gemini://ur.gs/blog/straw-bales/

Part of our long-term plan is to get some land and build our own house(s) on it, in a manner that's suitable for self-sustainable living. Building houses is hard work, of course, and the modern building industry is rather problematic, so I've been looking into "alternative" methods of building for a few years.

The Centre for Alternative Technology is a bit of a hub for that in the UK, and I was signed up to both their "Natural Housebuilding" short course and "Green Building" MSc pre-pandemic and pre-baby. This year, things felt stable enough for me to get back to it, so a friend and I went on their "Building with Straw Bales" course last week. Five rather intensive days of theory and practice that left me feeling quite capable of.

I'll detail the main things I learned day by day, but there's a canonical book if you want a reference manual, which I shall link to here as well.

Monday and Friday were both part-days, but add them together and you've got almost another full day. We were quite busy - the overall impression was of a course that should be twice as long or have half the content.

Officially, the course started at 5pm on Monday afternoon with a tour of the CAT facilities, where we visited the straw bale buildings they have on site, talked about how they're put together in broad terms, and then had dinner and a couple of hours of seminar-format conversation about the course and each other.

We did all that, but we actually got there Sunday evening and ended up helping the course organisers (Bee Rowan and Hassen Mzali) to set up. CAT weren't super-organised, and the course space ("the pole barn") was a mess. We swept out large quantities of old straw, uncovered the demonstration kit we were going to build on, and then started sorting the straw bales we were meant to be using for the course, which had been stored from the previous year under the barn's roof overhang.

Nobody thought to check for hazards, and we ended up breaking open a wasp's nest that had set up between two of the bales, leading to a swarm of angry wasps and 20-40 stings apiece for several of us. I got off lightly with just a couple, but it wasn't the most auspicious start to the week - although it made for a reasonable bonding experience. We ended up relocating the course (and the demo kit!) to the CHP building instead.

We spent this day getting started with the demonstration kit, which is used to demonstrate load-bearing straw bale construction. It consisted of a sole (floor / base) plate and a roof plate, with room for six bales between them. Timber uprights were fixed to the sole plate and passed through holes in the roof plate, simulating timber framing for doorways and windows, while a brace was present at each corner. These all mirror or represent actual construction details.

The overall process was explained to us - we build the walls using straw bales, which must be made to fit the lengths required, and placed in a running bond. Once the walls are completed, ratchet straps (5 tonne+ loading) are used to squeeze the three components together, giving a solid construction. The roof is then built, putting its load onto the roof plate (which distributes it along the walls). Sometimes the roof is sufficient to maintain compression all by itself, but you can also replace the trucker's straps with packing straps (similar loading) and leave them there permanently. Compression forces air out of the wall and makes it behave as a single unit.

The first layer of bales is held in place by upward-pointing green hazelwood spikes set into the sole plate. These go about ½-⅔s of the way through the bale, and give it some resistance to being knocked out of place. Two of these per bale, and yes, the Buffy the Vampire Slayer theme tune was playing in my head throughout.

The timber framing is used to keep openings square as the house shifts, and is generally useful for attaching heavy things to, since it's fixed into position. It's not load-bearing, in that it's not needed to hold the roof up, but it does take some of the load if it's there, of course.

Where a timber post is present, the end of the surrounding bales must be 'notched' - a space cut out - to wrap around it. Fit everywhere should be snug, but not too tight - forcing a too-large bale into an opening will just push other bales out elsewhere. We saw this in action, popping a corner brace of entirely at one point. Friction makes getting bales into place difficult, sometimes - this can be overcome by using plastic bags to temporarily lube the gap, or by putting bales in with their mates 'corner-to-corner' and pushing two in together - either horizontally or vertically. Or you can use ratchet straps to compress horizontally, making space to slip a bale into its place in the wall before removing them again.

Each bale must be checked for damp or mould and have each of its faces dressed. This means cutting away loose straw to get a flat surface (as much as possible). We used alligator saws (double reciprocating blades) for this, and also for cutting the notches. If a bale is too long or too short, we need a specific length, or if it's not dense enough, this can be fixed by modifying the bale, which we got to practice at length.

Techniques for bale modification centre on the two strings holding it together. If a bale needs splitting into two smaller bales, we use baling needles to insert two new strings. These are tied off with a trucker's hitch (to get tension), and the old strings cut and removed. This can turn a single 1M bale into two 0.5M bales; or into a 0.3 and 0.7; etc.

If a bale needs to be lengthened, we can cut the strings and add some straw from another bale. They generally consist of leaves of straw, so you can take slices quite easily. Extend the cut strings on the bale with a length of baling twine, then use a trucker's hitch to get tension again. Always add a centimetre or so to account for increased tension.

To shorten a bale by a small amount, such that the baling needles are silly (there's no real use for bales < 25cm in length), you can do the same - cut the strings, remove some straw, re-tie. You can also use a clamp to streamline the process.

We built some wall and tried out the dressing and bale modification techniques. It was slow going to start with, but things sped up after the tea break and we made some real progress.

The seminar covered properties of straw bales and some history. Fire resistance, insulation, acoustics, historic buildings in the UK and elsewhere. We also talked about foundations a bit - the straw and sole plate both need to be protected from water. Car tyre foundations is one option; a plinth wall is another.

This was a very practical day. We worked pretty much flat-out, modifying straw bales and building the walls up, until we hit five courses. Time and straw supply was somewhat against us, and the alligator saws were emitting smoke, so we left it at five bales high, rather than going the full six - the principles of compression are the same, regardless of height. You're going for ~2cm per bale.

At four bales high, longer spans of wall without a timber post were reinforced by knocking long green hazel wood stakes down through the wall. These go in at an angle and take some horizontal loading. Make sure the wall is good and level before adding these! We got to work on that with the subtle application of "persuaders" - huge wooden mallets - to the wall, to push individual bales out or in. Focus on getting one side - typically the outside - level, since bales have variation and the outer surface tends to need the best weatherproofing and visual appeal.

We tackled some awkward spots too - putting bales above and beneath windows, particularly. The framing around a window requires a long notch to be cut *along* the bale to receive the horizontal post(s) of the frame, which is enough of a faff that they brought along a pre-cut bale as a shortcut. In a real build, on a given storey, one does all the spaces above and below windows first, compressing them and getting the frames into place before moving onto the rest of the walls. Compression can be achieved with ratchet straps, and held in place with the horizontal parts of the framing, or a temporary horizontal member can be screwed into place and bottlejacks (or even car jacks) used to push against it and compresss the straw that way. Properly a faff, but necessary.

We took a break for a talk on coppicing with Sven, to talk about where our green hazel comes from, then we went back to it and prepared the building for compression, giving the walls a final finish with a hedge trimmer, stuffing gaps with straw, and putting the trucker's straps in place. We didn't have time to actually do it, though, so left it for the following day.

The evening was more of a personal lecture by Hassen, who walked us through his introduction to straw bale building, and the projects and people who have filled his life since. Very heartwarming.

Compression! We ratcheted the straps and watched the roof plate squash the bales down by around 10cm, took away the braces and admired our handiwork. Very good. Then we loosened it all off and took the house down.

All done, right? Wrong. We had two more straw-bale building techniques to learn, so went back over to the pole barn (the wasps had been removed in the interim) to get started as two separate teams.

Those indoors worked on the infill technique, where a timber frame is the loadbearing element and straw bales or SIPs just provide the insulation. The principles are otherwise similar to loadbearing, but there's no roof plate. Instead, the frame is built to the post-compression dimensions and a temporary metal plate, or set of plates, is stacked on top of the penultimate course of bales. Slight over-compression is achieved with ratchet straps against that plate, the final course of bales (uncompressed) is put into the gap, and then the tension released and the metal plates (very quickly, lest they get stuck) removed. The bottom courses of bales relaxing causes the top course to become compressed.

Us outdoors worked on a straw bale wrap instead. This is designed for retrofit - you have an existing structure and want to improve its insulation by adding straw bales, either to the outside or indoors. Space is generally at a premium in retrofit, so we stacked the bales 'on edge' (so ~350mm depth rather than ~450mm). This was all relatively simple, except achieving compression was fiddly - as it would often be in real retrofit. Rather than trucker's straps, we used packing straps directly, held against the existing wall with hooks (eyebolts would be better, we were told). Unlike in infill, the compression plate remains in place and would typically be wood - perhaps ply, although it needs careful design to avoid cold bridging.

We also had a workshop and some practical work on clay with Dieter, since clay plasters are commonly used internally in straw buildings. We were then set to work preparing various clay mixes for use the following day.

The seminar was, again, more of a lecture format, this time by Alex who did the Green Building MSc the previous year and had just finished up some research on moisture levels in straw bale walls during and following a storm - basically comparing lime plaster with a vapour-permeable membrane + wood cladding. Interesting stuff, but needs further research (of course).

The final day finished with lunch at 1pm, and was focused on plastering and rendering. We applied an initial layer of clay slip to the infill wall, which is generally called the 'discovery coat', and learned how to make 'rats' - twists of straw dipped in slip, used to fill any remaining gaps exposed by the discovery coat and get the surface as level as possible. Then we applied clay plaster by hand, throwing lumps of it onto the surface and feathering out the edges of each blob, before coming back over with a trowel to level the surface and 'knock it up' ready for the finish coat to be applied. Then we went outside and had a go at applying a finish coat to a "here's one we made earlier" piece of board.

Oh, and we took down both kits and cleaned up after ourselves, then got everything packed away ready for next year. A quick trip to the lime kiln to see how it's made and look at some lime putty rounded off the course - we didn't get the chance to play with that ourselves.

I had the long-term future in mind when taking this course, but came up with a bunch of ideas for things I could do in the short term while on it - and feel confident I could actually pull any of these off. Here's a summary of each.

The fences between us and the neighbours on either side are flimsy wooden things held up by concreted-in wooden posts. They need renewing anyway, and I'm considering lime-rendered straw bale walls, a bit under 2M high, as a cheap and sturdy option. They'd look great and the acoustic attenuation (around 54dB for this buildup) would do a great job of muffling waist-high noises like heat pumps and babies.

This project is about 12 x 6 bales, and maybe 1M³ of render, per wall. The foundation can be a small plinth wall; the really important detail is how to finish the top, which is something I'm not really clear on just yet. Here's a couple of similar projects:

Initial thoughts are an EPDM "hat" on top of the straw but below the render, along the length of the wall, perhaps with something approaching a weep vent to aid runoff of anything that penetrates the render.

We also have the boundary at the back of the garden, which just has an unadopted road on the other side. Noise does penetrate, though - we hear their kids playing, and they hear (and make comments about) ours. Since there's already a good brick wall there, with wooden cladding panels between pillars, I'm thinking a vapour-permeable membrane between that and the straw bales, and rendering just the inside face. Again, the hat needs some attention to detail. There are also runoff pipes coming into the garden from the road behind the wall, so they'd need some reworking.

Integrated into the rear straw bale wall, located centrally, we can create a D-shaped space for not many more bales. Perhaps 4M x 3M or so - whatever we can get away with without needing planning. The foundation and roof needs some thought, but I'm very confident we can get the structure in-between up!

We have a brick-and-block garage that's really too small for a car, and I've been thinking about converting it for a while. It's around 5.1M x 2.8M, with 2.6M to the roof trusses. Straw bales on edge on both long walls would reduce it to a ~2M space, which is probably too narrow. We could use straw bales on just the short sides, perhaps. One for more thought.

We're not allowed permanent boundaries along the front of the house, as there's a service trench right where the wall would usually go - and there's almost no space between house and street. So I'm imagining straw bales in a wheeled tray, compressed and lime rendered, at a height of 80cm-1M. This would cut out a small amount of road noise, but allow the baby and dog to go out round the front of the house. It would also be a good practice project - we didn't get to try lime rendering on the course, just clay, and I'm sure there are relevant differences.

My allotment already has a couple of sheds, but they're coming to the end of their lives and I have a friend with no sheds at all. Adding a simple straw bale building to each shouldn't be the end of the world, although it would need committee approval, of course.

Between them, the above projects could use up 500-700 traditional "flat 8" bales (around 1 x 0.45 x 0.35M in dimensions). These typically have a density of 80-120kg/M³, and are actually starting to get a bit difficult to find - modern straw bales can be much larger, and are only suitable for moving by telehandler. There is a supplier who will send 40 at a time via palletised delivery, at a cost of around £6.25/bale all-in, but you can get them much cheaper (£1-£3) from local sources. The ideal is meeting a farmer who's interested and willing to produce bales to order. Let's see what we can rustle up!

As a bit of a coda, Bee isn't doing much straw bale building any more; instead, she's focusing on the use of lime-stabilised soil in foundations as an alternative to modern concrete foundations, particularly in disaster relief efforts - she's spending a lot of time in Bangladesh working on exactly this. The basic idea is to amend soil with hydraulic lime, which makes it strong enough to pop small buildings onto, and resistant to flooding, earthquakes, etc. We had a very good chat about it on Sunday evening.

Lime-stabilised soil has been used in civil engineering for decades, but hasn't made its way to foundations in a big way yet. Something to watch for, and something I intend to do some experimenting with myself. There's a book, but it's not cheap - fortunately Bee had a copy with her so I got to read it in the evenings.

Straw bales

Prospectus

Monday

Tuesday

Wednesday

Thursday

Friday

Future projects

Garden boundary walls

Garden room

Garage infill

Movable walls

Allotment shenanigans

Sourcing bales

Lime-stabilised soil