The great tragedy of science – the slaying of a beautiful hypothesis by an ugly fact

The title is a quote from Thomas Huxley

I have a part time but long standing interest in various things relating to straw bale buildings, including how to improve the quality of building. Last winter I had what I thought to be a really clever idea, which I sought to test by doing a bit of thermal imaging of straw bale walls. My theory turned out to be false (which I was quite sad about), but the most interesting discoveries are often made my mistake and whilst I was fiddling about with the thermal imaging camera, I saw this:

IR wall plaster thickness
Thermal image of a straw bale wall, taken from the outside of the building. We can see several areas that are very obviously bale shaped.

Thermal imaging is often used to detect failure of insulation in buildings by detecting cold spots in walls. It’s a fairly good technique for the job, but there are a few common pitfalls that result in artefacts in the image and need to be thought about when interpreting images. If you are trying to detect heat loss from a building, you need to be sure that there is a decent temperature difference between the inside and outside air temperatures, and that the only significant source of heat is from inside the house. For this reason, images are often taken very early in the morning, in winter, before the sun hits any part of the building.

Because I wasn’t actually interested in heat loss and was looking for something entirely different, I didn’t worry too much about this requirement. The sun had hit this side of the building, and what we can make out in this image is quite distinctive shapes of some of the straw bales.

The reason we can discern the shapes of bales in this wall is almost certainly that there are significant differences in the thickness of the lime render that covers them. Render has a high thermal mass, and the sun has heated up the render, with the amount of heat being re-emitted (and therefore detected by the thermal imaging camera) being dependent on the thickness of the render. This image is taken from the outside of the building. Visually, the wall appears to be pretty flat, so the differences in render thickness suggest that the straw bale wall underneath wasn’t very flat before the render was applied.

One of the last steps in building a straw bale wall is flattening out the surface of the bales prior to the application of lime render or plaster. This is usually done with a chainsaw, strimmer, angle grinder or alligator saw.

shaving a wall
Straw bale wall being flattened out using an angle grinder. If you’d heard of farmers lung, you’d be wearing a dust mask when doing this…

The process is important because it results in a surface that is easier to key the first surface of render to, and it’s easier to get a nice flat finish if the straw bale wall is fairly flat to start with. The other reason to pay attention to strimming the wall nice and flat is that it will have a significant impact on how much render you need to apply in order to get a nice flat finish, and this in turn has an impact on labour and material costs.

not a flat wall
The initial render coat being applied to straw bales by hand. This wall has obviously not been properly strimmed, it’s really uneven as you can see from the surface on the right hand side of the picture.

Inevitably on straw bale building sites people want to get on with the next task, and flattening the walls is a lot less fun than daubing on the plaster, so it is completely understandable that sometimes the flattening gets a bit neglected. It would be possible to get a more quantified idea of how important this was by drilling holes in the wall to gauge thickness, but this isn’t my wall, so I haven’t done this. It’s unlikely that the varying render thickness is particularly important to the long term functioning of the wall, but it’s still a waste of materials. Incidentally, when I took images of the other walls of the same building, none of them had this effect to nearly such a great extent. It wasn’t possible to tell whether this was because they’d been strimmed beautifully flat or simply because of the building orientation the sun hadn’t hit them yet.

The moral of this story; if you’re building with straw bales, take time to flatten your walls properly before you render them, you will probably save yourself a lot of time and money.

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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