[Strawbale] RE: re re re The BIG question for Eric (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
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.
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