Load-bearing strawbales

Our research project Geen strobreed in de weg! Ontwerpen met dragende strobouw (Eng.: Not the final straw! Designing with load-bearing strawbale construction.) – funded by the Creative Industries Fund – was initially inspired through our membership of the Dutch Strawbale Association (Strobouw NL) and multiple, independent queries we received regarding the possibility to build a house in the Netherlands with load-bearing use of strawbales.

Strawbale

Building with straw has a number of advantages. Straw bales have excellent physical properties. Straw is a good insulating material and is permeable so it can regulate its moisture (hygrothermic behaviour). Straw bales are very economical and available in large quantities all over Europe, as a by-product of agriculture. Straw bales have excellent environmental properties, are renewable, plant-based and biodegradable. The composite wall is fire resistant without the need for chemical. The primary production of straw requires almost no energy (1 kgCO2/m³), and the material stores carbon (-135 kgCO2/m³).

In the Netherlands, an estimated 150 buildings and structures have already been realized with straw construction. These are usually timber-framed structures, in which straw bales are used as insulation in the walls, floors and/or roofs, often finished with lime or clay plaster. However, it is also possible to use straw structurally, where the bales along with the outer coats of plaster act as a composite sandwich. The construction method,  also known as ‘Nebraska’ style, has been popular in recent years, but dates back to over a century, with examples still standing, such as the Pilgrim Holiness Church, built in 1928 in Arthur, Nebraska, United States.

Hybrid methods, with timber posts, but where the straw is still providing for stability, also exist, such as CST (Cellule Sous Tension) and GREB (Groupe de Recherches Écologiques de la Batture). A few examples of load-bearing straw construction are known in the Netherlands, but these are temporary, small-scale and/or permit-free buildings, such as the Houtje Touwtje Strogebouwtje, built in 2016 in Breda.

Unfortunately, the wider uptake of straw-bale building is slowed down by limited technical understanding of its structural performance and other metrics, the lack of building codes and standards, and the absence of certification and warranty for straw products.

Literature study

In order to introduce load-bearing use of straw bales to Dutch mainstream construction, standard and guidelines, based on design rules and calculation models, are needed. To pave the way, available knowledge and relevant studies on load-bearing use of straw bales was collected. This survey focused on structural properties, test results and any numerical modelling, and includes an inventory of the experimental work and their testing set-up. An initial outcome of the research was presented at the European Strawbale Gathering (ESBG) 2021 in Limburg, organised by Strobouw NL. A draft version of the resulting state-of-the-art report is being submitted for publication in a peer-review journal.

Structural properties, from the literature review, are also entered as preliminary assumptions, indicating the mechanical behavior. The model gives an indication of the limits of load-bearing straw construction, for further design exploration.

Testing plan

To allow for permanent, load-bearing strawbale construction in the Netherlands, it is necessary to submit structural calculations for a building permit, in order to comply with the Dutch Building Decree (Bouwbesluit) and Eurocodes. Eurocode 0 allows for ‘design assisted by testing’, requiring a testing program to be defined. Based on the earlier literature review, a pilot program – consisting of testing a straw bale with and without plaster, the plaster itself, and an entire wall – was submitted to testing institutes, including SHR and the Stevin Laboratory at TU Delft, for feedback and budget estimation.

Contribute?

The outcome of this project was a literature study, parametric design model and testing plan. In order to carry out the testing plan, Strobouw NL requires further support in carrying out the tests and calculations, as well as acquisition of funding through relevant grants. Delft University of Technology (TU Delft) has expressed interest in this work through a series of student theses, followed by further research, if possible.

Once the properties of load-bearing straw construction become known, then informed design can take place, and the artistic potential of this building material can be more fully explored.