Unit 2: Straw bale house
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 con's of straw bale houses.
We will only find out about the real (dis)advantages of living in a straw bale house, once we have actually moved in and lived there for a number of years. However, we have read an awfull 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 the Green Swing development 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 haven't had any issues in this area, so don't let it put you off.