Editing Earth shelter (section)

==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 (or north or east, depending on latitude). 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 (or in-hill) 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 (about 8,000 pounds per square foot), excellent drainage and a low [[Frost heaving|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 (geographical feature)|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 (and north-facing in the Southern hemisphere) due to solar benefits. The most practical orientation in the [[Tropics]] nearest the equator is north-facing toward the [[Apsis|aphelion]] (or perhaps northeast) 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.

===Building materials===
The selection of construction materials should consider the type of structure, site characteristics, climate, soil type, and design. Stronger, longer-lasting building materials are required for structures that are buried deeply. Waterproof and insulated materials should also be utilized to withstand the pressure and moisture of the surrounding ground. For instance, concrete and reinforced masonry, wood, and steel are all viable materials. Concrete is the most common choice for earth-sheltered buildings due to its strength, durability, and fire resistance. Cast-in-place concrete is employed for non-critical structural elements such as concrete foundations, floor slabs, and exterior walls with less than {{convert|6|ft|m|abbr=off|sp=us}} of earth cover. In contrast, precast reinforced concrete can be used for floors, walls, and roofs. Concrete masonry units should be {{convert|8|in|mm|abbr=off|sp=us}} or greater, with the use of “A” or “H” facilitating unit placement around vertical reinforcing bars, depending on the required structural integrity. It is typically advised to use Type S mortar, grout with a minimum strength of {{convert|2,000|psi|MPa|abbr=on}}, and a concrete slab with a minimum strength of {{convert|2,500|psi|MPa|abbr=on}} and {{convert|4|in|mm}} thickness.<ref>{{cite web|last1=NREL / DOE |title="Earth-Sheltered Houses" |url=https://www.nrel.gov/docs/legosti/fy97/6993.pdf |access-date=15 November 2022}}</ref> Brick or stone masonry reinforced with steel bars can be utilized for walls that will be subjected to lateral or vertical pressure from earth cover. Masonry generally costs less than cast-in-place concrete. Wood can be widely employed in earth-sheltered buildings for structural and internal work, including floors, roofs, and exterior walls. However, wooden frame walls, which must endure lateral pressure, are limited to a burial depth of one story when used as a structural material. Beyond this depth, the cost will rapidly increase while using wood as a structural material. Although wood can be less expensive than other materials, it lacks steel's strength, therefore it might not be the ideal option for structural material in some earth-sheltered dwellings, especially in moist soil type. Steel is used for beams, columns, bar joists, and masonry reinforcement. The advantage is that steel has high tensional and compressional strength, while the disadvantage is that it must be protected from corrosion if it is exposed to air, water, or other chemicals. It must be used effectively because it is also expensive.<ref>{{cite web |title="Earth-Sheltered Buildings" |url=https://ncma.org/resource/earth-sheltered-buildings/ |publisher=NCMA |access-date=15 November 2022}}</ref>

===Excavation===
In earth-sheltered construction, there is often extensive excavation done on the building site. An excavation several feet larger than the walls' planned perimeter is made to allow for access to the outside of the wall for waterproofing and insulation.

===Foundations===
Once the site is prepared and the utility lines installed, a foundation of reinforced concrete is poured. The walls are then installed. Usually, they are either poured in place or formed either on or off-site and then moved into place. Reinforced concrete is the most common choice. The process is repeated for the roof structure. If the walls, floor, and roof are all to be poured in place, it is possible to make them with a single pour. This can reduce the likelihood of there being cracks or leaks at the joints where the concrete has cured at different times. The foundation of the buildings designed by Vetsch are built conventionally.

===Walls===
Several different methods of external ([[Load-bearing wall|load-bearing]]) wall construction in earth shelters have been used successfully. These include [[Concrete masonry unit|concrete block]] (either conventionally [[Mortar (masonry)|mortared]] or surface-bonded), [[Masonry#Stonework|stone masonry]], [[Cordwood construction|cordwood masonry]], poured concrete, and [[Wood preservation#Pressure processes|pressure-treated wood]].<ref name="roy2006" /> Earthships classically use rammed earth tire walls, which are labor-intensive but recycle used tires.<ref name="roy2006" />

Oehler prescribed a very low budget method he termed "post, shoring and polyethylene". This involves buried wooden posts to act as a frame, shored up with planks, and with a [[Vapor barrier|waterproofing barrier]] of polyethylene sheet between the planks and the backfill, but no foundation and nothing on the floor except a plastic sheet and carpeting.<ref name="oehler1981" />

Untreated wood in direct contact with earth rots within five years of use in earth shelter construction, but can be used for structural support construction in a well-insulated and water/moisture-proofed home. Reinforced concrete is the most commonly used structural material in earth shelter construction. It is strong and readily available. Steel can be used but needs to be encased by concrete to keep it from direct contact with the soil which corrodes the metal. Bricks and concrete masonry units are also possible options in earth shelter construction but must be reinforced to keep them from shifting under vertical pressure unless the building is constructed with arches and vaults.

Unfortunately, reinforced concrete is not the most environmentally sustainable material. The concrete industry is working to develop products that are more environmentally friendly in response to consumer demands. Products like [[Grancrete]] and [[Hycrete]] are becoming more readily available. They claim to be environmentally friendly and either reduce or eliminate the need for additional waterproofing. However, these have not been extensively used in earth shelter construction yet.

===Roof===
[[Image:Mesh structure.jpg|thumb|right|Meshed metal stretch net construction]]
The roof of an earth shelter may not be covered by earth (earth berm only), or the roof may support a [[green roof]] with only a minimal thickness of earth. Alternatively a larger mass of earth might cover the roof. Such roofs must deal with significantly greater dead load and live load (e.g. increased weight of water in the earth after rain, or snow). This requires stronger and more substantial roof support structure. Some advise to have just enough thickness of earth on the roof to maintain a green roof (approximately 6 inches / 15&nbsp;cm), since this means less load on the structure. Increasing the amount of earth on the roof past this gives only modest increases in the benefits while increasing costs significantly.<ref name=roy2006 />

Despite being underground, drainage of water is still important. Therefore, earth shelters do not tend to have flat roofs. A flat roof is also less resistant to the weight of the earth. It is common for earth shelter designs to have arches and shallow domed roofs since this form resists vertical load well. One method uses finely meshed metal bent into the intended shape and welded to the supporting armature. Onto this mesh concrete is sprayed forming a roof. Terra-Dome (USA) is a company specializing in construction of earth-sheltered houses and sells a modular system of concrete domes intended to be covered by earth.<ref name="terra-dome">{{cite web |title=Terra-Dome Corporation - Earth Sheltered Housing |url=https://www.terra-dome.com/ |publisher=Terra-Dome Corporation |access-date=29 January 2019 |language=en}}</ref> Others advise the use of [[Timber framing|timber frame]]d, [[gable roof]]s of pitch at least 1:12 to promote drainage.<ref name=roy2006 /> The roofs of Earthships tend to be [[Mono-pitched roof|mono-pitched]], classically using [[Viga (architecture)|vigas]].

===Waterproofing===
On the outside of the concrete, a waterproofing system is applied. The most frequently used waterproofing system includes a layer of liquid asphalt onto which a heavy grade waterproof membrane is affixed, followed by a final liquid water sealant which may be sprayed on. It is very important to make sure that all of the seams are carefully sealed. It is very difficult to locate and repair leaks in the waterproofing system after the building is completed. Several layers are used for waterproofing in earth shelter construction. The first layer is meant to seal any cracks or pores in the structural materials, also working as an adhesive for the waterproof membrane. The membrane layer is often a thick flexible polyethylene sheeting called [[EPDM rubber]]. EPDM is the material usually used in a water garden, pond and swimming pool construction. This material also prevents roots from burrowing through the waterproofing. EPDM is very heavy to work with and can be chewed through by some common insects like fire ants. It is also made from petrochemicals, making it environmentally unsustainable.

There are various cement-like coatings that can be used as waterproofing. The product is sprayed directly onto the unprotected surface. It dries and acts like a huge ceramic layer between the wall and earth. The problem with this method is, if the wall or foundation shifts in any way, it cracks and water is able to penetrate through it easily.

Bituthene (registered name) is very similar to the three coat layering process only in one step. It comes already layered in sheets and has a self-adhesive backing. Applying it manually is the same as with the layering method, in addition, it is sun sensitive and must be covered very soon after application.

Eco-Flex is an environmentally friendly waterproofing membrane that seems to work very well on foundations, but not much is known about its effectiveness in earth sheltering. It is among a group of liquid paint-on waterproofing products. The main challenge with these products are they must be carefully applied, making sure that every area is covered to the right thickness, and that every crack or gap is tightly sealed.

[[Bentonite]] clay is an alternative that the most environmentally friendly. It is naturally occurring and self-healing. The drawback to this material is that it is very heavy, difficult for the owner/builder to install, and subject to [[termite]] damage.

Bi-membranes have been used extensively throughout Australia where 2 membranes are paired together—typically 2 coats of water-based [[epoxy]] as a 'sealer' and stop the internal vapor pressure of the moist concrete exploding bubbles of vapor up underneath the membrane when exposed to hot sun. The bond strength of epoxy to concrete is stronger than the internal bond strength of concrete so the membranes won't 'blow' off the wall in the sun. Epoxies are very brittle so they are paired up with an overcoat of a high-build flexible water-based acrylic membrane in multiple coats of different colors to ensure film coverage—this is reinforced with non-woven [[polypropylene]] textile in corners and changes in direction.

===Insulation===
One or more insulation board or foam layers are added outside the waterproofing. If the insulation chosen is porous, a top layer of waterproofing is added. Unlike the conventional building, earth shelters require the insulation on the exterior of the building rather than inside the wall. One reason for this is that it protects the waterproof membrane against freeze damage, another is that the earth shelter is able to better retain its desired temperature. There are two types of insulation used in earth shelter construction. The first is close-celled extruded polystyrene sheets. Two to three inches glued to the outside of the waterproofing is generally sufficient. The second type of insulation is a spray on foam (e.g. [[polyurethane]] solid foam insulation). This works very well where the structure's shape is unconventional, rounded or difficult to get to. Foam insulation requires an additional protective top coat such as foil or fleece filter to help it resist water penetration.

In some low budget earth shelters, insulation may not be applied to the walls. These methods rely on the earth's U factor or thermal heat storage capacity below the frost layer. These designs are the exception however and risk frost heave damage in colder climates. The theory behind no insulation designs relies on using the earth's thermal mass to store heat, rather than relying on a heavy masonry or cement inner structures that exist in a typical passive solar house. This is the exception to the rule and cold temperatures may extend down into the earth above the frost line making insulation necessary for higher efficiencies.

===Backfilling===
After previous construction stages are complete, the earth is backfilled against the external walls to create the berm. Depending on the drainage characteristics of the earth may not be suitable to place in direct contact with the external wall.<ref name="roy2006" /> Some advise that topsoil and turf (sod) be put aside from the initial excavation and be used for the grass roof and to place as the topmost layer on the berm.<ref name="roy2006" />

===Finishings===
In the earth houses designed by Vetsch, interior walls are furnished using loam rendering, providing superior humidity compensation. The loam rendering is finally coated with lime-white cement paint.<ref name=vetsch1994>{{cite book |author1=P Vetsch, E Wagner, C Schubert-Weller |title=Erd- und Höhlenhäuser von Peter Vetsch = Earth and cave architecture |date=1994 |publisher=A. Niggli |isbn=9783721202823 |language=de|oclc = 441647358}}</ref>