This article is focused on the investigation of the mechanical and thermal properties of composite material that could be used for the production of plaster or plasterboards. This composite material is made of gypsum and reinforcing natural fibers. The article verifies whether this natural reinforcement can improve the investigated properties compared to conventional plasters and gypsum plasterboards made of pure gypsum. From this composite material, high-strength plasterboards could then be produced, which meet the higher demands of users than conventional gypsum plasterboards. For their production, natural waste materials would be used efficiently. As part of the development of new building materials, it is necessary to specify essential characteristics for their later use in civil engineering. Crushed wheat straw and three gypsum classes with strengths G2 (2 MPa)—gypsum class I., G5 (5 MPa)—gypsum class II. and G16 (16 MPa)—gypsum class III. were used to create the test samples. Samples were made with different ratios of the two ingredients, with the percentages of straw being 0%, 2.5%, and 5% for each gypsum grade. The first part of the article describes how the increasing proportion of straw affects the composite’s mechanical properties (flexural strength and compressive strength). The second part of the article focuses on the change of thermal properties (thermal conductivity and specific heat capacity). The last part of the article mentions the verification of the fire properties (single-flame source fire test and gross heat of combustion) of this composite material. The research has shown that the increasing proportion of straw reinforcement caused a deterioration in the flexural strength (up to 56.49% in the 3. series of gypsum class II.) and compressive strength (up to 80.27% in the 3. series of gypsum class III.) and an improvement in the specific heat capacity and thermal conductivity (up to 31.40% in the 3. series). This composite material is thus not suitable for the production of high-strength plasterboards, but its reduced mechanical properties do not prevent its use for interior plasters. Based on the performed fire tests, it can be said that this composite material can be classified as a non-flammable material of reaction to fire Classes A1 or A2. From an ecological point of view, it is advantageous to use a composite material with a higher straw content.

The external walls of buildings account for a substantial part of the financial costs of the entire construction, and there can be a loss of up to 35% of thermal energy through them. By properly optimizing the price for the construction of m² of the external wall structure and its thermal technical parameters, interesting savings can be achieved. At present, there is no multi-criteria analysis for designing external wall structure compositions involving broader input options according to the user’s technical parameters and priorities. There is a large selection of special software in the Czech and European markets, but the software is focused only on the narrower area of design and ignores the issue of building material prices. The aim of this work is to create an algorithm that reliably finds the composition that best meets the user’s requirements using a wide database of materials and selected mathematical methods. This article presents an algorithm that would design the ideal composition of an external wall. This algorithm has two options for searching. The first is based on eight technical criteria and the prices of materials used in combination with user priorities. The second option is to find the best composition based only on the specified interval of the selected technical parameters. Materials databases and the use of existing computational methods, such as the Saaty method and the WSM—weighted sum method, applied to the algorithm are essential to find the composition. According to the assignment, the structures will be clearly quantified in values from 1 (best) to 0 (worst). The algorithm, which is based on the analysis of data, sources, and theories of multi-criteria decision-making, should, therefore, facilitate the design of the external wall. At the end of this article, there is a verification of the functionality of the algorithm on a case study. We believe that software that uses the proposed algorithm could be very useful for practice.