Fire testing of straw constructions to enable more Biogenic Building

As part of a so-called MUDP project, DBI has conducted three scaled-down facade tests of different straw constructions types. Consequently, adjustments will now be made to one or more of these constructions before further testing is carried out. The goal is to make it feasible to use biogenic materials in multi-story modular construction.

In an era of increasing focus on climate-friendly construction, straw and hay are intriguing building materials. However, their flammability poses a challenge. This became evident during the initial scaled-down facade tests conducted by DBI (Danish Institute of Fire and Security Technology) for the project ’The Biogenic Industrialised Construction – Construction, Fire and Architecture’, although the tests did produce promising results.

The project, in collaboration with CINARK at the Royal Danish Academy and other partners, aims to develop and fire-test prefabricated biogenic material constructions scaled for modular multi-story buildings. The project builds on the findings of the previous project ‘Thatched Facades for the Green Transition’, which demonstrated that it is possible to use clay as fire impregnation on thatched facades.

The latest project includes a literature review on biogenic materials such as straw, hay, hemp, seagrass, reeds, and grass, which has provided insights into their fire properties and market availability.

Three different constructions

The project focuses on entire external wall constructions, which are fire-tested in different combinations of wood, thatched facade solutions, and clay as a fire retardant. Modular elements, sized for easy handling on a construction site, are pre-thatched before installation, to avoid thatching the entire building shell onsite. Special attention is given to the joints between elements, as they significantly influence fire behavior.

“We also want to test whether it is possible to build constructions that can function without clay as a fire retardant. This is primarily because we are uncertain about the durability of clay over the building’s lifespan when exposed to rain, sun, frost, and other elements,” says Karlis Livkiss, Research Consultant at DBI, Ph.D., and responsible for the literature review and analysis of the test results in the project.

The first construction was tested without clay impregnation. Instead, it featured tightly packed thatching to partially expel air from the straw tubes, limiting fire spread. Seagrass was used to fill cavities at the back of the construction. The test showed flame spread in the first few minutes but quickly transitioned to smoldering fires inside the construction. Full combustion of the upper section only occurred after nearly 50 minutes.

The second design incorporated a clay board with hemp fibers, using shorter straw tubes glued to the clay, which is a technique from an old German thatching tradition. In this test, too, fire spread across the surface in the first few minutes but self-extinguished rapidly.

The third construction followed traditional Danish thatching techniques but was supplemented with moraine clay applied between the straws (tested in the earlier MUDP project). Furthermore, seagrass was added at the back. This design showed no flame spread initially but gradually developed smoldering activity that evolved into flames later in the test. The test was stopped after 60 minutes.

– In the second construction, the thatch protects the clay from rain, while in the third construction, the clay protects the straw from fire, explains Mia Fossing Frederiksen, Project Lead for DBI’s part of the project.

Further fire tests

The next step will focus on optimizing the construction(s) that performed best during testing, considering factors like buildability and durability against rain and moisture. Additional scaled-down facade tests will then be conducted before a full-scale test of the most promising construction.

“The tests were a success because we gained valuable insights into how straw burns, which allows us to identify the key challenges. However, the tested materials are inherently flammable, introducing risks such as smoke and smoldering fires. It remains uncertain whether we’ll achieve a viable solution,” says Karlis Livkiss.

“Our hope is to develop a construction that can serve as a basis for fire safety engineers to make decisions. The ideal scenario would be to enable the use of thatched facades in multi-story modular construction, allowing certified fire safety engineers to rely on the findings from our project,” adds Mia Fossing Frederiksen.

Read also: Building structures with more biogenic materials

Biogenic Industrialised Construction – Construction, Fire and Architecture

The MUDP project ‘The Biogenic Industrialised Construction – Construction, Fire and  Architecture’ is being carried out in collaboration between DBI, CINARK at the Royal Danish Academy, Tækkefirmaet Horneby (thatching company), Hemmed Tækkefirma (thatching company), EcoCocon Danmark ApS and Enemærke & Petersen. The follow-up group includes representatives from the Danish Environmental Protection Agency, FSB, Rønnow, Leth & Gori and Kasper Køppen Ingeniører.

The project is supported by the Environmental Technology Development and Demonstration Programme (MUDP) under the Ministry of the Environment, which grants funding for the development of new environmental technology for the benefit of the climate and the environment. The project’s total budget is approx. DKK 6 million and runs from January 2024 to December 2025.

Testing methods

The earlier project ‘Thatched Facades for the Green Transition’ involved 14 mini-SBI tests (Single Burning Item) to identify promising fire retardant methods. These tests were followed by scaled facade tests using moraine clay, which proved to be efficient as impregnation. Read more here

The current project ‘The Biogenic Industrialised Construction – Construction, Fire and Architecture’ begins with further scaled facade tests and concludes with a full-scale test of a modular thatched facade construction.