[Strawbale] Re: convection currents within a straw bale wall?

[Peter van Balen]

I think we shouldn't mix up "air tightness", which helps minimizing 
connection losses), and "vapour permeability" (often, incorrectly, referred 
to as "breathability"), which helps moisture control.
So yes, let it "breathe", i.e. try making/plastering/rendering outside of 
wall more vapour permeable than inside; but no, don't let it [air] move!

----------------------------------------
I wonder how many of these failed walls were plastered with a cement based
render/stucco?
I know the Americans like their straw bale buildings cement stuccoed.

I think if you are going to use an organic material in a buildings
structure, provision for air movement  through that structure becomes
paramount, even at the loss of thermal performance.

"sealing" in a straw bale wall with cement plaster seams like asking for
trouble unless it is a dry climate.

It would be interesting and very useful to have a reference to straw bale
wall "failures" to learn from, is anyone making a comprehensive record of
these situations?

let it move, let it breathe!

>From: "Michael lough" <michaelklough at sympatico...>
>Reply-To: mkl18 at pobox...,        European strawbale building
>discussions<strawbale at amper....muni.cz>
>To: strawbale at amper....muni.cz
>Subject: [Strawbale] convection currents within a straw bale wall?
>Date: Wed, 03 May 2006 13:07:49 +0000
>
>Mark said.
>
>There is a disparity between directly-measured U-values for strawbale walls
>and U-values calculated from measured thermal conductivity of bale samples;
>the best explanation anyone has been able to give for this is slow
>convection currents in - or rather around the outside of - the bale wall.
>The disparity was worst in the case of the least professionally plastered
>test in the US.  I would therefore expect the thermal performance of a
>system which uses cladding _instead of_ external render to be substandard.
>
>
>This seems akin to the "theory " that there is the possibility of  moisture
>produced within a bale wall being part of the convection current. And this
>being condensed at the top to moisten the outside top wall and in worst
>case scenarios (presumably in more humid climates) maintain a permananent
>risen damp within the straw bale wall which could if not in a few months
>probably over years produce the conditions for microbial activity .?
>
>I understand someone called Jenik is active with unfunded experiments in
>Europe to  test various methods of stopping this moisture transfer.  Do we
>assume by his activity and your acknowledgement of  a concensus on the
>probability of convection currents in a straw bale wall that there is room
>for concern in parts of the world where humidity is a factor such as
>Northern  Europe and parts of North America? There have been reports of
>"failed" straw walls in NE America with some  admitting the damage was
>likely  caused by dampness but no  admission of the dampness being caused
>by convection currents within a wall, therefore no remediation has been
>undertaken as it has not been proved. Which of course means here  "business
>as usual".
>
>
>Care to comment?
>
>
>Thankyou
>
>Michael Lough
>
>
>
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>





------------------------------

Message: 2
Date: Wed, 3 May 2006 23:53:20 +0100
From: "Oliver" <obswann at btinternet...>
Subject: [Strawbale] Tadelakt on strawbale
To: "European strawbale building discussions"
<strawbale at amper....muni.cz>
Message-ID: <000b01c66f04$600a5500$0301a8c0 at Oliver>
Content-Type: text/plain; charset="iso-8859-1"

Hi,

Does anyone have any experience of tadelakt on strawbale? Is there anyone 
out there who could run a tadelakt workshop in Scotland.

There are a variety of volunteer opportunities on www.naturalhomes.org. If 
you would like to post your workshop on naturalhomes.org to attract 
volunteer workers please send me details.

Regards,

Oliver
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