between deep soils and heated spaces. Wells's insulation recommendations increase as the depth of the soil decreases (a negative correlation). In addition to thermal mass, Earthships use passive solar heating and cooling. Large front windows with integrated shades, trombe walls and other technologies such as skylights or Steve Baer's "Track Rack" solar trackers are used for heat regulation. Earthships are positioned so that its principal wall, which is nonstructural and made mostly of glass sheets, faces directly towards the equator. This positioning allows for optimum solar exposure. To allow the sun to heat the mass of the Earthship, the solar-oriented wall is angled so that it is perpendicular to light from the winter sun. This allows for maximum exposure in the winter, when heat is wanted, and lesser exposure in the summer, when heat is to be avoided. Some Earthships, especially those built in colder climates, use insulated shading on the solar-orientated wall to reduce heat loss during the night. Current Earthship designs like the global module have a "double greenhouse" where the outside glass is angled towards the equator, and an internal glass wall forms a walk way or hallway as you step into the Earthship. This greenhouse is primarily used to grow food; it also creates a barrier for the 'comfort zone' inside the house. Ventilation Earthships structures have a natural ventilation system based on convection. A pipe extends from the interior of the house under the berm, cooling the air by the time it gets to the comfort zone. As the hot air rises, the system creates a steady airflow - of cooler air coming in, and warmer air blowing out though a smaller vented window in the greenhouse. Around the world Africa The first earthship in South Africa was built by Angel and Yvonne Kamp between 1996 and 1998. They rammed a total of 1,500 tires for the walls. The Earthship, near Hermanus, is located in a 60 hectare private nature reserve which is part of a 500 hectare area enclosed in a game fence and borders the Walker Bay Nature Reserve. The second earthship in South Africa is a recycling centre in Khayelitsha run as a swop shop concept. The centre was finished in December 2010. Another low cost house built with tires is in development in Bloemfontein. A project nearing completion in South Africa is a combined living quarters and the Sonskip / Aardskip