BUILDING WITH STRAW #2: CHARACTERISTICS AND ADVANTAGES OF STRAW
PHYSICAL PROPERTIES OF STRAW, AS A VIABLE BUILDING MATERIAL FOR ECO-FRIENDLY BUILDINGS
In the past, straw was used in buildings because it was an available resource, and the construction technique was simple and cheap. However, in the past few years, due to investments into research and development, the technique of construction with straw has evolved so much and therefore new technologies have been developed that emphasize the value of the physical and thermal properties of straw, in order to build resistant, durable and energy-efficient buildings. Therefore straw has the next advantages when it comes to building applications:
1. THERMAL INSULATION
Straw, through their composition, is a good thermal insulator, if they’re compacted into the construction elements of the building, and have a long lifespan if kept under good humidity conditions - below 25% (here it is worth to recall the 100 years buildings from the US and France mentioned in the previous article).
The thermal conductivity of straw can reach great values and varies according to multiple factors, like the constructive system (the way they are laid out in the structure), density, humidity, fiber orientation (parallel or perpendicular to the heat flux). According to this experimental study on different samples of straws, thermal conductivity (λ) varies between 0.38-0.92 W/mK. The difference is justifiable mostly by the straw density and the fiber orientation regarding the heat flux, parallel or perpendicular to the wall.
2. SOUND INSULATION
Straw is a good acoustic insulator and its performance can vary, depending on its density and the level of uniformity of the construction elements (ex. Straw must fill completely a wall, gaps must be avoided). As a result of an experimental analysis conducted by Eindhoven University of Technology according to ISO 140-3, that was conducted on a wall made out of straw bales, plastered on both sides with clay and lime, results are showing that the level of sound insulation when using straws is 55 dB, a value that is very close to human hearing sensitivity. Although it is low, the value is really good, in comparison to a regular wall of masonry with a high mass that records weaker results.
To those of you who have been in a straw bale house, you can observe that the sounds from the inside tend to be louder because they interfere very little with the background noise from the outside. When we are talking about the acoustic performance of a building, we must look at it as a whole: besides the construction elements (insulated walls, rooftop, efficient windows) we must take into consideration also the joints between the construction elements (ex. windows and wall) and air infiltrations, these definitely have an impact on the acoustics of a building. Hence, every execution detail is important.
The first straw bale houses were built around the mid-1800s in Nebraska, USA, and a big part of them have been refurbished and are visited by tourists in the present. You can discover more about the history of straws in this article. Restoration work mainly consists of replacing the finishes and the rooftop, the straw being in a good condition, despite the high levels of humidity in the area. Straw is a natural fiber that is rich in silica and can last for hundreds of years in favorable conditions of humidity, oxygen, and heat, conditions that can be guaranteed easily through good design, execution and maintenance of the building. Therefore, straw has successfully passed the test of time.
4. VAPOUR PERMEABILITY AND NON TOXICITY
Humidity and uncontrolled circulation of water vapors through construction elements are the main problems that affect the integrity of materials and the durability of a building. Straw is a permeable material when it comes to vapor and water in a liquid form, thus it doesn't need special protection such as vapor barriers. When it comes to straw-bale constructions, it is recommended that the interior and exterior finishes have in their composition materials that are vapor permeable, the most frequent being natural plasters based on clay and/or limestone.
Therefore, at high variations of humidity, the straw walls “breath” and support the circulation of water vapors, operating as a “buffer”. Besides all that, straw is a natural material and does not contain chemical compounds or other substances that in the long term could harm the health of occupants.
5. AVAILABILITY AND LOW COST
Straw is a byproduct in agriculture, and as a raw material for constructions is available in abundance, considering the fact that straw can come from various crops: wheat, barley, oat, rice, and so on. On average 2 million hectares of wheat are being cultivated annually in Romania, listing it as top exporters of the European Union. For each hectare, between 3-5 tons of wheat grains are harvested and 4 tons of straw produces, without counting other cereal crops mentioned above. Straw can be purchased as bales of different sizes and shapes, with a cost of approx. 40-50 EUR/ton.
6. LOW IMPACT OVER THE ENVIRONMENT
It is measured through the embodied energy in construction material, representing the total energy consumed for extracting the raw materials, manufacturing, and implementing that building material. A complimentary size that measures the impact of a material/process over the environment is the carbon footprint, which measures CO2 emissions corresponding to the energy embodied in the building material. Straw requires a very low quantity of energy to be harvested, transformed into bales, transported and/or processed in different shapes for constructions: bales, chopped and blown-in, panels, etc. and implemented.
As a comparison, a straw bale has an embodied energy of 0.24 MJ/kg, glass wool has 30.3 MJ/kg, and expanded polystyrene 117 MJ/kg. This means that straw has the embodied energy 126 times lower than glass wool and 487 lower than expanded polystyrene, reported to kg of material.
About the impact of buildings on the environment, we’ve talked about more extensively in this article.
Therefore, straw turns out to be very viable construction material, and not only for residential buildings. In Great Britain, a series of public buildings are built with straw, confirmation of the fact that straw is a very good alternative for durable and energy-efficient buildings. The only inconvenience of straw is that in order to be implemented in a typical construction as bales, they require a lot of extensive work on the construction site and they generate a lot of dust. But thanks to the available technology, many new construction systems have been developed, in which straw is used very effectively, clean and without great effort. Find out more in the next article.
This article is part of a series dedicated to straw and its building applications. Don't miss the other articles!