Since their appearance in the XIXe century in the United States, straw houses have grown in popularity and today represent an ecological alternative to traditional construction methods. Contrary to popular belief, straw is a resistant and highly efficient material from a thermal point of view. Still a minority in the French housing stock, the straw house market is developing gradually, solutions in kit are even proposed to facilitate its implementation.
The straw house: the general principles
A renewable material
The straw that is used for house building is a scrap of agricultural production of cereals (wheat, rice, barley or spelled). So it's a renewable material, which is produced near construction sites. Biodegradable material, straw can be obtained for a quite modest sum (even free). Nevertheless, the low industrialization of the construction methods penalizes its cost price and the final cost of construction.
Framed construction technique
France is a European leader in straw construction, thanks to the promotion and information work carried out by the French Straw Construction Network (RFCP) and the National Center for Straw Construction (CNCP).. There are as many straw houses built in France as in all the rest of Europe. Since 2012, the professional construction in straw is framed by the CP 2012 (Construction in Straw 2012) and is part of the Current Techniques (the "TC") that are eligible by the standard scales of the ten-year guarantee (and therefore the property damage insurance).
The straw construction is therefore framed by precise rules (approved by the Construction Quality Agency) and building professionals have a repository and a training opportunity thanks to the RFCP. It is also referenced in the appendix of the RT 2012 (the thermal regulation 2012). In 2009, the CSTB (Scientific and Technical Building Center) validated the construction of a straw school.
An ecological alternative
The straw construction is an ecological alternative that attracts more and more people, attracted by its low cost and its very good thermal capacities. The straw still suffers from an image of fragility (all children grow up with the head of the fable of the three little pigs) although one of the first straw houses built at the beginning of the XXe century (1921) still stands perfectly.
Straw boots and wood frame
To build a house from straw, it is pressed in parallelepipedic boots (a kind of big straw brick) 35 to 45 cm thick. These "bricks" of straw are then stacked within a wooden framethen covered and protected, either by a plaster cast with gypsum boards, plasterboard or wood, or by an applied rendering of plaster, lime mortar or earth.
Good fire resistance
While loose straw burns very easily, it has been proven that straw compressed in brick form has excellent fire resistance (there is not enough oxygen between the strands for combustion to take place). In this form of compressed boot, straw is more fire resistant than wood.
Protection against water and humidity
To best express its qualities (especially thermal), the straw must, like all plant materials, be properly protected from water (rainwater and other precipitations) and water vapor. This may be his main weakness. To ensure his good moisture resistance, do not implement straw bricks whose moisture content is greater than 20% (and therefore should not be exposed to rain during construction), it is necessary to provide an overflow of the roof and a system of gutters to avoid runoff, it is also necessary correctly ventilate the roof and the roof.
Due to the insulating properties of the material and because of the thickness of the walls, the straw has a total heat resistance index R for each wall which is 2 to 3 times greater than that of traditional walls made of bricks or concrete blocks.. A wall at least 45 cm thick provides an R of 6 compatible with RT 2012.
All the requirements for the implementation and design of the straw insulation as well as those for the application of coatings are listed and precisely described in the rules of the CP 2012.
Construction of a straw house
To build a straw house, there are two techniques:
- the first is to use straw as a carrier structural;
- the second to use straw as an insulating filling between the elements of a wooden frame.
Straw as a carrier
Straw used directly as a load-bearing material (whose load-bearing walls are only made of straw, therefore) does not require additional framing and can support the weight of the frame and the roof. While some examples of large-scale construction exist abroad (including a hotel in Switzerland and two-storey social housing in England), the straw-bearing construction is not recognized as a TC in France (Current Technology, eligible for insurance) and is therefore not widespread. It is nevertheless used for medium-sized buildings in self-construction and for a few larger public buildings (up to two storeys).
Straw on wooden frame
In France, the most common technique is that of straw used as insulation in a wooden structure. It can be a prefabricated box structure, a traditional structure of posts and beams assembled tenon-mortise or a wall structure type wood frame double or single.
In a box structure, compressed straw bricks are stored in boxes that can themselves be attached to a structure or support all loads. This technique can be associated with different types of structure (wood and / or concrete) to build multi-storey buildings. Straw is here mainly used for its insulating properties and its ecological qualities.
In a traditional structure of posts and beams, the straw is arranged between the main elements of the frame and is maintained by compression. The latter is done by the structural elements already in place or by the addition of a so-called secondary frame.
In the case of a wooden frame, the straw bricks are placed between the vertical uprights of the framework and are maintained by the implementation of horizontal battens. They can also be arranged without compression between the walls of a double frame type "GREB" (double frame).
To insulate brick houses
Straw can finally be used to isolate masonry walls in a classical way. In this type of use, the straw is most often glued to the interior walls using earth or lime and then held in place by a complementary framework.
Many artisans and builders offer kits to facilitate the implementation of straw as self-building material. To find out about trained and competent craftsmen and local professionals, the RFCP website offers a directory of recognized professionals and associations.
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