In the psychrometric chart, if environmental conditions are to the left of the comfort zone, then the body loses more heat than it gains, and heat must be provided to achieve thermal comfort. Passive solar heating provides this heat to the body by warming the space around it. In general, conditions to the left of the comfort zone can bene›t from passive solar heating if there is enough solar radiation to provide the necessary energy and if the building has the appropriate design features to capture energy, thermal mass to store it, and insulation to keep the heat inside. When outdoor conditions are below but still relatively close to the comfort zone, thermal mass or internal gains are suf›cient to achieve thermal comfort, and requirements are more lenient. But as the outdoor temperatures become lower, the building must be designed with more stringent requirements (e.g., better insulation, tighter and better windows, additional thermal mass). It is generally assumed that the lowest outdoor temperatures with which we can provide suf›cient heating to a building, using only passive solar strategies, is around 5°C (Figure 8.1). However, there are examples such as the Rocky Mountain Institute in which the building uses special glazing, super insulation, and large amounts of thermal mass to achieve thermal comfort with outdoor temperatures well below 5°C.