The use of technologically byproduct agricultural wastes in various segments of the brick and tile industry is increasing continuously. The additives, mixed into the raw clay ignite during the firing process, adding extra thermal energy from inside the mixture decreasing the energy requirements of the manufacturing process. Added to this, through the combustion of the bio-wastes the porosity increases enhancing the thermal insulation properties of the final product. We have investigated some common, agricultural wastes to determine their effect on the thermal properties of bricks. In our experiments industry relevant amounts of additives (sawdust, rice-peel, seed-shell) were added to the basic clay composition. We have prepared mixtures with additive concentrations of 0, 4, 7 percentage by weight. The preparations of the samples were (milling, drying and firing) following industrial standard procedures. Precise thermal conductivity data were gathered from all samples using a RAPID-K type static thermal conductivity measuring instrument. Our measurements show that by increasing the amount of the organic byproducts in the clay mixture it is possible to significantly decrease their thermal conductivity, leading to an improved insulation capability of commercial brick products. On the other hand, there was only a minor reduction in the mechanical strength found during previous works. The investigated agricultural byproducts were also ranked based on their effect on the product's thermal properties. It was found that the largest decrease to the thermal conductivity was caused by the sunflower seed-shell additive. Mixing 7 % wt. seed shell to the clay, we can decrease the thermal conductivity of the fired product from 0,27 W/m·K to 0,17 W/m·K (36%). We have found that under the same conditions the sawdust caused the least improvement, only a decrease of 0,27 W/m·K to 0,23 W/m·K (16%) was measured.