The wind in Melbourne can huff and puff but it won’t blow our (round)house down.
People respond to the word straw when I mention the Whistlewood Common roundhouse. Incredulity often fleets across the faces of my companions (unless they have watched Grand Designs). So, for this month’s post I spoke with Hannah Hunt, our designer from Straw Works. By the end of this post you should be a firm advocate of straw as a building material.
“It is exciting to see something you have worked on come into reality. We have got to know Whistlewood and it is great to see your vision come to life”
Hannah Hunt, Design Team Leader – Straw Works Ltd
In case you don’t know – I certainly didn’t before speaking with Hannah – everyone must pass these steps building and opening a new building:
- Get planning permission. For this our first architect designed our beautiful octagonal roundhouse, and we planned its location. The local planning office gave consent in July 2014.
- Develop technical drawings and specifications to demonstrate a whole bunch of stuff to a building inspector. We are coming back to this step.
- Create the construction drawings, so that we can plan the build and resource the materials. While this step doesn’t involve any consents, it is vital to the overall process.
- During construction receive inspections from building control. If we have made any changes, we agree them with the building inspector.
We have finished the first three steps. Now we are planning the build and resourcing materials.
What does building control (the building inspector) want to know?
Before we get into all that, I should mention that Whistlewood Common has engaged a private building control service. Our service provider is a Corporate Building Control Approved Inspector. This means they can give us all our consents. They work directly with the local authority and their experience with strawbales is invaluable. AND they help us make the right decisions. Their experience doesn’t just ensure the quality of our build but will also help us achieve our goals. If you are not sure what goals Whistlewood aspires to, please see December’s post.
So what does building control want to know?
- Does straw work?
- Will our building meet safety regulations?
- Will our building meet fire regulations?
- Will our building meet insulation standards?
- Will our building be breathable?
Here comes the technology…
Does straw work?
To know whether straw works, we need to know what it is going to do. In some straw bale constructions the staw is an infill material. These buildings have separate frames that are structural. In our roundhouse, the straw will hold up the roof.
That begs the question – how strong is straw? The answer is strong enough, with the right construction methods. Load-bearing straw walls can take the weight of a solid wood roof. But they work in a different way to ‘traditional’ walls. Most importantly straw cannot support a point load. Think of the rafters in your house, they bear down on the bricks. If we used the same construction in the roundhouse the bales under the rafters would compress. As a result the roof could end up uneven.
Instead, we use two methods to make the bales ‘strong enough’. The first technique is to use a roof or ring plate on top of the bales. The ring spreads the load of the roof evenly around the building. The second procedure is pre-compressing the straw.
Let’s step back a moment and refresh ourselves on the structure of the roundhouse. For more details see the first post in this series. Our foundations are made of tyres filled with pea shingle. A base plate rests on the rubbery supports, and we will build the straw bale walls onto the base plate. Hazelwood stakes will hold the bales in place, and the roof plate goes on top. To ‘squish’ all the air out of the bales, we will loop many ratchet straps under the base plate and over the roof plate. Then we will pull hard. Once the bales are compressed, we will replace the ratchet straps with tape. If we didn’t pre-compress the bales we would have to wait for gravity to do its job. The pre-compression also levels out the roof plate.
So, long story short, yes straw works. In our case, it will hold up the roof.
Will our building meet safety regulations?
Yes, we have engaged an extremely competent, experienced firm to design and manage the build. The roundhouse will satisfy all safety regulations from disability access to fire exits and all necessary signage.
Will our building meet fire regulations?
You might be forgiven for thinking that a straw building would not resist flames. The truth is that straw bales – once compressed – break the fire triangle. Now, what is the fire triangle I hear you cry? For a fire to take hold it needs three things – hence triangle – fuel, a source of ignition and oxygen.
Squishing all the air out of the bales deprives any fire of that all-important third ingredient. Hannah likens burning a straw bale to burning a telephone directory. If you apply a flame to a telephone directory the edges will char. While some of the outer pages may catch fire, the fire will run out of oxygen and splutter out almost as soon as you remove the source of ignition.
If you have ever watched a TV programme or film where the good guys pick up essential information from a poorly burnt stack of papers – usually in an open fireplace – you know what we mean.
Will our building meet insulation standards?
Given the previous section, I won’t describe straw as ‘toasty’, but it is a great insulator. If you want to get into the science here is a link to an online book. For a lay person’s version read on.
To achieve the same level of insulation as mineral wool, we need twice the thickness of straw. Taking your roof space where the latest recommendation is for 200mm (20cm or 8 inches), you would need 400mm of straw. We are using 450mm straw bales, so we have enough insulation.
We will see in a second that straw bales are breathable. So they let air pass through them, just not enough to sustain fire. As cold air comes in it picks up warmth from the air in the build and acts as a secondary insulator. The warm air increases the effectiveness of straw (and many other natural insulators) conversely mineral wool is not breathable. Just in case you wondered this effect is called dynamic insulation.
Finally, our walls will have two beautiful finishes – 30mm of clay on the inside and 30mm of lime render on the outside. 60mm is a lot of material. And while they are not great insulators in their own right, applying this much render and clay to the walls of the roundhouse further increases its ‘thermal mass’.
Will our building be breathable?
Modern construction techniques have improved insulation and comfort by sealing gaps and stopping drafts. But they have also compromised ventilation. In particular, they stop water vapour from leaving buildings. When this results in condensation the fabric of our buildings can be damaged, and moulds may start to grow.
Newer building regulations recognise this issue and set a standard for breathability in roof spaces. Inded, permeable roofing felt has been a requirement for some time. Now breathability is not the same as a draught. The latter is an uncontrolled air escape or ingress. Whereas breathable materials allow water vapour to pass through them. Straw bales are breathable, so is lime render and even clay render. This means we can expect excellent air quality in the roundhouse year round. And that we are unlikely to experience the problems associated with damp.
Another aspect of breathability covers the amount of toxins in the air. Many modern building materials ‘off-gas’. For example, formaldehyde escapes from glues in bonded wood products. To reduce formaldehyde levels we will avoid Oriented Strand Board and instead use Smartply and woodfibre board.
Sourcing the materials
As you know, we are using the most natural materials we can find and are keeping a firm eye on the distance the materials will travel. However, for the most part, finding products of the right quality is fairly easy. But when it comes to straw, we will have to inspect the bales before we confirm our purchase.
We need to make sure the bales are regular shapes and dense (not too floppy!). We can accept no loose straws, and each bale must be a similar size. The straw must be in good condition; it must be dry, stored well and have no signs of mould.
Common sense and our ‘five senses’ will help us inspect our bales. We will also use a straw bale standard, written by Straw Works, to make sure we get the right bales for the job.
In case you were wondering we will be using wheat, barley or rye straw. Straw is the inedible part of domesticated grasses (it is a stem, not a leaf) and is not a food source.
A corner piece
As you know our roundhouse is octagonal. That shape has eight exterior corner of 45°. However, you may have noticed that most buildings have right angles (90°) at each corner. Creating our unusual shape in brick would require experienced, talented bricklayers. In straw, it is pretty much impossible. Our builders can shape straw bales, but making sharp angles isn’t feasible. So instead, we are going to have beautiful, soft, natural, organic curves.
Heading for the door
When Hannah told me about the pre-compression my thoughts turned to the openings in the walls. She quickly put my mind to rest on the subject of windows. All our windows will be in the roof, but the four doors are going to need special treatment. The main door will be a traditional hinged door. The other three, leading to the stage, are a series of ‘French doors’. In both cases the door frames need to be strong and robust.
To keep the doors upright we cannot attach the door frames to the straw bales. Instead, they will be fixed to the base plate and, after the pre-compression, to the roof plate. We will cut notches into the straw bales to hide and insulate the frames. Enclosing the frames prevent ‘cold-bridging’ to further reduce heat loss.
Squish – the non-technical term for ‘compress’
Thermal mass – the amount of heat a building can hold
Oriented Strand Board (OSB or flakeboard – see this Wikipedia article for a good description and some pictures
Smartply – a brand of OSB that does not off-gas
Woodfibre board – an insulation material made with natural wood fibres
Cold-bridging – a weak spot in insulation
Next in the series
No building is an island – how will visitors get to the roundhouse?