In the northern hemisphere, the winter sun appears low on the southern horizon, providing heat and light directly on south facing walls. During the summer, the sun appears higher in the southern sky while the walls are shaded by the roof overhang or other structures. The placement of windows and doors on any south facing wall allows the sun's radiant energy to heat the interior. Well planned walls and floors hold the heat collected during the day and slowly release that heat during the night as the outside air temperature drops. The result, much less energy is needed to keep the building warm. This capacity is called thermal mass.

Thermal mass materials like stone, concrete, and adobe slowly absorb heat through the day, but as the temperatures drop in the evening the heat is slowly released, warming the surrounding air. To experience the thermal mass firsthand, walk barefoot on a concrete sidewalk after the sun has gone down. The sidewalk will feel warmer than the surrounding air. During the summer the sidewalk may still be "too hot" for barefeet until long after sundown. The next morning the sidewalk will be cooler than the current air temperature and will heat slowly as the sun rises. The Desert House utilizes this thermal mass principle.

Other examples of building using thermal mass principles include Montezuma's Castle in central Arizona and the ancient cities of Greece and Rome.