This paper reports on the performance of a rice husk ash (RHA) based sand–cement block. Its performance is compared with that of a standard commercial clay brick. The RHA-cement block reduces solar heat gain in buildings and the comparisons include an evaluation of room temperature, solar conduction heat transfer and economics. An appraisal of the two was conducted using two small rooms (floor area of 5.75 m²). One of the rooms was constructed using the RHA based sand–cement block wall; the other, which served as the reference, used a commercial clay brick wall. Experiments were performed throughout a period of one summer month (March) in Thailand. The results showed that the RHA based sand–cement block reduced solar heat transfer by 46 W. An economic analysis indicates that the payback period of the RHA block in tandem with a 1 ton, split-type air conditioner depends on the indoor set-point temperature. The payback period is 4.08 years when the indoor set-point temperature of 26 °C is taken.