BUILDING WITH STRAW #3: CONSTRUCTION SYSTEMS
FROM STRAW BALE HOUSES TO MODERN AND PERFORMANT BUILDINGS
1. STRAW BALE HOUSES
It is the oldest and well-known construction technique with straw and by far the simplest, from the technological point of view. Straw bales are used as thermal insulation in the house’s walls and in some situations can also be used to insulate the roof.
Rectangular straw bales of smaller dimensions (10-20 kg and have in general 45 cm width) are used as filling in the walls structure, which can be of several types: beams and posts of wood/steel or timber frame structure. While in the past, in Nebraska, straw bales had a structural function (Nebraska Style) due to lack of available regional timber and low seismic activity, nowadays, straw is being used just as thermal insulation, while the structure takes over the efforts and loads. Nonetheless, it is recommended to use bales with higher density, in order to increase the efficiency of thermal insulation and create rigid support for the finishes. The density of a straw bale varies between 40-80 kg/m3.
There are different ways and techniques for building straw bale houses, each having certain features, but the process is basically the same: the concrete foundation is hydro insulated before the straw bales are laid down within the structure (increased humidity is detrimental to the quality of the straw, on a long term). The bales are placed one on top of the other and the flatness of the bale walls is provided either through ropes to hold the bales together, either through metallic rods or wooden sticks. There are situations in which even the structure of the roof is designed to support straw bale insulation.
The straw bale walls are usually covered with clay or lime-based plasters, applied on a reinforcement wire mesh layer attached to the wall with screws or staples. These types of finishes that are permeable to vapors are used because the straw wall needs to “breath” and therefore the finishes must allow the circulation of vapors in both directions (interior and exterior). Amongst other benefits, natural finishes made of clay and lime help by adjusting the humidity and temperature on the inside because of high thermal inertia. Find out more about clay as a building material in this article. Therefore, in straw bale houses we must avoid finishes that are impermeable to vapors.
The costs for a straw bale house can vary, depending on the type of structure, the distance from where the straws are being brought, and other variables, but what is certain is that a straw bale house requires intensive work, both qualified and unqualified. However, this can be an opportunity for lots of organizations around the world who are building such houses, to involve volunteers and transform a construction site into a workshop dedicated to students, makers, and other people interested in this field. Even in Romania, we can encounter these types of entities, among them being 3econcept.ro, a company that organizes workshops and invests in research and development of new methods and technologies for eco-friendly, economical, and energy-efficient houses.
2. PREFABRICATED PANELS INSULATED WITH STRAW BALES
It is a more advanced method of using straw as a building material. Straw bales are introduced in a wooden frame, usually timber frame system, which forms a prefabricated panel ready to be shipped on the construction site. Likewise, in this configuration straw has the role of thermal insulation. Usually, the density of straw in a panel varies between 80-110 kg/m3, which not only assures a really good heat transfer coefficient, but also makes the panels more rigid.
Prefabricated structural panels filled with straw are covered in the majority of cases with rigid boards, plasterboard (or drywall/gypsum board), wood fiber panels, magnesium oxide, etc. for reasons of modularity, optimization. There are also situations where the classical plastering method with clay or lime is used, even though this means extra work on-site.
The building technology with prefabricated straw bale panels is a modular technology through which the efficiency of using straw in constructions is exponentially increased (unlike the straw bale houses), since these are processed with the help of specialized machinery. The panels can have different dimensions depending on the project, and they are executed indoors in a construction hall, being transported afterward to the assembly place. In this way, the construction time is reduced along with the on-site work and associated risks, and the performance of the entire building is increased. We’ve discussed thoroughly the benefits of prefabrication in this article.
Prefabricated straw bale panels gained notoriety in the last 20 years, a timeframe when investments in research and development have increased significantly, especially in the European Union. Among the benchmark companies that promote this technology is Modcell from Great Britain, who have built not only residential buildings but also commercial or public ones on multiple height levels.
3. BLOW-IN LOOSE CHOPPED STRAW
It’s another advanced technology of using straw as an effective insulation material. This implies that the straws are very well cleaned and sifted of dust and impurities in repeated processes, then they are chopped and blown into the structure or other structural elements that form a cavity, such as a wall, roof, or floor. The blowing process is done with special equipment, that ensures not only a uniform filling but a certain straw density, which can vary between 80-110 kg/m3, a range in which straws have the best coefficient of thermal transfer.
Every process can be easily automated, therefore human intervention is reduced. The chopped straw can be stored in good conditions, protected from humidity, for further applications, this being an advantage in the production process. Unlike prefabricated panels which are manufactured off-site depending on the project, chopped straw is a thermal insulating material that can be easily transported and implemented in other on-site construction projects, not only in the factory where the structure of the house is being manufactured.
Just like in the case of other constructive systems, the walls must be covered with materials that are permeable to vapors, allowing the circulation of water vapors in both directions. Thereby, we can use dry or plaster finishes, the kinds we mentioned in the paragraphs above. An advantage of loose chopped blown-in straw is that they fill homogeneously the wall in every corner and joint where typically thermal bridges can appear. The process of blowing straw into walls is simple, fast and it doesn’t require a lot of labor (usually two people are enough and can do all the work). Another advantage is the fact that loose chopped straw can easily be fitted not just into walls, but also in other cavities like roofs, lofts, ceilings, or any other horizontal, vertical or sloped building elements.
Based on the research made by Daniel Barbu-Mocanescu, we are using this type of technology and have developed natural thermal insulation - straw processed locally, cleaned, chopped, and blown into the structure at 80-110 kg/m3. It is the optimal insulation material for our modular NIDUS Homes, which complies with the manufacturing process and the energy efficiency requirements, and the prefabricated TRAROM Structure. Another example of western Europe is Iso Stroh, an Austrian company that processes straw with this technology for more than 10 years.
4. SEMI RIGID STRAW INSULATION BOARDS
Semi-rigid straw boards can be applied as an insulation cladding on any type of structure, either masonry, wood, or other surfaces. They can be then finished like any thermally insulating material, with the condition that they are vapor permeable. The boards are made of cleaned straw that are compressed between two layers of fabric, on standard or custom made dimensions and thickness, depending on the applications.
Naturami thermal insulation boards, developed and created by Daniel Barbu-Mocanescu, are a Romanian innovation unique in Europe, authorized for the most performant constructions. The thermal conductivity of the straw boards is 0.037 W/mK at a density of 137 kg/m3, according to the technical data. They are very versatile and can be used in any kind of project. The manufacturing process is automatized and thus human effort is reduced at a minimum. The boards do not contain additives or chemical substances, they are made only of straw processed mechanically.
They can easily substitute other thermally insulating materials under the shape of boards, like polystyrene or glass/rock wool, both by thermal performance, application and cost. But the greatest benefits of Naturami straw boards are for the environment and the occupant's health. The carbon footprint of Naturami is insignificant when compared with other conventional insulation materials and it does not emit volatile organic compounds over time. The boards behave very well on humidity variations and have a prolonged lifespan if they’re implemented correctly in the Thermo system. More than that, at the end of a building’s life, the straw boards can be reused in other industries, like the production of pellets or biofuel, unlike glass wool or polystyrene which are harder to recycle.
Therefore, it is an innovative technology that takes full advantage of the technical characteristics of the straw as performant thermal insulation and is environmentally friendly.
5. COMPRESSED STRAW CONSTRUCTION AND SOUNDPROOFING BOARDS
For these types of products, straw is carefully selected, cleaned, and dried. Afterward, it is heated to a certain temperature and pressure is applied, which will allow the release of lignin from the straw fiber, a natural binder that glues together the straw fibers that form the board, which is then sealed with paper or cardboard. Usually, an additional formaldehyde-free synthetic binder is added to improve the physical characteristics of the boards. For this particular reason, these products are named OSSB (Oriented Split Straw Boards).
OSSB boards can replace regular OSB boards, having similar structural characteristics in both directions. The density of the boards is above 350 kg/m3, thermal conductivity is 0.099 W/mK and they’re vapor permeable. Aside from that, straw boards are easy to handle and secure with screws. Ortech Industries is one of the companies that produces rigid straw plywood.
Another company that produces this kind of straw boards is EkoPanely from The Czech Republic, whose production process is 100% ecological, without other chemical adhesives, by wrapping the straw boards into recycled cardboard. Besides all the proprieties mentioned above, straw boards are a good sound insulator and significantly improves the quality of the indoor environment, for special applications like studios, performance halls, schools, kindergartens, etc.
In conclusion, straw as a construction material is very versatile and can be used in multiple configurations: from load-bearing material in old houses in Nebraska (an outdated solution nowadays) to a very performant thermal insulator shaped into semi-rigid boards or loose filling and up to semi-finished sound insulator suited for any type of building.
Straw-based products are mainly processed through mechanical processes, without additives or other compounds (with little exceptions) and these products and processes have a very low embodied energy and carbon footprint, compared to other conventional materials. In other words, straw-based constructive systems have proven a viable alternative to conventional solutions, taking into consideration the characteristics and advantages of straw as a construction material. We have approached this topic in detail in this article.
This article is part of a series dedicated to straw and its building applications. Don't miss the other articles!