This paper presents indoor temperatures measured during the summer season in about 40 Swedish houses spread across the country. The houses were built around 2010, which means that they have been subject to modern requirements for low energy use. In addition to indoor temperatures, ventilation air flows were measured, and the type of ventilation system was documented. The residents have answered a questionnaire regarding their experience of the indoor climate, and regarding actions they have taken to mitigate overheating, for example how and how often they open windows. The indoor temperatures in summertime are correlated to the building’s geographical location, various building parameters such as the floor area and type of ventilation system, and the residents’ perception of the indoor climate. The measurements were carried out during the summer of 2018, which was an unusually warm summer in Sweden and comparable to summers according to different future climate scenarios. The results can therefore provide valuable information on what indoor temperature conditions can be expected in the future. Simulations were used to study the cooling need and how different parameters, and passive cooling strategies influence the cooling need. Examples such as increased ventilation, solar shading and customised architecture of the resulting indoor temperatures without active cooling is analysed.