[Strawbale] convection currents within a straw bale wall?
michaelklough at sympatico...
Wed May 3 15:07:49 CEST 2006
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
Care to comment?
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