[Strawbale] Strawbale dome - Aerial lime?

Christopher Fraser chrisf at goop.org
Thu Feb 9 13:43:08 CET 2012


We are currently starting up an open innovation / learning / 
sustainability experimentation site in Extramadura, Spain. We are 
loosely inspired by Open Source Ecology though our range of interests is 
wider than the Global Villiage construction Set. You can find some 
information about the project at http://openeland.org.

We've constructed a 6m metal frame dome, which we intend to cover in 
straw bale. This is an interim workshop / accomodation structure until 
we build more permanent buildings with proper foundations, planning 
permission etc.

I have no prior experience in strawbale construction but I've been doing 
a lot of reading on the web and as far as I can tell what seems most 
appropriate given our budget, requirements and resources on and around 
site is a clay render finished with lime.

The only lime I can find locally is powdered "aerial" hydrated lime 
("Cal Aerea Hidratada CL-70-S"), which I believe is the same as fat 
lime. All the different types of lime are confusing, but I believe this 
is suitable, e.g. according to this:


I was planning on following this guidance, but have a queries about what 
they suggest:


They don't recommend using hydrated lime, but slaked quicklime - by 
which I assume they mean liquid rather than powder form. Is the local 
hydrated lime going to be a big issue? It's only a interim structure and 
it's fairly dry here (seasonally winters are the wettest with an average 
of 150mm/month, though it's very dry this year).

One issue is the overnight temperatures are dropping below freezing at 
the moment. Should we be worrying about this? It may have lifted by next 
week, and it's a very dry cold at the moment.

One thing I'm not clear about is if I just mix the lime powder with sand 
and water and apply it to the straw? I know there are lots of different 
opinions about this but any thoughts about the aggregate mix ratio in 
this circumstance? (See below about the sand). I was assuming 2:1 for 
now. If there are any test suggestions I can do I'd be very interested 
to hear.

The lime comes in 18kg roughly 60cm x 30cm x 10cm sacks (an over 
estimate as the edges are rounded). If we were mixing 2:1 and covering 
around 60m2 to say 1cm do I just do the straight forward maths 
(something like 60 / (0.6 x 0.3 x 10 x 2) which is about 20 bags with a 
few spare)?

The site has some very sandy patches, which are at least 60% fine sand 
mixed with small rocky particles (around 1mm), quite few small rocks 
(5mm and up) and some silt. I would like to use this if possible. Does 
anyone have any advice on how to prepare this? I assume I'd want to wash 
all the silt out and screen with something a 3mm screen it to remove the 
small rocks. It's much finer than beach sand though, so should I think 
about getting some less fine builders sand too?

In terms of the earth for the clay, I dug away some top soil and made 
some test mud and it seemed to be fairly sticky and sun baked into a 
reasonably tough clay-like tile. My test piece was only about 50mm x 
100mm x 15mm but it didn't seem to crack.  I'll try some bigger pieces 
but that much is looking good so far.

I did a soil separation test and it produced a slightly ambiguous result 
in that after the sand settled (not more than 15%) I was never able to 
make out a clear distinction between silt and clay - the fluid part was 
always opaque and the boundary indistinct. I'll try it again with less 
soil and some detergent but I'm assuming all this means the soil is 
reasonably cohesive and there's least some clay.

In terms of wall construction the plan was to dig a 10cm deep bale width 
trench around the edge of the dome, with a deeper, shallower 
drained-to-sky channel below that, fill the bottom with sand, hammer in 
some rebar for the initial row of bales, stack up rocks until they're 
about 5cm above ground level and then top with a plastic layer. I'll try 
and construct it so where the rebar punctures the plastic any trapped 
water will drain out. I know this is not ideal, but I'm hoping this will 
be sufficient to survive a few years. The ground is fairly dry and well 

So, any comments or suggestions on any of the above would be gratefully 

In terms of the project itself, we're very interested in sustainable, 
resilient, open source, permaculture type ideas so if you have a project 
you're interested in building, teaching about or experimenting with 
please get in touch. The plan is during the summer we'll be running a 
number of residential workshops on things like earthship and straw bale 
building (we have people interested in teaching the former, very 
interested in hearing from people who would like to do the latter). The 
goal is that everything is free or very low cost. The only real 
requirement for use of the land is that projects should be open and 
documented. We're also interested in developing action learning / 
demonstration permaculture growing spaces. Of course, in the more short 
term if you happen to be nearby and are interested in participating in 
building this or other structures please get in touch as well!



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