Installation of stone wool as thermal insulation in the roof assembly can be adopted to store heat in the living space, if the building is exposed to cold weather, and, inversely, to retard heat from entering the living space, if it is exposed to hot weather. In spite of the effectiveness of stone wool as a roof insulation material, during installation, it can cause irritation to the skin and can be hazardous to the lungs. Therefore, incorporation of stone wool with other materials to form a rigid board, without compromising its effectiveness as a roof insulation material, is imperative. Strength properties of a stone-wool-fibre-reinforced high-density polyethylene (HDPE) composite roof insulation material were studied. Granular silica aerogel, which possesses an ultra-low thermal conductivity, was added as filler to reduce the thermal conductivity of the composite. Hot compression moulding was performed to prepare samples of the composite with varying silica aerogel content of 0, 1, 2, 3, 4, and 5 wt. . Findings suggest that 2 wt. is the optimum silica aerogel content as it resulted in the highest flexural strength and modulus, which is 24.4 MPa and 845.85 MPa, respectively, even though it reduced the tensile strength and modulus by 10 and 4.45 respectively, relative to 0 wt. , which can be considered as inconsequential. Higher silica aerogel content above 2 wt. may result in poor interfacial adhesion and low compatibility to the stone wool fibre and HDPE, which further reduces the tensile and flexural strengths and moduli of the composite. © Published under licence by IOP Publishing Ltd.