Back from Ariège

Dorian

Well, this time it looks like we've made up our minds: we're going with loadbearing straw bales as the building system for tour house!

Of course, it was fairly predictable that the training course we just completed over 4.5 days in Ariège with the Mieux Vivre Habitation (MVH) coop would reinforce this idea: one might expect that people providing training on this construction system would be convinced of its usefulness, and that their enthusiasm would be contagious.

On the other hand, being able to learn in depth about the limitations of such a technique, and even experience it first-hand by building a demonstration module, could also have led us towards other solutions...

I will try to summarise the lessons that reassured us or reinforced our chosen path. But first, a few words about the training itself.

Meeting the experts

The MVH training centre is located in a beautiful corner of Ariège: a few buildings nestled in the bend of a small, winding road; all around, as far as the eye can see, a splendid forest, rustling with birds. As soon as you arrive, you are immersed in the atmosphere - a small, multicoloured building of about 10m² with a single-slope roof stands at the entrance to the car park, covered with different coatings and built on a tyre foundation. You have to get closer and walk around it to see a few strands of straw sticking out from under the roughcast and the bales from the inside. This is a module used as a support for some of the eco-construction training courses given there: earth plasters, quicklime plasters, tadelakt, etc.

Once past the dry toilets, you enter the building where the theoretical sessions take place. As soon as you enter, you feel the inimitable comfort – so difficult to describe – of an earth-straw building. Whatever the outside temperature or humidity, you feel good here: the warm atmosphere of the earth that makes up the floor slab and the wall coatings – decorated with beautiful curves in plant shapes. Exposed beams, large windows opening onto the sun and the forest...

MVH has been offering training courses in eco-construction for seven years, aiming to popularise different techniques that combine energy efficiency, the use of local and bio-based materials, and the promotion of traditional craftsmanship passed down through generations. Since 2015, the partners have also been offering their services on various eco-friendly renovation and construction projects, from design to execution and supervision. One of them, Vital Bies, is a renowned specialist in masonry heaters.

The icing on the cake is that they offer discounted rates, tailored to each person's means, whether or not they have funding from their financial institution, including reduced rates for people receiving minimum social benefits.

Suffice to say that this summer, when we stumbled upon their website while looking for more information about loadbearing houses, we knew we couldn't pass up the opportunity... Even though it promised to be quite an adventure, given that it would be impossible for us to get there by car, and there isn't a single bus stop in the vicinity. We were already mentally bracing ourselves to hitchhike from Cazères and miss the first morning of training! Fortunately, everyone rallied round to support these two poor pedestrians: carpooling on the first day with our fellow trainee Sarah, arranged by the team; and for the daily trips to and from our accommodation, 6km away, at Bertrand and Séverine's Bambou Créations, another great training location - we were able to combine the use of mountain bikes kindly rented at a modest price by our hosts, carpooling in rainy weather with Laurent, another kind fellow trainee, and romantic walks through the woods. As a bonus, we picked delicious chestnuts for dinner :heart:

Because yes, apart from transport and accommodation, you also have to be self-sufficient when it comes to meals. Bearing in mind that the nearest grocery store is 5 km from MVH, and there's quite a steep hill to climb! Without a car, it's a bit of a challenge. (Note: we found out, a little too late, that we could have rented a caravan directly at the training venue.)

So that's the context... but what about the training itself? We'll get to that! But first, what is loadbearing strawbale building? A quick technical detour for those wondering...

Straw in all its forms

Straw used in construction can come from different types of cereals - wheat, triticale, rye, barley, rice, etc. Other plant straws, such as hemp (hemp shives), lavender, flax and sunflower, are also used.

As Luc Floissac points out in his book La construction en paille, (see our bibliography article), straw is an excellent insulating material: it is highly efficient thermally, inexpensive, healthy, adaptable, unappealing to insects and rodents, and highly fire-resistant (when well compacted). it also has an excellent social and environmental record, as it is a renewable resource, its use in construction allows for the storage of large amounts of carbon, and it is a by-product of cereal production - therefore, it does not compete with food production, and its commercialisation even provides a significant additional income for farmers. Finally, as long as it is kept away from moisture, it is also a very durable material: buildings hundreds of years old around the world, often constructed with mixtures of earth and straw (e.g. with cob infill), attest to this. It should be remembered that nearly half of the human population nowadays lives in buildings made of eco-sourced materials of this type, and that concrete only began to dominate the building sector after the Second World War.

It is therefore not surprising that straw is (finally) being rediscovered at a time of widespread ecological crisis. According to Floissac, ‘with only 10% of wheat straw, all the homes built each year could be insulated with straw’ (I assume he refers to French buildings and straw production). Unfortunately, probably due to preconceived ideas and inertia in the construction industry (not to mention the political weight of Lafarge-Holcim and other cement manufacturers...), ‘less than 1/1000 of new homes are built with straw’ (in 2012).

There are many ways to incorporate straw into buildings, both in renovation and new construction. Broadly speaking, there are two main types of use:

In the form of bales. Cereal straw is harvested using a combine harvester and immediately pressed into bales using a baler; or
In bulk. This straw is crushed and used either as bulk filling for insulating boxes or in the form of ‘vegetable (or vegetal) concrete’ (made from a mixture of straw, water, and earth and/or lime) which can be used as plaster on wooden structures (formwork) or in the form of prefabricated panels.

Ever since we started dreaming of building a house, we've been particularly interested in straw bales... and not just because we like growing vegetables: straw bales have the advantage of being easy to find, standardised in size, and easy to handle – especially ‘small’ bales (which are generally 37 cm high by 47 cm deep, with a length of about 1 metre) – which makes them easy to design and install.

But one question still bothered us until this training course: straw bales are all well and fine, but what technique should we use to build a house with them?

When we talk about straw construction, people around us generally think we want to build:

a ‘wood-frame building’ – i.e. placing straw bales between wooden uprights that support the roof structure;
a ‘post and beam’ building - where a main structure (with posts and beams) supports the roof frame, and a secondary frame is filled with straw; or
‘boxed straw building’: in which a structure (load-bearing or not) composed of bales (often made of wood) is filled with straw, and is usually prefabricated in a workshop and then assembled on site.

These are the most commonly used methods, including in the construction of the houses in the Écopernic eco-development near where we live in Pau. Each has its advantages. However, none of them are quite right for us... because they seem too expensive, requiring technical expertise that is beyond our reach (for example, to build a post-and-beam structure or prefabricated boxes); and because they use a lot of wood, which is not necessarily very environmentally friendly (this is particularly the case with timber frames).

Our very small budget and our desire to build our own house led us to another set of techniques, less common in France (but well known abroad): load-bearing straw. A building of this type is not structured mainly by wooden studs, posts or boxes, but by the straw bale walls themselves (and their render), which are chained together and support the roof structure. The straw is therefore both structural and insulating. These techniques have the advantage of being quicker to build yourself, requiring fewer tools and less wood (they do, of course, have their disadvantages). They are often referred to as the ‘Nebraska technique’, in reference to the Sand Hills region of Nebraska (United States) where, at the end of the 19th century, farmers who had no wood but were equipped with the first agricultural balers discovered the usefulness of straw bales for building houses.

Our first encounter with load-bearing straw was through the ‘paillourte’ style house: a round house (shaped like a yurt), built with straw bales and topped with a ‘reciprocal’ roof structure. We may come back to this in other blog articles... For now, we refer you to David Mercereau's excellent website, who has meticulously documented the construction of his own paillourte, to find out more!
Unfortunately, during our discussions with the town hall and the planning department of the beautiful town of Lys, where we would like to settle, we were made to understand that any building permit application for an ‘atypical building’ (i.e. not rectangular) would be systematically refused - as would any roof that was even slightly too flat... This is because it must comply with an architectural charter on which the local authorities relies instead of a local urban planning scheme (PLU). In short, we began to wonder whether straw bales would still be affordable for us, both economically and technically, on a rectangular building.

On the financial side: we are still preparing our budget... On the technical side: the training at MVH greatly encouraged us!

So let's talk about this training.

The art of stacking bales

Our training course was led by Julien Noury, who has over 15 years' experience in straw construction. He is also one of the authors of the French Professional Rules for Straw Construction - the reference work on the subject in these parts, approved by the state authorities, which everyone in the industry refers to; and he has also co-authored the Professional Rules for Load-bearing Straw Construction... which have not yet been published. In fact, the above-mentioned book only covers the use of straw as an insulating filling material (generally in a timber frame), but says nothing about load-bearing straw. This explains to a large extent why there are not more construction projects using this technique in France: without official guidelines, it is very difficult for builders to provide ten-year insurance for such projects, which means they have to find a design office.

The first half of the programme is theoretical. After looking at photos of a few examples of buildings constructed in different countries, we moved straight on to the fundamental technical aspects: how to choose the right bales (tip: fibre length is very important), how to store them, and how to start stacking them on a slab while protecting them from moisture and termites. Over the following days, we also learned about the construction of the bottom and top wall plates, the dimensioning and layout of the walls (i.e. the organisation of the straw bales within the walls), and air and wind tightness management. In short, the essential points that make this technique unique.

While the mornings were devoted to these theoretical points, the afternoons were spent in the field: under a large shed in the centre, we learn how to assemble a 10 m² load-bearing straw module step by step, all together. The bottom and top plates (i.e. the pieces of wood forming the wall framing), as well as the boxframes (into which the doors and windows are fitted) were prepared in advance to save time. We had to learn how to cut grooves for the straps that would compress the straw bales from top to bottom, then start building the walls by cutting the bales to the right length, which was quite satisfying! It was sometimes difficult to slow down, which is necessary to avoid having too many gaps between the bales, or walls that are a little too bulging... We also learned how to test the elasticity of the straw bales in order to calculate their compression coefficient. And how to drive bamboo pins into a wall using a persuader (an enormous wooden hammer, the straw bale builder's best friend). Finally, we checked with a laser metre that all the walls were perfectly plumb... which required tightening the straps around the module, little by little, until everything was at the right height. And then, vertically, we trimmed the walls with a brush cutter.

At this point, we would normally move on to installing the roof frame, then applying the plaster. But unfortunately, the training course came to an end – so after a quick triumphant souvenir photo, we dismantled everything! In just over an hour. As someone I know might say, ‘it's quicker eaten than cooked...’

A few points that stood out for us, among many others:

Always choose bales with longer fibres.
Having scaffolding around all four walls is much more practical!
Forget traditional roof structures with two or three trusses. For load-bearing straw, the roof must be supported by lots of small trusses that distribute the weight evenly along the walls.
Julien does not recommend using stakes (or pins) through the bales... unless you have to work on top of the walls (and in the corners).
The articulation of the wallplates and boxframes is crucial...
... as is the perfect alignment of the plates and bales with each other. Don't go too fast!
Beyond 8m in length, a load-bearing straw wall made of small bundles must be reinforced with a cross wall or a post.
Never force the bales into the wall!
Swing shutters are complicated to install on straw walls...
Rather than extending a bale by adding straw from another bundle, it is better to attach a small, recut bale to it.
Ventilation is essential.

See here for all of our notes! They will probably be a bit cryptic to the uninitiated, sorry...
And here for photos.