Owner Building a Strawbale House
in the Bega Valley,
NSW,
Australia.



Journal

Building

Photos

Tools

Plans

Statistics



Join our
Mailing List



Strawbales

Straw bale wall construction - we use the post-and beam (non loadbearing) method, using a steel frame.

Bale Walls

Standard small (800 long x 450 wide x 350 high mm) straw bales stacked on top of each other (flat side down) in a brick running bond.

Footings

We have a raised concrete plinth with lengths of plastic conduit embeded in it at 300mm centres. The plinth is 450mm wide, 90 mmm high, the conduit about half way up the plinth. Reo was placed in plinth above conduit, tied (with LIGS) to mesh fabric of slab, then the monlithic slab (footings, slab and plinth) poured in one go. On top of the plinth we layed a plastic damp proof course. On the outside of the building we put galvanised 40x40x2.5mm angle iron, bolted to the slab, to act as a render stop.

The Wall

Our straw bales are layed in a brick running bond, one layer cut side out, next layer cut side in (to minimise the wall leaning towards the cut side). Four or five courses high (about half-way up the wall), we put on a layer of 4 bar 11mm trench mesh, put fencing wire over trench mesh, down through every second hole (600mm centres) in the plinth, tighten with a gripple on each side (best way to get even tension). Then we install more wire under the trench mesh, lay on the next four or five courses, another length of trench mesh, then wire and gripples (squeezing the two runs of trench mesh together). Finally add more wire and gripples around the whole lot, using the remaining holes in the plinth.

The walls but up to the columns (steel ladder-frame, 65mm think and 300mm deep). Where the bales but up against each side of a frame we stuff extra straw over the frame edge (burrying it in the wall) and cover this with aviary mesh (half by half inch mesh of .9mm galvanised wire), later on to be rendered over. Diagonal bracing is removed before the bale wall goes up, the wall is compressed, the line of the diagonal bracing is marked, then all the compression wires are removed and a groove cut into the bale surface with a chainsaw to the depth required and slightly wider than required. The diagonal bracing is installed, covered with extra starw, this is covered in aviary mesh held on by roberta pins (so named because they are a bit bigger than a bobby pin), and wire and gripples re-installed. The whole lot gets tightened up, and later on gets rendered over.

On the subject of roberta pins, make them pretty long (200mm or 8 inches into the bale) so they get maximum friction (and hence holding power). Also curve the wire with a changing radius curve (again, maximum friction) with the most radical curve towards the tips of the pins.

Other methods to cut and trim bales include a standard hand saw and a special cutting wheel to fir on a grinder, which mimics a round chainsaw, usually called a "lancelot wheel". Note that cutting straw bales is dusty work, so a standard dust mask may not be adequate. We prefer a positively ventilated full face helmet with ear and eye protection built in. My favourite is the Triton powered respirator - not too expensive, but it does the job well.

When you need to trim the bales you will need to keep tension on the straw, so inserting a replacement bale string or two is required. Read the text books on how to do this, but note it is quite straight forward.

Moisture Sensors

If you have a straw bale wall, why not build in some moisture sensors, so you can track any potential problems (mostly, damp straw) ? You can do this in many ways, but I wanted a low-cost solution that also automated the data collection. I chose to put a data acquisition (DAQ) unit onto a standard personal computer, and attach wires between the computer and some moisture sensors built into the bale wall. This way I get regular readings taken and plotted on a web site, for all to see.

Alternatives - you can install wood blocks into the wall (with short leads sticking out of the wall surface) and use these to take readings with a commercial wood moisture meter. Quite accurate, and the moisture level of the wood should be in equilibrium with the moisture level of the straw bale. Check out the Masonary Heater Associations study (commissioned by the Canada Mortgage and Housing Corporation) for moisture sensor options. One of these options is to use a relative humidity sensor to approximate the moisture content in the straw bale.

Aha ! I can do this. Lets see, quick google search for "humidity transponder" (which is a relative humidity sensor with supporting electronics to turn the measurement into a simple analog voltage or digital signal). Lot of options, for speed and simplicity I chose a Honeywell unit. I can attach this to one of several different data acquisition electronic kits, I chose a kitsrus unit number 118 - very small, takes 0-4v input, 12 bit output, 8 sensors can be connected.

The parts I have so far are enough to get me started. The DAQ provides stabilised 5 volt output, so I can feed this to the sensors. Wiring the system is easy, I just use some spare CAT5 cable to hook it all together. The sensors have a common connetion (0 volts), a supply (+5 volts), and an output (0 to 4 volts). Software is provided with the DAQ unit to automate taking readings. The sensors are soldered onto some of the CAT5 wires, heat shrink used to insulate them, then inserted into some old tube I had lying about (with a few holes drilled into it) making a small air pocket, and silicon is squirted up the pipe from teh sire insertion point to seal it off. This is pushed into the straw bale wall (using a steel rod to ease the way).

Special mention of the Smartec system, which looks like it would be cheaper and give great results, but somebody needs to put some effort into planning up how this would be assembled - so hint hint go for it!