Earth shelter
An earth shelter, also called an earth house, earth-bermed house, earth-sheltered house, earth-covered house, or underground house, is a structure with earth against the walls and/or on the roof, or that is entirely buried underground.
Earth acts as thermal mass, making it easier to maintain a steady indoor air temperature and therefore reduces energy costs for heating or cooling.
Earth sheltering became relatively popular after the mid-1970s, especially among environmentalists. However, the practice has been around for nearly as long as humans have been constructing their own shelters.
Definition
- "Earth-sheltering is
a generic term with the general meaning: building design in which soil plays an integral part." This definition is problematic however, since earth structures are not usually considered as earth shelters as they are above ground. - "A building can be described as earth-sheltered when it has a thermally significant amount of soil or substrate in contact with its external envelope, where “thermally significant” means making a functional contribution to the thermal effectiveness of the building in question.
- "Structures built with the use of earth mass against building walls as external thermal mass, which reduces heat loss and maintains a steady indoor air temperature throughout the seasons."
- "A residence with an earth covering for its roof or walls."
- "Homes that have been built underground, either partially or completely."
- "The use of earth cover to moderate and improve living conditions in buildings."
Design and construction
Design
Earth sheltered homes are often constructed with energy conservation and savings in mind. Specific designs of earth shelters allow for maximum savings. For bermed or in-hill construction, a common plan is to place all the living spaces on the side of the house facing the equator. This provides maximum solar radiation to bedrooms, living rooms, and kitchen spaces. Rooms that do not require natural daylight and extensive heating such as the bathroom, storage, and utility room are typically located on the opposite side of the shelter. This type of layout can also be transposed to a double level house design with both levels completely underground. This plan has the highest energy efficiency of earth sheltered homes because of the compact configuration as well as the structure being submerged deeper in the earth. This gives it a greater ratio of earth cover to an exposed wall than a one-story shelter would.The soil type is one of the essential factors during site planning. The soil needs to provide adequate bearing capacity and drainage, and help to retain heat. With respects to drainage, the most suitable type of soil for earth sheltering is a mixture of sand and gravel. Well graded gravels have a large bearing capacity, excellent drainage and a low frost heave potential. Sand and clay can be susceptible to erosion. Clay soils, while least susceptible to erosion, often do not allow for proper drainage, and have a higher potential for frost heaves. Clay soils are more susceptible to thermal shrinking and expanding. Being aware of the moisture content of the soil and the fluctuation of that content throughout the year will help prevent potential heating problems. Frost heaves can also be problematic in some soil. Fine grain soils retain moisture the best and are most susceptible to heaving. A few ways to protect against capillary action responsible for frost heaves are placing foundations below the freezing zone or insulating ground surface around shallow footings, replacement of frost-sensitive soils with granular material, and interrupting capillary draw of moisture by putting a drainage layer of coarser material in the existing soil.
Water can damage earthen shelters if it ponds around them. Avoiding sites with a high water table is crucial. Drainage, both surface and subsurface, must be properly dealt with. Waterproofing applied to the building is essential.
Atrium designs have an increased risk of flooding, so the surrounding land should slope away from the structure on all sides. A drain pipe at the perimeter of the roof edge can help collect and remove additional water. For bermed homes, an interceptor drain at the crest of the berm along the rooftop's edge is recommended. An interceptor drainage swale in the middle of the berm is also helpful or the back of the berm can be terraced with retaining walls. On sloping sites, runoff may cause problems. A drainage swale or gully can be built to divert water around the house, or a gravel-filled trench with a drain tile can be installed along with footing drains.
Soil stability should also be considered, especially when evaluating a sloping site. These slopes may be inherently stable when left alone, but cutting into them can greatly compromise their structural stability. Retaining walls and backfills may have to be constructed to hold up the slope prior to shelter construction.
On land that is relatively flat, a fully recessed house with an open courtyard is the most appropriate design. On a sloping site, the house is set right into the hill. The slope will determine the location of the window wall; the most practical orientation in moderate to cold climates is a south-facing exposed wall in the Northern hemisphere due to solar benefits. The most practical orientation in the Tropics nearest the equator is north-facing toward the aphelion to moderate the temperature extremes. Just outside the Tropics, the most practical way to avoid afternoon heat excess may be an east-facing house or, if near a west coast, exposure of the east end and the west end, with the two long sides embedded in the earth.
Depending on the region and site selected for earth-sheltered construction, the benefits and objectives of the earth shelter construction vary. For cool and temperate climates, objectives consist of retaining winter heat, avoiding infiltration, receiving winter sun, using thermal mass, shading and ventilating during the summer, and avoiding winter winds and cold pockets. For hot, arid climates objectives include maximizing humidity, providing summer shade, maximizing summer air movement, and retaining winter heat. For hot, humid climates objectives include avoiding summer humidity, providing summer ventilation, and retaining winter heat.
Regions with extreme daily and seasonal temperatures emphasize the value of earth as a thermal mass. Earth sheltering is most effective in regions with high cooling and heating needs and high-temperature differentials. In regions such as the southeastern United States, earth sheltering may need additional care in maintenance and construction due to condensation problems in regard to the high humidity. The ground temperature of the region may be too high to permit earth cooling if temperatures fluctuate only slightly from day to night. Preferably, there should be adequate winter solar radiation and sufficient means for natural ventilation. Wind is a critical aspect to evaluate during site planning, for reasons regarding wind chill and heat loss, as well as shelter ventilation. In the Northern Hemisphere, south facing slopes tend to avoid cold winter winds typically blown in from the north. Fully recessed shelters also offer adequate protection against these harsh winds. However, atria within the structure can cause minor turbulence depending on the size. It is helpful to take advantage of the prevailing winds in the summer. Because of the limited window arrangement in most earth shelters, and the resistance to air infiltration, the air within a structure can become stagnant if proper ventilation is not provided. By making use of the wind, natural ventilation can occur without the use of fans or other active systems. Knowing the direction, and intensity, of seasonal winds, is vital in promoting cross ventilation. Vents are commonly placed in the roof of bermed or fully recessed shelters to achieve this effect.