Building envelope in houses is accountable for the enormous heat gain. This numerical study focuses on the influence of lightweight aggregate in the concrete for the heat gain in the building due to the sun’s radiation. Four lightweight aggregates, such as gravel, stalite, lytag, leca and argex were selected for inclusion in concrete and silica fume where it was added to the mortar. Transient thermal characteristics subjected to regular thermal excitation were solved using the admittance method through a computer simulation program. From the result, it is observed that argex concrete wall showed a better thermal performance with a higher time lag (6.218 h) and a lower decrement factor (0.5051). Mortar with silica fume showed a better thermal performance than a mortar with a higher time lag (0.60 h) and a lower decrement factor (0.991). © Springer Nature Singapore Pte Ltd 2021.