[Strawbale] convection currents within a straw bale wall?

[Michael lough]

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