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



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Rendering

You can render straw bales in either cement, clay or lime; or you can clad them in any cladding you want (but rendering is good as it provides a good seal against vermin and fire). We have chosen lime render (using standard bagged hydrated lime and rendering sand) mostly as cement is reputed not to "breath" as well (i.e.: the lime render allows moisture vapour to escape from the bales) and clay suffers some erosion problems (we get wind-driven rain a fair bit). The lime render will penetrate about 15mm into the straw and will be built up about 40mm proud of the bale surface (and will be applied by a spray-rendering pump).

Above windows we use stud walls instead of straw (which require a substantial lintel). This saves us from extra building effort, and we will super-insulate the stud walls. The stud walls are supported by hardwood uprights on each side, which also double as the window buck (rough frame to hold the windows or door). On the really wide window (4600mm) we added steel strap as a triangular brace to transfer weight loads from the lower middle of the stud wall to the upper outside uprights, as a 40mm coat of render will weight a substantial amount.

Stud Walls

We want a seamless appearance to the render from the straw bales to the stud wall, so we covered the stud wall with cement sheet (7.5mm thick "Duratex", specifically designed for render finishes), full height hardwood uprights on the sides, angled bracing to shift loads from the centre bottom of long spans to the top of the side uprights. An example is the kitchen servery window or the south wall (check out the south wall after rendering and windows went in).

The cement sheet is covered by aviary mesh (1/2 inch square galvanised mesh, 1mm wire), held off the face of the sheet by metal bottle caps, held onto the sheet (and the sheet held onto the studs) by screws. Holding the mesh off the sheet means that the mesh will be fully embedded into the render, which greatly improves its strength.

Electrical

The cables are run through the roof cavity and into conduit run down the walls to metal masonry mounting boxes, pinned to the bales with roberta pins, and the conduit plus pins lightly hammered into the bale face.

General Advice

Spend a lot of time getting the walls straight and plumb. You will spend a lot of time with render (whatever method you choose), and the simple truth is that it is easier, quicker and cheaper to get the original straw bale walls correct than it is to correct (hide) problems using the plaster. Bale frenzy gets to us all, but resist the urge to get the wall up super quick, and concentrate on getting the wall up correctly. Use a long (and very straight) piece of wood (or any other reliable straight edge) and a spirit level to check and re-check that your walls are plumb and (if they are supposed to be straight) that they are straight. Whipper-snip the walls with the same process, checking that the final trimmed and cut walls are still plumb. Use the bale hammer a lot - compacting the surface is good, and it helps get the walls straight.

Where you add straw (stuffing holes or filling cavities) make sure you stuff tightly as otherwise the render will sound hollow over that spot. We have areas where we cut a large chunk out of a series of bales (to accomodate a column) and stuffed the small gap with straw and covered the lot with mesh, but we didn't stuff the gap tightly enough and the final rendered wall sounds hollow in that spot when you tap it, even with over 5cm (2 inches) of render.

Spray Rendering

LOCATING SAND, LIME AND EQUIPMENT

When getting sand delivered, consider getting it dumped out onto big bits of builders plastic sheet (like the left overs from under the concrete slab). This helps to stop it getting contaminated with small stones, sticks and the like, so you can use it down to the last shovel full.

Also when ordering materials try taking a just-in-time approach. Only order as you need it, as the process takes weeks and weeks, perhaps months if you are doing it by hand. Having a lot of sand and lime around that you need to keep clean and dry is difficult. Estimates are also just that - estimates. So how much you really need isn't clear until you get closer to the end. Don't take an estimate, even from the most knowledgeable person, as a hard and fast quantity.

PUMP HOSE LENGTH POWER WATER, BARREL

The Mix

When we rendered our house we measured the sand out in shovel fulls, the lime was half of a 20Kg bag (which just about filled a bucket) and about one and a half buckets of water. This recipe was provided by the renderers, who had a lot of experience and knew how to quickly put together a good mix. The beginner should measure everything by volume (using buckets is good) - so find a source of large strong buckets (check out some of the stores in your area, not to buy buckets, but to find a store that gets their materials delivered in buckets and hence ends up throwing out good, strong buckets).

We used "normal" hydrated lime. This is easy to get, and whilest purists argue that you need to use lime putty aged for several years (a) reality gets in the way of this and (b) modern testing seems to indicate that hydrated lime is almost as good, possibly due to improved and consistent manufacturing techniques providing uniformly crushed fine hydrated lime dust. You buy the hydrated lime by the pallet load (a bit over a tonne at a time, 54 bags), and don't be put off by the thought of manually having to load and unload the pallet (onto your ute, or moving it around the work site) as it is easier than you think.

SAND TYPE OPTIONS

You should not add all the water so that you can adjust the mix at a later stage. Start with an empty (clean) mixer, add most of the water (say one and a quarter buckets). Then start shovelling in the sand. Let the water and sand mix together for a while until it looks consistent. I like to get the sand looking wetter (sloppier, thinner - whatever your terminology) than I want the final mix. Then add the lime, watching out for the dust that will blow out of the mixing bowl (use a mask, position yourself upwind). You can adjust the mix by adding more water or sand, but do not add too much more sand (no more than half a shovel full) unless you also add more lime (you don't want the lime mix too weak or the render will be too friable).

The final mix you are looking for has several indicators (whilst in the mixer bowl with the mixer going) that help you to know it is just right. Choose whatever indicators you feel comfortable using. If the mix is too wet it will run off the wall, will run out of the spray applicator with no pump assitance, and will dry to create a weaker wall. If the mix is too stiff the spray applicator will not be able to apply it.

***does this description apply if you are using a different pump system? or doing it by hand?
  1. slump at leading edge of bowl - should be rounded. If too wet this leading edge will run fairly flat to the bowl, when stiffer it starts to drag the leading edge under the mix and the mix looks like it is slumping over the leading edge, forming a rounded edge.
  2. flow over mixing paddles - too wet and it flows over easily. As the mixture stiffens the flow over the paddles starts to have a rounded look to it, similar to the leading edge of the bowl (but slightly different).
  3. average slope of mix in bowl - I liek about 30 degrees of slope for a good spray application mix, or 45 degrees if you want to hand apply the mix.
  4. drag of mix up the bowl - the mix (once the lime is in) becomes "sticky" and hence gets dragged up the trailing edge of the bowl. This should happen, but I can't really describe how much - when you get a mix you really like take note of how much it happens.
  5. sound and/or sight of mix dropping off paddles - you should hear the "plop" of the mix dropping off the mixing paddles. The mix should cling to the paddles, and some should loose its grip as the paddles continue around in the air above the mix. Too wet and it won't adhere, too dry and it won't fall off.
DIAGRAM OF MIXER BOWL

Let the mix rotate for a few minutes (minimum) to get it airated and fluffy. If you are not going to use the mix right away you should turn off the mixer until you need it, then when you need it turn it back on and add a little bit more (a handfull) of water, as the mix will have dried out whilst just sitting there. If you are not going to use it for a while (say, not until tommorrow), level the mix in the bowl and cover with water. If you are not going to use the mix for many days (a) you should not have made it yet nad (b) you can store it under water for quite a while, but I wouldn't store it in the mixer in case you do something wrong (like not enough water for the period of non-use) and it sets in the mixer - use a bucket, tub, waterproof wheelbarrow or plastic lined depression in the ground.

Applying It

by hand / hawk by professional pump by home-made unit

Have plenty of spare hands available if you can. There is lots of basic, unskilled labour involved and so more hands will speed up the process. This was true for us even though we had a professional team on-site. Remeber that when you get people onsite you need to be prepared for locusts. We went through almost 3 litres of milk a day - in tea & coffee - about 75 litres during the rendering! We didn't keep track of cake, biscuits etc, but the milk was amazing.

Trowelling Off

when to trowel pressure and blade angle clean tools

Scaffold

We thought about the maximum heights (3.5 metres outside) we needed to reach and got scaffold to handle that - forgetting that we can reach to 1.8 metres fairly easily with a trowel. It turned out that having some smaller, easier to move around scaffold would have been good. When rendering a normal room height you don't actually need to reach that far up, so scaffold to get you up 1 metre from the floor would be plenty.

Build your own sprayer

You should look at a home built spray unit and a commercial equivalent.