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Re: GSBN:plaster and straw question



Dear GSBN...

Below is a comment from Joergen Munch-Andersen.

Joergen has been a reliably constructive resource for the development of SBC
in Denmark all the way.
I keep passing on the good stuff from GSBN to him, while always thinking
that he ought to be on the list himself, while at the same time thinking
that there is no way he'll have the time, but as he has just suggested it
himself, I would like to suggest that he is invited in ?

This is what I wrote on this list about him on May 7th:
... Joergen Munch-Andersen, whom some of you might have met at
the ISBBC 04. Joergen is Senior Researcher at the Danish Building Research
Institute
(www.sbi.dk) and has for more than 5 years consistently been constructively
critical and he has made a huge difference towards making SBC  an accepted
building method in Denmark.
Furthermore I should say that Joergen has on several occasions shown that he
is ready to put in time.

Regards,
Lars


  I believe that what Bruce calls speculation is the way it works. In our
report on straw bale building there is a section on experiments with straw
bale walls with clay and lime plaster, written in English. Cement is of
course more dangerous, but we subjected our wall to very severe moisture
conditions for a long time, and still nothing happened with the straw.

The report can be found at

<a  target="_blank" href="http://www.sbi.dk/byggeteknik/konstruktioner/serlige-konstruktioner/halmhuse/halmhuse/";>http://www.sbi.dk/byggeteknik/konstruktioner/serlige-konstruktioner/halmhuse/halmhuse/</a>
click on download pdf to the right and go to page 46.

I cannot quite guess how the wood/plaster walls are constructed,
especially where the grade D paper is placed (I assume that it is some sort
of a vapour barrier). But if it is on the inside of the wood, rain water
penetrating the plaster is trapped between two vapour barriers. If it is on
the outside, moisture from the inside is absorbed by to wood. So it is
deemed to fail. In a straw wall the mass of the cellulose is much bigger, so
it can absorb more water.

I hope this can be of help to somebody.


J&#xBF;rgen Munch-Andersen
SBi, Danish Building Research Institute
Dr. Neergaardsvej 15
DK-2970 H&#xBF;rsholm
tlf +45 4574 2388
fax +45 4586 3575
jma@...





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








--
Jo, Asger og jeg er pt I Australien. Vi er i Danmark igen midt november.
Juli, august og september kan vi kun traeffes paa nedenstaaende mobilnummer,
samt modtage post via nedenstaaende adresse.

Jo, Asger and I are in Australia until mid November this year. July, August
September our contacts are:

jomorandin@...
larskeller@...

Lars, Jo og Asger
5 Crouch Street North
Mount Gambier
5290 SA
Australien

Mobiltlf: 0061 405 366 455
--


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