[Strawbale] RE: re re re The BIG question for Eric (Michael Lough)

Michael lough michaelklough at sympatico...
Tue May 16 19:05:17 CEST 2006

Eric Larmett said

The concept of interstitial condensation is well known. This is
to occur when the outer face is sealed.


Condensation as a result of warm atmospheric humid air contained within a 
bale wall meeting cooler humid air either as a result of an interstitial 
convection current within the bale wall "circulating" the air (or possibly 
as a result of simple thermal transfer from a warm interior through an 
impermeable interior plaster or stucco to a more humid air space within the 
bale wall ) do both rely on the ability of the outer "skin" of the bale wall 
to wick out moisture during a drying period. I think I have this right.

It is generally assumed by many that a bale wall will if wetted by rain from 
directly above (through the middle)  that it will not dry out before 
microbial activity begins, it is further assumed by many that a bale wall if 
wetted into the sides of the bale wall  by  up to 4 inches or so will dry 
out if uncoated (or coated with a sufficiently vapour permeable coating like 
Lime/sand only render or earthen plaster) but possibly not or at least not 
sufficiently well enough or quick enough in some regions of the world to  
guarantee that the bale sides will dry to below the around 20  percent 
moisture level  (recognised as the point where microbial activity is going 
to begin) in time.

The central question is of course will the addition of Portland cement to a 
render mix "tip the scale" in some regions (perhaps over a fairly long time 
period ) to creating a less than adequate drying regime and thereby creating 
the circumstances that prelude rot, it could be argued that keeping moisture 
out is as important as letting it out after it has become damp but this does 
not allow for humid interstitial "air simple" to exist within a bale wall 
and for this to be able to be condensed by warmth through an interior 
plastered wall over time and consequently have no means of getting out ( if 
the outside wall is coated with an insufficiently vapour permeable coating 
such as one containing Portland cement).

The idea of rainscreens is not unique in the straw bale world but it is 
recognised that the bales on the outside should still be coated with 
something like earthen or even lime/sand render but then it will  possibly 
not have the thermal efficiency of a thicker coating nor preclude the 
possibility of in bale convection currents still meeting colder humid air at 
the outside edge (dew point)  or further toward the middle (or at the top of 
the bale wall) depending on the geographic location of course.This is an 
inexact science still (part of its "charm" of course) that has not stood the 
test of time (using Portland cement in a mix in the North East of North 
America or perhaps in cooler wetter parts of Europe. Britain comes to mind 
of course.
As far as I understand it has been concluded that Portland cement containing 
render is "allowed and therefore must have been the subject of code approval 
scrutiny in Canada and therefore testing but I have yet to see this and 
therefore I am interested in any research on this subject.

Michael Lough

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