[Strawbale]Some loadbearing compression ideas/questions

[Chris Mowatt]

G'day Wolf

Could you share the practical problems that you have experienced with the
dip method. In a previous post some time ago, you said something about the
difficulty of carrying a dipped bale up a scaffold. Was it the additional
weight, the mess, the slipperiness or something else that made it difficult?
Did you try the wet method, the dry method or both? Presumably Tom Rijven
must have found a way to get his walls straight.

I like the theory of French dipping bales in a clay slip. It does sound
messy and potentially dangerous to use the wet dip method. I can imagine
that carrying a slippery bale up a slippery ladder onto slippery scaffold
boards would not be a pleasant experience. It sounds more practical to use
the dry method where bales are dipped in a clay slip and allowed to dry
before using them in construction. I was planning to use this method, but
apply wet clay slip liberally with a paintbrush at the joins as the bales
are assembled. The clay slip would then act like a primitive glue. I am
thinking that gluing the bales like this will make the wall stable enough to
apply the render/plaster/stucco without pre-compression.

You mention the creep you get even with pre-compressed bales. Have you yet
got around to re-designing your top-plate to reflect the findings of the
recent University of New South Wales test with regard to the additional
strength gained by a wider top-plate that rests on the render/plaster/stucco
skins? If so, do you still experience creep?

In Dan Smith's EBNet research
(http://www.ecobuildnetwork.org/pdfs/Creep_report.pdf) he load tested a
number of walls made of various different types of bale. Only two walls were
rendered, one earth render and the other cement/lime render. The most
telling line in the report is "The cement lime plastered wall, at 400plf,
showed no noticeable settlement at all.". This was despite the fact that
"The walls were not carefully or uniformly “stomped” into place or otherwise
precompressed". "The earth plastered wall, at 100plf, however did show some
settling, 1 ½” in the initial weeks as the first coat of plaster was still
curing. We had provided some precompression, with the load applied for 1.5
weeks before plastering." It is not possible to tell from the report whether
the load was applied to the straw or across the full width of the wall
including the render/plaster/stucco skin. It is my belief that a rendered
straw bale wall where the top-plate rests on the render/plaster/stucco skins
should not suffer significant settlement/creep.

Dan Smith's research seems to suggest that, with your earthen renders, you
might be well-advised to leave putting on the roof for a week or so until
the earthen render has had a chance to cure. In this way you might get less
settlement.

Chris Mowatt
Isle of Arran, Scotland