The effect of oven wall chamber insulation on the Nusselt number and heat loss reduction in a specially fabricated direct-fired natural convection oven was studied. Fiber glass and aluminum-reinforced fiber were used as wall chamber insulation materials. Three sets of experiments, namely uninsulated and insulation with each of the two insulation materials were performed and a data acquisition system was used to continuously monitor temperature and environmental changes from the oven’s four sides, ambient, and inside using a heat temperature sensor (REX-C700-FK07-M-AN) and thermocouple (OM-HL-EH-TC). The temperature profiles, the Nusselt number, the Rayleigh number and the heat transfer coefficient were used to assess the heat transfer characteristics of the oven to ascertain the heat loss reduction potential of the insulation materials. Within the range of the Raleigh numbers studied, the average Nusselt number is highest for the uninsulated and higher for the insulated with aluminum-reinforced fiber and least for the insulated with fiberglass. The average percentage reduction in heat loss as a result of insulation was 3.8% and 16.7% for the fiber reinforced aluminum and fiberglass, respectively. Based on the findings, which are consistent with earlier related research, installing thermal insulation materials in oven walls could reduce heat loss, appreciably. The least-square correlation of the Nusselt numbers and the Rayleigh numbers resulted in high values of adjusted R-square values, which could be used for the optimization of natural convective heat transfer for uninsulated and insulated ovens.