Well-insulated structures are like tea cosies. How quickly the tea cools will depend on how big the pot is, how thick the tea cosy is and how cold the air is outside it. The effectiveness of the insulated envelope depends on a number of variables, not least the area of the envelope in relation to the heating requirements of the occupants in it and the available internal heat sources.
For example, take the tent dwellers of the Mongolian steppes of Siberia to the deserts of Saudi Arabia. The Turkoman yurt (Figure 1.6) is a tent built on a framework of bent wood, with a felt cloth tent, in which ten or more people can live through the freezing winter months. Heavy clothes are worn, even indoors, and a single central brazier provides heat. The yurt is an airtight structure into which there is very little infiltration of cold air from the driving wind. The area of the whole floor is only in the region of 15-20 m2 so the body heat of the people in the tent makes up a significant amount of the heat they need to warm the family.
A Yurt of the Turkoman tribe of Iran (Andrews, 1997, drawn by Susan Parker).
You would probably not survive a severe Mongolian winter in a 300 m2 gher with one small brazier fire and ten people. Insulated envelope buildings need constant heating, from a heat source or from other internal gains such as machines and body heat. The yurt works well because its occupants go to bed very early to conserve heat and light, and sleep at night under thick quilts, often with more than one to a bed, saving considerably on heating. Considerations of the thermal performance of building envelopes are covered in Chapter 2.
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