WHO 2018, p. 34:
4 Low indoor temperatures and insulation / 4.1 Guideline recommendations / ... For countries with temperate or colder climates, 18 °C has been proposed as a safe and ....
WHO 2018, p. 54:
5 High indoor temperatures / 5.4 Research recommendations / Table 5.2 Research recommendations: high indoor temp / Current state of the evidence / Few high-quality studies have assessed the direct effects of indoor temperature on health..