Permafrost is any soil or rock maintained at or below 0 degrees C for a minimum of two years. As long as more heat is removed than is added, a permafrost mass will be maintained. AFI's two-phase thermosyphons operate when the condenser is colder than the liquid in the evaporator or when the air is colder than the ground.

Thermal design of a foundation to maintain permafrost must extract the heat from the building or seasonal thaw from above, geothermal heat from below, heat from warmer soils surrounding the site, buried water and sewer lines, runoff from building roofs, and surface drainage. The design must also allow for seasonal thaw without affecting the foundation.