[Strawbale] convection and humidity in straw bale walls

Jan Hollan jhollan at amper....muni.cz
Mon Jan 9 18:56:57 CET 2006

In April I wrote some quantitative info on the increase of thermal
flux through straw layers into this list, and promised to describe
air permeability measurements at least... I failed to do even that.

I just prepared some experiments with injecting gypsum to divide a bale
into three vertical chambers to have three convection cells in a series
instead of just one. I have to proceed this way further (learning at first
how to postpone any decline of gypsum fluidity, by milk or vinegar
perhaps). We have also built a wall insulation (in an old mill in Krtiny,
near Brno) with three chambers using untreated sheep wool or straw, pushed
into the chambers (divided by cardboard) by hand; I should measure its
performance by thermography soon. Then I had a lecture on porous
insulation (in Czech, http://amper.ped.muni.cz/jenik/passiv/slama/) and
some interesting ideas emerged from the subsequent discussion.

As a demo house should be built in Hostetin (a Czech village near to
Slovak border) this year, with straw used as an insulation in some its
parts, with the goal of achieving passive standard, I'd be forced to
develop an easy technology to produce three-chamber straw walls. With
three convection cells across the thermal flux, there is almost no
``reciprocity failure'' of the thickness - thermal resistance relation
(and U=1.0 W/m2K can be reached), whereas with one cell there is.

On moisture problems, I have nothing to say apart from repeating my old
standpoint that letting moisture diffuse into the cold parts of
construction from inside the building helps nothing, and that an interior
should reach the basic passiv-standard airtightness, n<=0.6, for our
climate. Roland Meingast (lehm.at) achieved that using clay-impregnated
flax strips in his house in Tattendorf.

jenik, lagging behind his urgent tasks

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