modern building material, and cob is more insulative than 
steel), it gives a false impression to someone who's trying to figure out 
the larger question of what cob is and how it stands against other building 
methods.  If Mike had written only to me, I might not be so insistent, but 
there are a lot of people who are trying to figure out how to build their 
homes, and who get information from this list, and I did not want anyone to 
rely on Mike's assertion and thereby build inappropriately to the climate 
or situation.

> Just like strawbale has some mass.

    Yes, strictly speaking, the bales have some mass, but I don't think 
that they matter much for mass in comparison to the stucco.  The stucco is 
what gives strawbale its mass; the bales provide the insulation.  It's the 
combination that give strawbale walls their peculiar performance 
characteristics.  For instance, the R-value of strawbale walls may be lower 
than first thought, but they still perform better than their R-value would 
suggest *because of the thermal mass*.

    I've heard some people say that they're going to build a cob wall and 
then put straw bales around it and stucco outside of that.  No doubt 
they'll have a well-insulated building with lots of thermal mass, and if 
the other details are right it'll be a good house.  I decided for myself 
that the walls were thick enough at 22 inches (bales plus stucco on both 
sides), and I decided to put my thermal mass in the floor, in the form of 
six inches of gravel topped by a hollow-core passive solar floor, topped by 
four inches of earthen floor.  Why?  Lots of reasons, including the fact 
that we get taxed on exterior footprint, so the wall thickness is taxed as 
usable space.  So I figured that if I could get the same benefit with more 
usable floor space, then I'd do it that way.

    Now I'm rambling.  Sorry about that; see above for my main point.  Best 
wishes to all you cobbers and all you owner-builders.

-Speireag.
0>>>>>>(---------------------
Speireag Alden, aka Joshua Macdonald Alden
Joshua.M.Alden.91 at alum.dartmouth.org
Usually found somewhere in the wilds of New Hampshire.
Nach sgrìobhaidh thugam 'sa Gàidhlig?