Cob and Cordwood

[M J Epko]

I dropped in at Rob & Jaki Roy's the other day & spent a few hours. Rob 
had just returned from a research trip to England for his next book, 
which isn't about cordwood. Or cob. (Cob's been enjoying a recent spate 
of published materials, eh? Just like SB has. Look for another cob book 
in the fray in a year or so, this one from Ianto Evans, to be published 
by Chelsea Green. My recommendation, as always, is that people should 
get 'em all. We're talking about *houses* here - $80 or $100(US) on 
these books is darned cheap in the long run even for the most destitute 
of us: even if it's only one passing phrase in one of the books that 
prevents a life-threatening, or just plain annoying, situation. But then 
again, there didn't used to be any books about it, and there's a heckuva 
lotta really good old earthen structures in this world.)

Anyway, we did talk a little bit about substituting cob for concrete in 
cordwood. There's a few concerns, but without knowing any of the 
specifics of a given project it's impossible to comment, really. Let me 
preface the notes of caution (not objection!) he raised by saying that 
he has indirect knowledge of a contemporary loadbearing cob-cordwood 
house in Wales that's evidently doing well. He doesn't know just when it 
was built, or how big it is, but understands that they simply 
substituted cob for the concrete, and used a typical insulative filling 
like the atypical one that Rob endorses. It wasn't double-wall. He's 
been told that the house is doing fine. Maybe somebody from Cob Cottage 
knows more, since there's some overlap-of-techniques going on.

That said... in an infill capacity (in a post-and-beam frame), he sees 
no real problem with cob-cordwood. It ain't structural, so delving into 
the unknown in that instance without the benefit of tests or experience 
isn't very alarming. For loadbearing, the concerns were primarily that 
the cob isn't going to create the kind of stable matrix with-and-around 
the cordwood that concrete does, let alone be a cohesive whole. If all 
the wood in a cordwood wall rots out, the concrete will still hold up 
the roof; and that same stability is beneficial when it comes to uplift 
(might want to think about strapping the roof to the foundation for this 
project, even though it's not for habitation), and straight-line winds.

Cob walls are stable because they're monolithic. Cob Cottage folks (and 
Shannon) have said in the past that using big boulders and hunks of wood 
in cob walls weaken them. (What about windows and doors, huh? Huh?... 
grin...) But I guess I see that point. With a cob-cordwood structure, 
assuming 18"-wide loadbearing walls (kinda big for this project), 
there'll be two 6"-wide (or so) bands of cob, maybe ranging from four to 
six inches thick... one band of cob-mortar on the in-side, and one 
out-side... and an insulative material (or airspace, not as good) 
in-between, all tied together with the log-ends, which are exposed on 
both sides. If the loads (live and dead) on the cob matrix exceed its 
ability to not crush, there will be settling as the cob crumbles. Given 
that this structure is a sauna (following Rob's basic principles of 
design and use in his book on the topic?), it's possible that the 
interior and exterior bands of cob-mortar may behave differently over 
time... weather-events notwithstanding. That all depends a lot on the 
batch of cob, though. Using cob-mortar all the way through, eliminating 
the core thermal break, would *possibly* make it more structurally 
stable, but would kill the thermal resistance (Rob uses Malcom Wells' 
pet phrase, "thermal nosebleed"), which would be an especially bad thing 
since we're talkin' sauna here.

Um, that's all that's coming to what's left of my mind just now. Is 
Bekcy Bee doing your workshop, or is that a different-but-similar thing 
somewhere else?

*

Oh, regarding the bark-falling-off-as-the-tree-dries thing, it's my 
understanding that its species-dependant. Some kinds of trees are easier 
to debark green, and almost impossible afterward... while others do 
indeed shrink away from their skins when they die.

*

Say, Charmaine, the hard-copies of your catalog arrived at Earthwood... 
did you send one to TLS? If not, could you? HC66 Box 119, Hillsboro NM 
88042 ... thanks... If you already did send one, I'll look it over when 
I get back there.

*

Please direct any responses to duckchow at mail2.greenbuilder.com and not 
the address this came from. I'm on the road (currently Annapolis, MD, 
staying at Sam's farm for a couple weeks, and ready for the Natural 
Building Colloquium East). I'll be back in three weeks or so, but am 
checking in on things when the opportunities come along.


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