Hokkaido is distinguished from the other island of Japan by having colder winters and cooler summers. The average monthly minimum is about -10 °C and summer daily average maximum around 25°C, so that efficient space heating becomes the dominant aspect But on hot days the temperature may reach 35°C, though for short period of time. There are few houses constructed to prevent overheating ID summer. I have designed and built a house for my family ID Sapporo which has cavity walls of concrete blocks insulated with urethane 100mm thick and deeply recessed, but operable widows. I detected the thermal performance of my house to solve overheating problems at design stage.