Cob: Insulation

[Mark Piepkorn]

At 02:59 PM 9/6/00 EDT, Patricia L. MacKenzie wrote:
>I'm given to understand the incompatibility between the old and 
>new cob or between cement and stackwood, shrinkage is mentioned.

	Argh... I don't have most of my books and stuff with me, so if I make any
name references, they're probably going to be close-but-wrong. I'm also not
subscribed to the cob list presently, so chances are good that this isn't
going to post.

	Far's earth-on-earth goes, thickish layers can and do delaminate one from
another. A good physical bond (or "key") is important whether it's layers
of cob, or plaster, or whatever earthen application is being pursued. If
the first layer is completely "cured" or dried (whether it's a week old or
a hundred years old), particularly if it's quite a thick layer, simple
mechanics becomes relatively important. 

	The dried part will suck much moisture out of the new part very quickly,
and as we know, drying cob or earthen plasters out too fast can make
problems... including poor integration with and adhesion to the substrate.
With structural earth, getting the old part saturated enough (not just on
the surface) so that it won't pull the moisture out of the patch too
quickly could compromise its structural capacity. Don't want that! Part and
parcel with that is that with a fiber-reinforced material like cob, the
straw/fiber bits aren't going to incorporate into the old part very well,
contributing potentially significantly to a weak joint. 

	There's a preservation society in Devon which has put out quite a bit of
material - including how to repair damaged cob; that is, stitching together
new cob with old. I'm talking about significant repairs. That info is
reprinted, at least in part, in the Cob Reader that Cob Cottage put out
some few years back. I don't know if they still sell it or not. Shannon, do
you know?

>If the new is incompatible, what is the suggested material 
>for cracks, repair or fill in?

	I don't know that I'd say it's incompatible. For fixin' old unstabilized
earth, I'd go with new unstabilized earth. Same stuff, just employed
differently.

	To briefly address stackwall, I'd go with an infilled post-and-beam frame,
substituting cob for cement. I'd also earth-plaster both sides of the
cordwood. I'd build the stackwall part (whether single- or double-wythe,
though I'd tend toward single per Rob Roy's method) with the log-ends well
proud of the cob 'mortar' to give an excellent physical key for thick
straw-rich earthen plaster.

>I have to assume we are not going to some company like DuPont 
>for an elastomeric material

	Could.   :)   Wouldn't, myself.

>I personally think cob, mud and or cement is incompatible 
>because of age and therefore some type of new surface to 
>adhere to is required - 

	Fair enough. One of the beauties of natural building is that there's SO
MANY ways to do it, SO MANY philosophies and opinions... and more often
than not one is just as right as another. New cement on old, though, that's
a rule: a new layer of cement won't physically incorporate with an old one.
Check out one of the appendices in the back of the book Spectacular
Vernacular (by Carolee Pelos) for a biting comparison of earthen materials
and cement materials.

>like uhm, nails driven into a wooden frame and then 
>gooped on?

	Ah, here's one reference I do have with me because it's brand new. Keely
Meagan has a new booklet out called Earth Plasters For Straw Bale Homes. In
it, there's a sidebar quoting Ed Crocker, former director of Cornerstones,
an earth-structure (principally adobe) conservation group. "... His strong
words come from the experience of seeing first hand the long-term results
of mixing metal and mud. Ed notes that the whole point of porous, breathing
plasters is that they allow air and water to pass through them. Add a
corrodible material, even galvanized metal lath, and you eventually end up
with rust." And this guy's a desert-dweller.

	I can neither confirm nor deny this. All the latter-day cob stuff talks
about peppering wood with nails to get that ever-so-important mechanical
key. Would some kind of wooden dowel system be better? Or is it better to
just accept that the house isn't going to last a hundred years (or even
fifty) without a major repair or two?

	Keely's well-presented 50+ page booklet is US$12 plus shipping; email her
at <keelymeagan at hotmail.com> for more info.

>I'm sure the log cabin people don't use anything organic.

	Not these days, not most of 'em. Not unless PermaChink has been
reformulated.   :)