Passive cooling by means of pressured heat exchange pipes was developed in Alaska by the U.S. Army Corps of Engineers in 1965 to preserve foundations in ‘warm’ permafrost. It has been widely applied in Alaska, northern Canada and Russia to preserve and cool permafrost. The original simple 20mm vertical pipe with a radiator in the air has evolved into other designs that include: thermopiles, sloped pipe thermosyphons and flat loop evaporator pipe (flat loop) thermosyphons. The flat loop thermosyphon was developed and field tested in Canada in 1994 to allow the construction of buildings directly on the ground, slab-on-grade. Subsequently this design started to be used also for buildings with crawl space.