materials Article Heat Transfer in Straw-Based Thermal Insulating Materials Dániel Csanády, Olivér Fenyvesi and Balázs Nagy *   Citation: Csanády, D.; Fenyvesi, O.; Nagy, B. Heat Transfer in Straw-Based Thermal Insulating Materials. Materials 2021, 14, 4408. https://doi.org/10.3390/ma14164408 Academic Editors: Nadezda Stevulova and Adriana Estokova Received: 15 July 2021 Accepted: 4 August 2021 Published: 6 August 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Department of Construction Materials and Technologies, Budapest University of Technology and Economics, 3 M˝ uegyetem Rakpart, 1111 Budapest, Hungary; csanady.daniel@edu.bme.hu (D.C.); fenyvesi.oliver@emk.bme.hu (O.F.) * Correspondence: nagy.balazs@emk.bme.hu Abstract: An analytic-empirical model was developed to describe the heat transfer process in raw straw bulks based on laboratory experiments for calculating the thermal performance of straw-based walls and thermal insulations. During the tests, two different types of straw were investigated. The first was barley, which we used to compose our model and identify the influencing model parameters, and the second was wheat straw, which was used only for validation. Both straws were tested in their raw, natural bulks without any modification except drying. We tested the thermal conductivity of the materials in a bulk density range between 80 and 180 kg/m 3 as well as the stem density, material density, cellulose content, and porosity. The proposed model considers the raw straw stems as natural composites that contain different solids and gas phases that are connected in parallel to each other. We identified and separated the following thermal conductivity factors: solid conduction, gas conduction in stem bulks with conduction factors for pore gas, void gas, and gaps among stems, as well as radiation. These factors are affected by the type of straw and their bulk density. Therefore, we introduced empirical flatness and reverse flatness factors to our model, describing the relationship between heat conduction in stems and voids to bulk density using the geometric parameters of undisturbed and compressed stems. After the validation, our model achieved good agreement with the measured thermal conductivities. As an additional outcome of our research, the optimal bulk densities of two different straw types were found to be similar at 120 kg/m 3 . Keywords: natural fibers; raw barley and wheat straw; heat transfer; thermal conductivity; porosity; physical properties 1. Introduction The amount of built-in thermal insulations is continuously growing these days [1] due to thermal insulations usually applied to newly built and refurbished buildings because of the strict energy performance regulations that have come into force in recent years to decrease carbon emissions and energy use. In 2019, households represented 26% of the final energy consumption of the EU, of which 64% is from space heating [2]. The energy demand of buildings can be reduced by thermally insulating the houses. Nowadays, even the previously insulated buildings should receive additional thermal protection to meet regulations or repair previous building construction mistakes that cause defects and mechanical, chemical, environmental, or hygrothermal deteriorations, or accelerate natural aging [3–6]. The manufacturing energy consumption and carbon emission of materials also need to be addressed. Artificial materials usually have higher manufacturing energy consump- tion and higher carbon emission than natural-based thermal insulations. With the carbon sequestration considered in the calculations, the embodied carbon content of a building construction insulated by natural thermal insulations can achieve negative carbon emis- sions compared to artificial insulations such as mineral wool, expanded polystyrene, or polyurethane foam. This is possible because cellulose-based insulations ′ embodied carbon contents are lower than those of artificial insulations [7] due to their composition or lower Materials 2021, 14, 4408. https://doi.org/10.3390/ma14164408 https://www.mdpi.com/journal/materials