[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

GSBN:plaster and straw question




-------------------- Original Message Follows --------------------

On Jul 2, 2006, at 2:23 PM, Pinyon Engineering wrote:

Hi list

I have been working on revising the Detail Book that CASBA put
together back in 2000

I have a question:  Why can we apply cement plaster directly to bales
but not to wood?  aren't they both cellous?


An ongoing question, subject to lots of discussion over the years.

Bob Platts, a very capable engineer in Canada, opened up various
cement-plastered bale structures of up to ten years of age (as I
recall;  it was in The Last Straw a long time ago).  He was looking for
any sign of problems at the straw-plaster interface, and didn't find
any -- no sign of alkaline attack (from the high pH cement), nor any
sign of water vapor condensing on the inside of the outer plaster skin
(as was widely expected in a heating climate like Canada -- warm moist
air moving outwards, dropping in temperature in the wall "cavity"
(bale), and hitting the dew point at or near the back of the plaster.

There should have been a problem, and there wasn't.  Maybe Bob just
didn't look hard enough, but, as I said, he's a pretty smart guy.

General speculation is that straw, unlike its more dense cousin, wood,
is able to disperse any water or condensation by capillarity, then let
it dry by any or all of several mechanisms before decay can set in.  In
a Canadian house in winter, for example, there can be water there for
quite a while at the interface, but so long as it remains below 40
degrees F or so, the microorganisms don't wake up and start to work.
By the time it warms up enough, the water dries up.

That is the speculation, anyway.

At a wood/plaster interface, the water is trapped, and remains in place
as the temperature rises, so the critters start a-molding.  I saw
abundant evidence of just this behind every failed stucco wall (about
30 of them) that I inspected after the 1989 San Francisco earthquake.
Even where there were two layers of grade D paper as per code, water
had gotten behind it (the paper) and there was, typically, lots of rot
in buildings over ten years old.

Bruce King, PE, rotting away the time
Director, Ecological Building Network  ( www.ecobuildnetwork.org )
Publisher, Green Building Press  ( www.greenbuildingpress.com )
209 Caledonia St.
Sausalito, CA 94965  USA
(415) 331-7630
bruce@ ecobuildnetwork.org