For non-loadbearing applications, the 15 cm blocks are usually used. They are lightweight, easy to install, and are cost-effective. The fire-resistance rating of the 15 cm block is about 1 h, while that of a 20 cm block is about 2 h. This reduced fire-resistance rating is due to smaller cells which leads to more convective and radiative heat transfer inside of the block cells, as well as the reduced face cells that cause the blocks to heat up quicker. With the use of lightweight insulation materials as cell fillers, an improvement in the fire-resistance rating was achieved for the 15 cm block. These materials were able to reduce the convective and radiative heat transfer in the cells. For the experimental and numerical analysis carried out, vermiculite, and gypsum were able to improve the fire ratings by at least 1h.