You can imagine the long list of materials required for a project like this. Walls, roof, floor, paint, kitchen benches, plumbing etc, etc. It has been difficult process at times with the answers not black and white but more often various shades of grey. We’ve explained about our selection criteria elsewhere on this website. In this section we’ll go through some of the materials that we will be using for the project.
What do you say when friends ring you with the offer of a couple of trailer loads of old wire-cut bricks. Only thing is, they still need to be cleaned and the cement based mortar does not particularly make that easy… This was only a start. We collected lots more (with lime based mortar) after this and only needed two more pallets to be able to build all walls for Unit 2 out of recycled bricks.
- About 700 bricks in total
- Reds, wire-cut
- Cement based mortar 🙁
First of all, we did not demolish an existing house on our property. This was a house in East Fremantle.
We were given the opportunity to salvage material from a house in East Fremantle. Previously a heritage listed home, the owner now had approval to demolish it. A shame in a way, but an opportunity for us to salvage some unique materials. Salvaged items include doors, window frames, floor boards, skirting boards, weather boards, beams, posts, bearers and lintels. A total of over 7 cubic meters of timber! And also about 25kg of nails…
The shed in the backyard is a little replica of the house we demolished.
The quick facts:
- Demolishing time: 4 days
- De-nailing time: heaps 😉
- People involved: ~14
- Floor bearers / joists
- Ceiling bearers / joists
- Rafters / hips / ridge
- Wall studs / plates
- Doors / window frames
- Roof tiles / tin sheets
Other salvaged Items
Other salved items used and found for the ‘Genesis’ project included:
- Bricks (see above)
- Timber, timber and more timber
- Light fittings
One of the ideas behind the Green Swing project is to try different materials and find out how they perform. Unit 2 will be mainly built with straw bales for the exterior walls (except for the garages at the front).
If you are interested in building with straw bales for your house, please seek advice from an expert. Yes building with straw bales can be done by owner-builders, but the techniques that are practiced are different depending on who you talk to (use wire netting?) and the type of building you are after (2 storey? wide verandahs?). The last couple of years a lot of experimenting has been done with the renders and the type of finishes (cement render?, lime wash? etc). We would strongly recommend to not just visit a workshop (where you only see the exciting process of stacking the straw bales or rendering them), but to do a proper course where all the aspects of the building process are covered and tailored for building with straw bales.
Pro’s and cons of strawbale homes
We will only find out about the real (dis)advantages of living in a straw bale home, once we have actually moved in and lived there for a number of years. However, we have read an awful lot of information on the subject and spoke to a lot of people in the (straw bale) industry about this. These are the advantages of building with straw that pushed us over the line to give it a go as part of this Green Swing project:
Advantages of building with straw bales
- Insulation – depending on the thickness of the render and how compact the straw bales are (not too compact!), the walls can have a thermal resistance (R-value) between 6-9 W/(sqm x K). Make sure you put some proper windows in, seal doors, insulate ceiling/roof and you will have a well insulated house.
- Bio-degradable – Very often houses get knocked down before they reach the end of their life. These days houses aren’t recycled as we used to do. Although some concrete can be used as an aggregate in new concrete, most materials end up in landfill when the house is bulldozed. At least the straw/clay/lime combination is bio-degradable.
- Sound barrier – It is amazing how quite it can be (depending on window/door openings) inside a straw bale house. If you have every had a chance to play with straw bales as a kid you will probably remember. During a straw bale course we (Helmuth & Eugenie) were standing on either side of the wall (no roof, no windows) and we could hardly hear each other. When rendered the walls become even more sound absorbing. Some interesting studies have been done in this space.
- Low embodied energy – compared to tradional building materials (like bricks, concrete) straw bales (and also mud brick) have an extremely low embodied energy as they are a waste product from wheat farming. Of course, one could argue that farming itself is very intensive and as a result, the straw has a high embodied energy. However, right now farmers don’t farm wheat for the building industry, they do it for food production. Straw bales are more or less a waste product and quite often gets burned.
- Carbon storage – When wheat grows it absorbs carbon, rather than burning it (or decomposing it, producing methane), we can store it in our walls. When rendered with a lime render, the lime will become harder and harder over time, as it absorbs carbon from the atmosphere.
- Excellent moisture regulation – This property is probably what sets straw bale walls apart from any other building material. It actually regulates the quality of air in your house in terms of moisture content (particularly when rendered with a clay render).
- Local product – In WA there is an abundance of straw bales available.
- Easy to work with – Not only is straw a very forgiving material (very easy to correct mistakes), it also is very easy to shape it so you can create interesting features if that is what you are after. Cracks that (will) appear in the clay/lime render are easy to fix.
- Thermal mass – Straw bale walls need to be sealed to make them fire proof and the render will also give a lot of additional strenght to the overall wall. The positive side effect is that you will always have a thick coat of render on the inside of your house, adding to the thermal mass. The additional thermal mass can help make your house more energy efficient.
- Great sense of security – Straw bale walls look impressive. They are wide and you can design window seat in these walls. They give a great sense of security, like living in a castle with thich walls.
Disadvantages of building with straw bales
Of course there are some disadvantages when you choose to build your walls with straw bales. These are the biggest problems:
- Moisture/Water – The biggest problem for straw bale walls is moisture/water. Straw decomposes quickly when it gets wet. However, the latest techniques in straw bale building make sure that you can safely build two storey walls will limited protection (eaves) and keep your straw bales dry.
- Space – particularly on a small block, straw bale walls take up a lot of space. Calculate with at least 600mm wide walls. On bigger blocks this is less of an issue, but we had to really design the layout of our rooms carefully.
- Paints – You can neither finish the outside nor the inside of the walls with a “sealing” paint (like an acrylic paint). It will seal the walls and they will start sweating, causing the walls to become moist and they will start decomposing very quickly. This means you are limited in the choices of finishes. But really, who wants to paint a beautiful earth/lime rendered wall anyway?
- Local Government/Councils – We had been warned about councils not being supportive of alternative building materials like straw bales. However, so far we only have had a possitive experience. Again if you are interested in building with alternative materials, just make sure you approach the building process professionally and involve experts. It helps to design the structure of the house to the straw bale walls are not “load bearing” (we think the structural infill method is probably making things a little easier).
- Insurance – Again, we were warned that finding an insurance company for a straw bale house would be difficult. We did not have any issues in this area, so don’t let it put you off.
Having done a little bit of rendering (courses / helping out others), I find this is one of the most time consuming tasks when building a strawbale house. Some (owner-)builders invite lots of people for a workshop to share the load. I am too scared to do this as the render is a crucial part of the final straw bale wall. Although the straw bale walls in unit 2 of our development are of the type “structural in-fill” the render is still very important to the overall performance of the house. Without the render the strawbales wouldn’t have a very high insulation value: air could pass through very easily. The render plays a critical role:
- Entraps the air in the bales so the wall gets its high R-value of between 6-9 m²·K/W;
- Protects the bales so they don’t get wet or become a fire hazard;
- Regulates moisture: it absorbs moisture from the atmosphere from within the house;
- Regulates moisture: it absorbs moisture (if there is any) from within the bales;
- Gives the final shape to the walls: curvy or straight?
There are some excellent books on applying an earth render to your strawbale walls as well as information on the web, but the problem is that clay (unlike with cement) is different where ever you get it from. Some clay does not mix well with lime, others are very expansive when absorbing moisture (causing cracks) and some are not very sticky when applied. The best is to do some test renders your self. This is what I have been doing to find the best recipe for me, so if you are in WA and can get your hands on the same ingredients, you are lucky 😉
1. Study the subject
Make sure you understand the basics of an earth render: it is important and fun. You need to be able to “read” the render: when to add sand, when to add clay and when to use lime (wash).
- Dave Morris from Strawtegic operates a render pump in WA and is very knowledgeable when it comes down to lime renders and straw bale building in general.
- I found this book extremely useful: “Design of Straw bale buildings – State of the Art” by Bruce King, publisher: the Green Building Press (2006). An almost scientific approach to designing straw bale houses including different types of render.
- Some good recipes / discussions : Natural Plaster & Lime wash
I am sure you will find other sources through Google.
Basically, what you are trying to get an understanding of is the following:
- Earth renders don’t contain (portland) cement. They typically contain clay or lime or a combination. And of course sand as the aggregate.
- Earth renders are applied to straw bale walls in layers (or coats).
- First coat is called the base or discovery coat (no shaping of walls is done at this stage) and have a high content of clay compared to the sand.
- Second coat is to cover everything and to shape the wall where necessary (second coat could consist of multiple layers!)
- Third coat is a protective coat and generally has a high content of lime.
- If the render cracks, you add sand; if the render is very “dusty” you add clay/lime.
- Subsequent coats typically have more sand (less cracking!) and top coats have high content of lime (weather proofing).
- Clay based renders are excellent moisture controllers, but easily erode. They need protection (eaves, lime coats).
- A lime wash can be applied over the final coat (very thin)
What ingredients are you going to use for rendering your strawbale wall? There are different types of sand, lime and clay. You can add binders (wall paper paste, actigel) and reinforcements (monofilament). Unless you know someone who has tried and tested a recipe, it is best to do this testing yourself.
Clay versus Lime
A lot of people use a pure lime render on straw bale walls and have had a lot of success with this. I wanted to use a clay render for the base coats and also the second coat because of the unique characteristics of clay when it comes to:
- Moisture control
- Repairablility (clay render doesn’t undergo a chemical reaction when it cures)
- Safe to use (unlike lime)
- Extremely low embodied energy
- Hydrophylic (absorbs moisture from its surroundings) and when wet it becomes a moisture barrier.
Lime renders (looking through a microscope) have quite an open structure and absorb a large amount of water very quickly. Clay absorb water as well, but very slowly and slowly give it of to the atmosphere again. Having said that, there is still a place for lime in my render. Clay renders are quite soft and erode easily. Lime renders on the other hand become harder over time (as it carbonises: lime absorbs CO2 and moisture from the air and turns into limestone). So, we apply a final coat of just lime render to the straw bale wall. This lime render is quite open in structure so if water enters and hits the clay render behind it, the clay becomes waterproof as it absorbs water. When the rain stops, the clay can dry out quickly because of the open structure of the lime render. We can further protect the render with:
- Installing eaves above render;
- Protecting the lime render by applying a lime wash possibly with some rice flour paste. Rice flour paste is still “permeable” (lets vapour through), but helps waterproofing the lime render.
Another product worth looking at is ActiGel. As you can see in my results, it makes the render a lot harder but also stickier. Actigel is an extremely highly purified version of attapulgite. This is achieved by using a unique and patented process developed by Active Minerals. A typical attapulgite would not work in the same way at all. It is not a cheap product, but since you only need a very small amount of it (0.1% of the total render) there is no significant additional cost.
3. What is the plan Stan?
Before you start throwing mud at straw bales, you need to prepare and plan. First you need to write down what combinations of ingredients you want to test. The base recipe that I started with was: 1 part clay, 3 part sand. And then I started experimenting, using different types of sand (brickies sand in WA contains 9-10% clay) and different types of clay as well as reinforcement techniques like netting (nylon/wire) or monofilament.
The idea is to first find the best recipe / ingredients for your base coat. Then pick the best 2 base coats and work on the 2nd coat, pick the best second coat for testing the final coat. For every test run I keep an eye on the following: cracking, compression strength, sticking to the previous layer (delaminating?) and finally erosion (garden hose test). Based on these aspects I pick the best render. Keeping in mind that cracking for the base coat is less of an issue than in the subsequent coats and the hose test is something that you really only need to apply to the final coat. In fact the cracks (if not too many and too big) provide a good grip for the next layer to prevent delaminating. I have provided my base test template below.
4. Executing the tests
I found it easiest for all the tests to keep the amount of sand consistent. I always measure by weight (use a kitchen scale), not volume. I first mix the dry ingredients then add the water. If you are planning to use a render pump to apply the render, it is probably wise to add a little more water to your render to resemble the slurry coming out of the render pump.
When you are ready to apply the render make sure you first fill a section of an egg carton with render. You will use these samples later for a compression test.
It is best to test for stickyness by applying render to a real straw bale. Make sure you wet the bale before you apply the render. There are three aspects that are important when you check for stickyness.
- First of all the render has to stick to the straw bales and shouldn’t fall of the bales easily when you apply it. If the render doesn’t stick, increase the clay content, perhaps a little more water can help or add an additive (flour based additives like wallpaper paste, or something like actigel).
- The second thing you want to know is how well does the dried render stick to the bales. Pull off sections of render after a week or so to see how well it has bonded with the bale.
- The last thing that is important is how well do the different layers bond with each other. If the one layer peels of the previous layer then there is a problem… You can try to improve the mechanical bond by scoring the previous layer with grooves (this should always be done), the previous layer should be wetted before the next layer is put on. If that still doesn’t work, you need to look at the consistency of the render that you are putting on.
With earth renders that are high in clay content, cracks start to appear quite soon after the render has been applied. Again, best is to test the cracking by applying the render to a straw bale then wait for a couple of days to let the render cure and inspect for cracks.
Although a compression strength test of the render may not be necessary in case of structural infill method (straw bales are non-loadbearing), it is still useful to do this test in my opinion (and it is quite fun to do).
I used the following method to determine which render was the strongest under compression. All tests samples had no reinforcement in them (ie. straw, hair or monofilament – pure clay/lime/sand).
Take a long flat piece of timber that is stable when laid flat on the floor and lifted on one end. You are going to put render samples on one end and the other end is still resting on a flat surface / ground. Before you put a sample of render under one end of the timber, inspect it and make them roughly all the same size (rub over paver to shave of a little) and have two flat surfaces at top and bottom. Put the sample under the timber and start stacking bricks one at the time directly above the sample. Use the same method of stacking for each sample and keep the bricks in the same location directly above the sample. Keep stacking until the sample crumbles to bits. Count the number of bricks that the sample was able to support (don’t count the one that broke the camel’s back). I used bricks of about 4kg and have had samples that supported 16 bricks (some didn’t even support the timber by itself without bricks)!
Test the render with a garden hose by pointing the water stream directly at the render. I know this will probably never happen in real life, but some showers in Perth are real down pours and can last for quite a long time. It is primarily the “driving” rain that you are simulating with this test. Rain coming straight down is less of a worry if you install eaves…
In my tests I found pure clay renders are very soft and extremely easy to remove pointing a garden hose at it. Apply lime render to it, and it is a lot harder. Give it a limewash (with some riceflour paste) and you need to point at it for quite a while before the render is affected. However, I still need to do more testing on the second and final coats of the render and right now I am leaning towards using a penetrating sealer (like they use on limestone), which is permeable and still allows water vapor to pass through, but is waterproof (ie. water droplets can’t get through). More on this will follow later.
5. Test results
You can find a results of some testing that I have done using the above methods here – render tests – jan 2012 (test plan for base coat). Here is the summary.
|% brickies sand||% clay||% actigel|
|base coat:||75% (incl 10% clay!)||25%||0.1%|
6. Local resources
Most ingredients are Clay (unlike lime) is not very easy to get hold of in WA. Here are some sources that you might want to try:
- Some brick makers have some good clay for rendering but you need to ring around and find your way around the receptionists. Ask to talk to people in the lab department that should do the trick. Contact The Green Swing if you are interested in clay from them and we can possibly get you in contact with some people that were extremely helpful to us.
- You may be able to get access to Kaolin clay which is a beautiful, pure and very fine kaolin clay.
- To do a proper render test, you need some bales to test the render on. Allan Fowler (our supplier) also supplies CityFarmers shops around Perth with straw bales. Straw from 2011 (harvested and baled early 2012) were of a superior quality (we have been told …). We were lucky!
- If you are interested in doing something different with your render, there is the option of using Tadelakt. An absolutely beautiful waterproof (but permeable) finish. Contact ConcreteMode for more information
- If you are interested in using “Attapulgite” (or Actigel as it is commonly known) contact Active Minerals. They are a Queensland based company.