Underground houses may be known by different names – green home, earth integrated home, earth contact home, earth sheltered home, hobbit house and many more – but they all have one thing in common: they offer sustainable benefits that can save 50 to 75 per cent on heating and cooling costs compared to traditional dwellings.
While many types of structures are now considered underground homes - the term applies to buildings with one or more walls below the earth's surface - the earliest underground homes were natural caves or caverns where primitive people used to live.
In recent years, architects and designers have been studying and reconsidering the many advantages of earth integrated architecture. With the earth as insulation, heating and cooling requirements are significantly reduced, especially in windy areas. In addition, underground homes can offer protection from wild storms, earthquakes and tornadoes.
Underground architecture is also unobtrusive, preserving clear views. It can be covered with gardens or lawns and, in most cases, the house is designed in a way that preserves the natural environment.
Compared to houses built with traditional materials such as wood, these buildings are more fire-proof as they typically make use of concrete and because of the insulation provided by the green roof.
Though maintenance costs for underground buildings are typically very low - they never have to be painted and the exposed exterior area is very small or completely nonexistent - initial construction costs are higher than those of a traditional building.
Different construction systems are being used to build underground homes. One of the adopted systems consists of a steel truss framework covered in spray-on concrete, tar and a layer of plastic for water protection. The foundation is built conventionally and the interior walls are furnished using loam rendering, which reduces problems associated with humidity. The loam rendering is coated with cement paint.
Concrete reinforced with steel is used in those earth homes that are designed more like conventional homes covered with earth. Large quantities of steel and shotcrete are being used by some construction companies for dome-style underground homes.
Some underground green homes make use of a modular construction system in which the components are manufactured with composites and designed to be covered with earth. These materials are 100 per cent water-proof and are quick and easy.
As with any building system, a major challenge with underground homes is to ensure comfortable temperatures year-round. However, earth sheltered construction has the benefit of passive solar principles that can provide large energy saving solutions by applying the concept of passive annual heat storage system (PAHS).
PAHS collects heat in the warm seasons by cooling the home naturally and stores the heat the earth’s soil naturally, returning that heat to the house in winter. In addition, PAHS uses natural heat flow methods and the arrangement of building materials to direct this passive energy from the earth to the building all without using machinery.
The basic concept notes that basements and other underground areas tend to be cooler and - by extension - keep people cooler. Heat is drawn away from the human body to the surrounding air, which then transfers this thermal energy into the surrounding walls/structures, which have lower temperatures than the air.
This effect works both ways: if the air inside the house is warmer than the surrounding walls, heat will be drawn out of the air into the walls, cooling the air and warming the walls. Likewise, if the interior air temperature is cooler than the walls, heat will be drawn out of the walls into the air, warming the air and cooling the walls.
In underground homes, PAHS also uses the absorption and re-emission of solar radiation at ground level to regulate interior temperatures year-round.
The use of the earth’s relatively stable temperature can provide occupants comfort with minimal energy costs. The earth moderates the temperature changes between night and day and scientists have determined that a time lag of approximately 133 hours occurs at a two-foot depth. The time lag occurs proportionately, so that an eight or 10-foot depth has a time lag of 2,100 to 2,200 hours or about 90 days. As a result, underground homes can create savings of 50 to 75 per cent over normal heating and cooling energy requirements.
The use of the earth as a large-capacity heat storage makes it possible not only to reduce the buildings’ demand for heating and cooling energy, but also to help to preserve the local microclimate. As a result, underground houses have little impact on the natural environment compared to traditional homes.