How much will it cost? Strawbale homes are labour intensive, and if being built by paid labour this will add to the cost. Strawbales are relatively cheap so people equate this to a whole house being cheap, however walls may only be 14-20% of the cost of a building. The cost of the walls includes construction grade bales (which may be up to $9 each), the cost of materials to apply compression to the bale walls, and the cost of rendering 3 coats. A contract built house can cost from $1750 square metre upwards depending on the level of finish and the home design.
The high level of insulation that strawbale construction provides is not easily or cheaply replicated in other building methods. To insulate other forms of construction to the same level (R8) as strawbale may make them more expensive than strawbale construction. Strawbale homes offer long term cost savings because of the high level of insulation they provide, reducing heating and cooling costs.
I believe a very energetic, resourceful hands on owner builder could build a house for around $800 per square metre, however most of us are not as energetic and resourceful as we would hope to be, and could build a house from around $1300 per square metre.
In general I find that owner builders take on the challenge of creating a hand built home, whether using second hand building materials or all new, and while the finished home is a great credit to their skill and ability it may not have been an inexpensive house. It is more than likely a nicer house than they could have afforded to have contractor built. They may reap their reward when they sell their property.
What about water?
Moisture is the biggest risk to strawbale construction. Wet bales will decompose and this will reduce their strength, reduce the level of insulation, may create unhealthy levels of bacteria, and create costly and inconvenient repairs. Attention to detail and building a house with well-designed water barriers is essential.
Bath and shower areas- I do not recommend that shower areas or baths be installed adjacent to strawbale walls without additional methods of moisture control. Laundries and kitchens sometimes flood internally so I also recommend placing strawbale walls on a plinth or up stand (to keep them above floor level by 50 -100mm), this plinth will reduce wicking of the water up into the bales.
Good Design- strawbale buildings should include aspects such as appropriate eaves and verandahs. Landscaping can also be designed to divert rainwater and storm water away form the dwelling (i.e. bushes planted to divert driving rains from directly hitting walls).
A mad scramble to protect bales from rain. Always a good idea to have the roof on first.
What about fire?
Strawbales once erected and rendered are highly fire resistant. The tight dense bales with their 36mm coating of what is predominately sand (with cement, clay, lime or other additives) are in a state where there is minimal oxygen within them and very little air that is able to flow into the wall. Setting fire to a completed strawbale wall is like burning a telephone book, it is not until the wall is torn apart and air is available to feed the fire does it burn freely.
Building in bush fire areas- Strawbale construction can adequately satisfy the Australian Standard (AS 3959) building in bush fire prone areas, up to and including moderate risk. Strawbale buildings in high bush fire risk areas at present need additional construction requirements, and in these cases strawbales provide insulation only. We have worked with CSIRO to test a number of strawbale wall samples up to and beyond the requirements (of 29kw/m2). Building with strawbales in extreme bush fire risk areas, (as per AS 3959) is approved on a case-by-case basis. The successful fire test results from CSIRO (of 29 kw/m2 or greater) have deemed strawbale a suitable construction material in moderate bush fire areas.
Fire rated construction- while international test results give strawbale walls a 4-hour fire rating, there is resistance to accepting this in Australia without repeating the tests here. We will as an industry continue to undertake testing in Australia. Any building requiring a fire rated boundary, party, or other fire rated wall may need to consider additional or other construction options in order to meet these requirements.
What about rodents in the walls?
Rodents and insects may decide to live in a strawbale wall, just as they may live in the cavity of a brick veneer wall. If you purchase good dense strawbales with no seed in them then there is no sustainable food source for rodents and they have to come out of the wall and forage elsewhere for food and water. Attention to detail when rendering the walls (not leaving any exposed straw, even in the ceiling space) will reduce the potential of infestation.
What about termites?
Under the Building Code of Australia (BCA) no matter what material you build your home out of you are required to undertake some protection from termites. Strawbales are cellulose fibre (like trees) and to my knowledge there is NO species of termites that live solely on straw, though some species may eat straw. Strawbale walls are at no greater risk than the timber in doorframes, furniture or roofing members.
Is it hard to get council approval?
As with any home you will need to have building approval from your council. We have found no problem with council approval where good documentation is provided.
Are there any "off the shelf" plans available for strawbale homes?
There are some design companies offering generic building plans, with plans that suit several building materials. Our experience with these is that for owner builders or builders not experienced in strawbale building there is insufficient construction detail provided. Predominantly strawbale homes are individually designed.
Can I use my local builder?
Yes you can. We can consult with your builder about overall design and construction considerations eg. The timing of eaves lining and window installation. We can and often do subcontract for the installation of rendered strawbale walls to other builder's projects. Lance is a licensed builder and can be your builder, but this does depend on the location of your project.
How long does it take to build a strawbale home?
Once the construction of your strawbale building commences it is likely that it may take 6-9 months to complete. This of course depends on many factors including the site, design of the building, the complexity of the design, the size and required finishes. Many owner builders can take longer than this.
What is the insulation rating of a strawbale wall?
Some literature will state that strawbales have an insulation value of R40 or higher. These R-values are stated using the American method of calculating R-values. The Australian (and metric) method results in a much lower number, but still with the same level of insulation. The Australian rating gives strawbale an R-value of between 8 and 10. Timber framed insulated walls are around R1.5, insulated brick veneer R2, uninsulated brick veneer R0.5, mud brick less than R1.0, rammed earth less than R1.0 and timbercrete R1.0.
So what is R and what does it mean?
The higher the R value the greater the resistance to heat loss/gain through the building material. As a general guide solid materials transfer heat at a greater rate (have a lower R value) than materials that have air trapped inside them. Air trapped in each straw of the bale slows heat transfer thus giving a higher R-value.
Thermal mass is not measured in the same way, but has some importance to a well designed home. Thermal mass relates to the ability of a material to store heat and coolness. It is important to make thermal mass work as part of a comfortable energy efficient house; it needs to be inside the building and well insulated from the extremes of outside temperature. Stone, brick, mud and concrete slab floors are high in thermal mass. Rendered strawbale external walls combine an internal 36mm thermal mass skin (the render) that is insulated from the outside by the straw and this provides some thermal mass.
What can I render the walls with?
It is important to clad the exposed wall surface with a good sound water resistant render, usually in 3 coats. Earthen or lime renders are very satisfactory, however all renders need to be kept in good repair. Cement renders are sometimes used. The inclusion of mastic filled expansion joints is desirable to allow for the building to move and reduce uncontrolled cracking in the render. The addition of, or post construction application of water repellent agents (as used in mud brick or rammed earth) may be desirable under some conditions.
Is it easy to build with strawbales?
While the technology behind strawbale construction is simple to understand, it is, as I have been known to say, not idiot proof and is still a hard days physical work. While stacking strawbales to form a wall is quick and relatively easy, it is however only part of constructing a strawbale wall. Wall compression, netting around openings, rendering and installing control joints-whilst all the time paying attention to detail are necessary to get a good quality and long lasting strawbale building. I highly recommend that people attend a workshop before commencing to owner build. The hands on experience that can be gained is invaluable.