The purpose of this study is to reveal the situation of heat insulation in Japan in the 1930s, when the idea of insulation was scarcely known, and to describe innovative insulation methods proposed by certain architects who pioneered this concept during that decade. Furthermore, we examined the reasons why heat insulation did not become popular in those days.<br> The heat-insulation methods used in Japanese houses in the 1930s can be classified into two types: wet-type and dry-type construction methods. In most applications of the conventional wet-type construction method, Celotex was used as a heat insulator and was applied only to the roof or/and the ceiling. This implies that heat insulators were used for the purpose of heat shielding.<br> Amongst the dry-type construction methods, a new method called “Trockenbau,” which was mainly influenced by the methodology of Bauhaus, gained significant attention. Ken Ichiura, Kameki Tsuchiura, Chikatada Kurata, and others, who were the main innovators of Trockenbau, introduced the concept and materials of heat insulation to the architecture field through their designs and research activities. They also played an important role in promoting further research on heat insulation. In houses constructed using the Trockenbau method, Celotex was applied to the ceilings and walls as a finishing material and rice husks were used to fill the hollow sections of the walls in some cases. However, their interest in heat insulation remained confined to Trockenbau and did not extend to the idea that the introduction of heat insulation would improve the quality of life in Japan.<br> On the other hand, some architects attempted to employ their own distinctive methods of heat insulation. Mamoru Nakamura and others worked on the wet-type construction method, whereas Antonin Raymond, Yoshiro Taniguchi, and others worked on the dry-type construction method.<br> Mamoru Nakamura applied double heat insulation. Rice husks were filled into the spaces between structural members, making an “entire house built within boxes of rice husks;” the thermal performance was enhanced by the interior insulator. Nakamura believed that houses should offer usability, security, thermal comfort, and beauty and that comprehensively designed houses enrich the quality of life.<br> Antonin Raymond used Celotex not as a finishing material but rather between studs placed inside wall bodies, thereby forming hollow layers partitioned by Celotex to further enhance the heat-insulation performance. Raymond recognized the importance of heat insulation as well as the importance of facilities in modern architecture. Yoshiro Taniguchi inserted tin plates in addition to Celotex inside wall bodies. His attempts to improve the thermal environment of houses were much more scientific than TrokenBau. The statements given by Taniguchi provide deep insights into heat insulation and reveal that he envisioned the advantages of heat insulation, which is a standard notion today.<br> Despite the endeavours of these individuals, heat insulation was not widely understood at the time. This may have been because material development and technology improvement were hindered by the influence of WWII, and the Japanese were hesitant to adopt heat insulation owing to Japan's indigenous customs and thoughts.