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Earth sheltered building
Earth sheltering is the use of earth against building walls for external thermal mass, to reduce heat loss, and to easily maintain a steady indoor air temperature. This can reduce the absolute amount of heat needed by a building.
- "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."
- "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."
Approximately 50% of the heat from the Sun is absorbed at the surface. Consequently, the temperature in the upper layers of the earth changes dependant on the day/night cycle and to a lesser extent weather (e.g. in full sun the surface receives more heat, whereas if it is very cloudy less energy reaches the surface). The passing of the seasons has greatest impact on the temperature in the superficial layers of the earth. The air temperature fluctuations at the surface can be rapid (e.g. the sun setting or a rainstorm). However the earth takes time to soak up heat and to release it (earth is like thermal mass). Although the very superficial layers of soil possesss so many roots of plants and air spaces it acts more like insulation. But past this layer, the subsoil is hard and compacted and dense. Soil is stated as having an R value of about 0.65-R per centimeter (0.08-R per 1 inch), or 0.25-R per 1 inch. Variations in R-value of soil is also greatly dependant upon soil moisture level, with lower R values as moisture level increases. The deeper down you go, the slower and more blunted the response to surface air temperature changes becomes. At a certain depth, the temperature in the earth is constant all year round. Seasonal temperature fluctuations have so much depth of earth to permeate, they even out, and this roughly equates to the mean annual surface air temperature over winter and summer.
Some put the depth of this deep earth constant temperature at 6 meters, and others at 15 meters. Also, this temperature will be different depending on geographic location. For example, 7 degrees Celsius in Montana, USA. The deep earth constant temperature in different parts of the UK can be between 8 - 11° C. Another sources suggests that the deep earth constant temperature (which is described in terms of the amplitude correction factor, the amplitudes referring the fluctuations in soil temperature between winter and summer) also varies dependant upon soil moisture level (4.25m, 5.5m and 6.7m for dry , average and wet soil respectively).
Below this the temperature of the earth gradually increases by about 2.6 degrees Celcius per 100 meters because there is also heat rising from the interior of the earth.
There are 2 main types of earth sheltered building (see also diagram). Earth berming is where the home is built close to the original grade and earth is mounded against the sides of the house. A thin layer of earth may be laid ontop of the structure to form a living roof. When passive solar design is applied to the layout of an underground house, the side of the structure facing the equator is left un-bermed to allow for collection of solar radiation. A chambered underground house is where the entire house is below the original grade, and this type is more rare. Earth berming a home provides 90 - 95% of the thermal energy advantage of a completely below grade home. Earth bermed houses also require less excavation and their roof must sustain less load, menaing they will tend to be cheaper and easier to design and build.
Earth shelteres are usually single story, perhaps because the structure already has to contend with the weight of earth above it that adding an extra story mandates excessively expenisive load bearing elements. However, some two story earth shelters have been built, sometimes because of a steeply sloping hill site which makes this form work well.
Advantages and disadvantages
Earth sheltering can increase the thermal mass of a building. However, in temperate and arctic climates we must make sure that we add a insulation layer. The thickness of this depends on the depth of the soil (see Passive solar house) If no insulation layer is put in these areas, heat will leak out into the soil at times of the year when the earth cools down.
As most heat tends to leak out via the roof (as well as via cracks, see Passive solar house) rather than the side walls, it is best to cover the roof with soil so as to attain maximum efficiency. However, to many people this could have a claustrophobic effect. In these cases, covering the walls upto only 1,6-1,8m allows a window to be put in so that (when standing up), this claustrophobic effect can be eliminated. Besides this method, it is also possible to cover the walls entirely, but implement a glass, flat roof.
Factors to consider
- The expense and energy of building earth-sheltered buildings compared to conventional buildings. More effort is required to remove the earth, but effort is saved by not needing to provide an attractive finish to the surfaces which are covered by earth.
- The different form and visual impact - e.g. earth sheltered buildings may have less impact, and be less conventional in appearance, and create less shade (thus allowing more sun and space for plants to grow - see Urban agriculture).
- The earth may provide temperature buffering and/or insulation.
- Lower set buildings are much more vulnerable to flooding - important if there is any significant risk of flooding (consider even theoretical 1 in a 1000 year floods, noting that actual risks may be higher than theoretical risks, if a risk factor is overlooked).
A benefit claimed for earth sheltering is that it provides a moderate temperature buffer between the house and the environment. Is this accurate?
It is true that the temperature of the earth is often more moderate than the air - e.g. 10 deg C (50 deg F) when the air is -25 deg C (-10 deg F). However, heat transferW is a function not only of temperature difference, but of the thermal conductivity of the medium.
Question: Over the range of temperatures experienced by a house in a given climate (say, a cold, temperate or tropical climate) what is the difference in heat flux between a wall exposed to air and a wall exposed to earth? This might be expressed in a formula, and depicted in graphs.[Suggested project] Of course this would be expected to vary by soil type and water content.
- Earth Sheltered Houses page on Lowimpact.org
- AJ Anselm (2012). Earth Shelters; A Review of Energy Conservation Properties in Earth Sheltered Housing, (chapter in: Energy Conservation, AZ Ahmed, IntechOpen, DOI: 10.5772/51873
- British Geological Survey (BGS) website 
- Earth-Sheltered Houses: How to Build an Affordable Underground Home. R Roy. New Society Publishers, 2006
- Passive annual heat storage: Improving the design of earth shelters. John Hait. 2013
- Mechanical and Electrical Equipment for Buildings. Walter T. Grondzik, Alison G. Kwok 2014
- What is Earth-Sheltered Housing? Earthwood Building School
- W (Earth Sheltering article on Wikipedia)
- Efficient Earth-Sheltered Homes on Energy.gov
- The Architectural Use of Underground Space: Issues And Applications, Kenneth B. Labs
-  Mother Earth News archive of articles on Earth Sheltered Houses