Editorial: Exploring hempcrete's potential: mechanical properties and sustainable building practices

This Research Topic was developed to provide an opportunity for authors to publish their study related to hempcrete in a focused collection. The fact that the built environment uses traditional building materials such as steel, concrete, iron, and aluminum, and thus produces excessive amounts of CO₂, has led researchers to seek ways to reduce the carbon footprint of construction materials, which necessitates the exploration of low-carbon alternatives. As a promising bio-based material, hempcrete is being recognized as a desirable lightweight construction material, which is currently mainly used as a crop-based insulation material for building construction. Made of hemp hurd, lime, and water, the mixture solidifies into walls, but these walls still require structural framing to carry gravity and lateral loads. Given the general interest by designers and builders to learn more about this product, this Research Topic aimed to explore developments in the field that will contribute to a better understanding of the physical, mechanical, and building science properties of hempcrete, as well as desirable attributes that help designers and builders to have more confidence in its use as a sustainable construction material.

Four papers have been published in this Research Topic, and the discussion below summarizes their key points. The first paper discusses teaching undergraduate students about hemp-lime, as students in the Sustainable Biomaterials department at Virginia Tech have been interested in learning about this product. The second paper also relates to learning about hempcrete, as students at Kansas State University designed a Zero Carbon Microhome to demonstrate how affordable homes can be built using hempcrete while achieving net-zero carbon over their lifespan. The third paper contributes to a better understanding of how a constructed hempcrete home performs in terms of thermal resistance, indoor air quality, and energy efficiency. Finally, the fourth paper investigates how the thermal resistance of hempcrete can be improved. This small collection of papers presents a succinct state-of-the-art status of hempcrete for home building.

The paper titled “Hemp-lime as a tool for undergraduate research and teaching experiences” (Hindman and Hammett) shares the experience of teaching a course about hemp-based products and mycelium, as there is growing interest among students in learning more about these biomass products. In particular, while there is plenty of literature related to properties and testing of hemp and hempcrete, there is very little discussion on teaching a formal course on this material and its application in construction. Accordingly, this paper shares the details of the course, including how a combination of lectures, laboratory testing, and interaction with industry professionals can bring a well-rounded understanding to students. In particular, the paper details the experiential learning approach that helps students gain a much better understanding than a lecture-based course alone. The paper explains the effectiveness of the way the course was developed as a combined lecture and project-centered learning approach to cover an extensive literature review on growing hemp, production of hempcrete (or “hemp-lime”), development of material/mechanical properties for hemp hurd, fibers, and hempcrete, construction with hempcrete, and advantages and disadvantages, including LCA, among others. It is of interest to note that two of the students' projects for the course related to how to create hempcrete blocks that would be load-bearing and what would be the mechanical and thermal effects of including Portland cement as an additive in the mixture besides the main binder, lime. The paper concludes that the way this course was developed and offered was a success story, which can be modeled by others who wish to teach such a course.

Next, the paper titled: “Integrating hemp in a replicable model for affordable, low carbon housing: a zero carbon microhome prototype for the Kansas Flint Hills” (Gibson) discusses the design of a microhome using hempcrete for the floor and hemp fibers as batt insulation for walls to demonstrate the use of hemp to build a net zero energy and net zero carbon home that is also affordable. The project discussed in the paper centers around the Net Positive Studio at Kansas State University, where architecture students design and build affordable, net-zero-energy homes as part of their Master of Architecture program. The paper discusses the Zero Carbon Microhome (ZCMH), which is intended to be replicable for practical home construction within a specific budget range. The paper presents how emissions from hempcrete and hemp fiber insulation can be considered in carbon assessment toward a net-zero carbon home. The study presented follows the basic steps in the program, including energy modeling, LCA analysis, and the use of photovoltaics as a local energy source to ensure a net-zero energy home. The paper describes variations in the mixture design by using Portland cement and hydrated lime at different ratios to determine the design with more desirable thermal conductivity. The design included prefabricating the raised floor (to avoid using a concrete slab-on-grade and reduce the carbon footprint) and using hempcrete for insulation, while using only hemp fiber for the walls and roof. Through energy modeling, the study compares the performance of a hempcrete- and hemp-fiber home with that of a home using conventional foam insulation. The paper also describes the energy modeling and analysis that uses the thermal resistance properties of the hempcrete in establishing the net-zero energy outcome, considering the reduced energy use due to the high thermal insulation provided by hempcrete and hemp fiber batt and the use of a PV array. The authors conclude that the project provides evidence that the use of hempcrete and hemp fiber, integrated with other sustainability design options, can lead to affordable, net-zero-energy homes.

While hempcrete's thermal properties have been documented through material testing, there has been a lack of such information from thermal measurements of hempcrete walls built as part of hempcrete homes. The paper titled “In-situ measurement of residential buildings with hempcrete walls: a case study” (Memari et al.) addresses these knowledge gaps by presenting the results of in situ measurements of the thermal resistance of hempcrete walls in a retrofitted house. The paper describes in detail the instrumentation used, following the heat flow meter method, to determine the R-value for the hempcrete walls of the house. For the total wall thickness of 11.75 in., consisting of a 34 in. hemp-rich plaster, 10-in. thick hempcrete, 12 in. air space, and 12 in. thick rough-sawn hemlock board siding, the measured R-value is reported as 17 or approximately R-1.45/in. This is about 10 times the thermal resistance of concrete with the same thickness. The paper concludes that although this R-value is lower than that of many other insulation materials, the advantages of hempcrete should still be considered. These include low embodied carbon, no use of synthetic materials, thermal mass from the relatively thick hempcrete walls, and energy efficiency due to the airtightness of the whole house.

Finally, the paper titled “Development of a novel hemp-based building material with enhanced thermal performance” (Babaahmadi and Tariku) presents the results of a research project aimed at increasing the thermal resistance property of hempcrete for situations where higher thermal resistance, comparable to conventional foam insulation, is needed. This may be the case when the wall thickness cannot exceed a certain limit or for high-performance building applications; in such cases, an enhanced version of hempcrete would be required. The paper explores the use of alternative mineral binders and aerogel beads, with the latter known for its very low thermal conductivity (e.g., 012 to 0.02 W/m.K) compared to hempcrete (e.g., 0.05 to 0.14 W/m.K). The paper evaluates the use of magnesium oxychloride and magnesium phosphate, in addition to conventional lime, as binders. The paper reports results of varying the binder-to-insulation ratio and varying the percentage of aerogel as the hemp substitute. The result of the study shows that the combined effect of both these binder materials and the incorporation of aerogel granules leads to a significant reduction in thermal conductivity. The paper explains the mechanism by which these materials influence thermal conductivity. For example, as the binder content increases, the mixture turns more toward a solid matrix, making the hempcrete denser and thus increasing its thermal conductivity. On the other hand, adding aerogel granules at increasing rates, e.g., substituting hemp by 20% to 80%, increases the pore percentage, leading to lower thermal conductivity. More specifically, the results show that mixtures with a higher binder-to-insulation ratio led to an increase in thermal conductivity, while replacing 80% of hemp with aerogel resulted in a significantly lower thermal conductivity on the order of 0.02 W/m.K, which is close to that of foam insulation. The results of the study also show that there is good bond formation between magnesium-based binders and hemp hurd (or shives), which helps reduce the amount of binders needed. The study also concludes that the use of hemp fibers improves the behavior of the composite, as the fibers work as a reinforcing network.

Overall, the papers published in this Research Topic have addressed a formal university course on aspects of hempcrete; the design and construction of homes using hempcrete and hemp fiber; in situ testing of a built hempcrete house for thermal resistance; and the advancement of the state of the art in increasing the thermal resistance of hempcrete. Aside from the original contributions presented in these papers, the literature review sections provide a wide spectrum of the essentials of engineering and construction aspects of hempcrete, particularly published studies on material properties, mechanical properties, moisture- and thermal-related properties, and design and construction aspects. Using hemp fibers and hempcrete for construction has only recently begun to attract attention, and there is still a long way to go before the material, and its use for construction, become ubiquitous. This collection can serve as a succinct introduction to hempcrete for construction.

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Footnotes

Edited and reviewed by: Claudio Ferone, University of Naples Parthenope, Italy