This week I have been harvesting house bricks

Ok, so they weren’t conventional house bricks, but straw bales; I was out in the field harvesting Miscanthus using our very own baling machine. This was a source of much excitement to me; baling machines have mysterious things going on inside (which I attribute to goblins, which are probably close relatives of the ones that are inside sewing machines), so for us to have bought one at work, and then to be able to play with it was pretty awesome. I have named her Betty, and she has seen a lot of baling action with her previous owners, but is still in decent condition.

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Betty (Jones MT10) in action at Gogerddan, April 2018.

The dimensions of straw bales used for buildings are based on the ‘small square baler’ machines developed in the 1940’s.  These have been used to produce bales that are typically around 35cm x 45cm x 101cm (height x width x length). As all straw bale builders know, the use of these baling machines has decreased, with most farmers either producing large square bales or round bales, both of which are more suited to modern agricultural production and minimise labour costs. Having hugged rather a lot of bales in the field this week (carrying bales by holding them upright against your body is easiest if you’re not particularly strong), I can see why any innovation that decreases the labour required will be popular with farmers. But it is definitely having an impact on the availability of straw bales for building projects, and this presents us with both a problem and an opportunity.

The scope to change the size and characteristics of bales are rarely debated or discussed by straw bale builders. Whilst this demonstrates impressive confidence in the ability of the builder to make design adaptations on site, this effectively means that the product designer (i.e. the person producing the bales) has no design brief. If you were to start from a blank sheet of paper, what would your ideal straw bale look like? What length, height and width? What strand orientation? Could we have a bale with squarer corners? What density produces the best U value? Would we want it pre-treated to improve resistance to fire, mould or rodents? Can we have triangular bales for gable end walls?

The fact that small bale production is on the wane is an opportunity – we can imagine a future where straw was being baled specifically for building, so quality control would be higher and the properties are what was specified by the builder. Some solutions are really simple – straw bale builders spend significant time splitting bales to get shorter bales that allow walls to be built using the Flemish bond approach. Yet standard small square baling machines can easily be adjusted to produce shorter bales; so we could relatively easily shift to a supply model where we ordered bales that were the correct length to start with. This is taken as a given in other building trades; you can go to a timber merchant with a cutting list and they will provide you with components pre-cut and sized to your order. Whilst there will inevitably be some adjustments required on site to a few bales, specifying bale lengths could significantly reduce work load and improve accuracy and build quality.

 

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The vertical bar dictates the bale length as it moves down, via a link to the baling needles. The metal link with the bolt acts as a stopper; by moving the stopper, the bale length can be altered.

Whilst the height and width of a bale are constrained by the dimensions of the baling chamber in the machine, bale density can easily be adjusted, but when I ask a straw bale builder what density of bale they would like, their answers usually relate to the likelihood of the strings popping rather than any considerations of how density might affect long term properties such as insulation value.

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The U profile beam can be moved up or down by turning the handles. This changes the bale density.

Strand orientation can be varied to some extent using current machines, and if baling was done as a static process (e.g. in a controlled facility with a feed conveyor dropping straw into the machine), better control could be achieved. This is somewhat chicken and egg; we don’t yet have a clear understanding of what the ‘best’ strand orientation would be.

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The tines on the drum pick straw up from the ground and into the chamber above. Some control of strand orientation is possible by changing the direction of swathed material on the ground, or by dropping material directly into the chamber.

An obvious step is to base the timber frame dimensions on a known bale length; this is widely known as a good idea but rarely practiced. There are many projects on which moving a window or door by 100mm could have resulted in significantly fewer bales needing to be cut down to size rather than placed as whole or half bales. Sometimes this is because the builder doesn’t know what the bale lengths will be until they turn up on site, long after the design is finalised, and in other instances, the frame designer doesn’t realise the impact of minor design changes on the amount of work required on site. Another argument for producing bales specifically for the building market? Perhaps, and whilst specialist building bales will almost certainly cost more, maybe we need to let go of the idea that straw bale building is cheap, and consider the cost of bales relative to the costs of other materials on site, and in relation to labour costs.

Shall we do away with bale altogether and move towards prefabricated structures? It clearly has its advantages in terms of control of build quality and the scope to do multi-layered wall build ups in the comfort of a factory environment without weather concerns. Several approaches to straw bale prefab are in use in the UK, including Modcell and Ecococon. The Modcell panels are in essence a timber frame panel with straw bale infill, but Ecococon have taken the bold move of moving away from the bale as a unit, instead using a chopped and blown straw manufacturing approach. This removes a lot of constraints in terms of panel dimensions, and in the future might be used to optimise strand orientation (in order to both provide a good surface to key in a render coat, and also to maximise thermal performance). At a first glance, prefabricated approaches seem expensive, but the cost of a standard straw bale building are enormously sensitive to assumptions on labour costs, and there is a lack of data on how much many building projects cost in practice.

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Ecococon panels. [link]
Undoubtedly, there is room for more than one approach to bale building in the future. Perhaps it’s time for straw bale builders to ponder their potential role as product designers. What does your dream bale look like?

This post is based on work undertaken as part of the BEACON project, which is funded by the European Regional Development Fund via the Welsh Government. If you’re interested in building with straw, the SBUK website is a useful starting point.

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The knotters. Don’t ask me how they work, it’s not wise to get close enough to them when they’re operating to learn exactly what’s happening.
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Box holding sisal twine to string the bales with. Also the cleanest and most convenient place to store lunch.
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Baling needles underneath the machine. Unlike sewing machines, balers are pretty easy to thread.

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