Living under a Rock

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Underground living isn't a new idea. Living in sub-terrain structures might have been the most popular habitable living in the past, hence “caveman” refers to ...
Green Building Methods

Living under a Rock In recent years, many Americans have opted for more efficient homes that use less resources and have minimal impact on the environment. One of the options to reduce the carbon footprint and build structures that become part of the surrounding environment is to build subterranean homes that use less energy to cool and heat, and do not stick out like a sore thumb but become, literally, part of mother earth. It goes without saying that global warming and climate change are posing a challenge to our survival as humans, as well as to animals and plants of our planet. Carbon dioxide is a major contributor, and powerplants are pumping millions of tons of carbon every year into the air we breathe, trapping the heat and causing severe weather. In that case, an underground structure is the best shelter from natural disasters and turbulent weather patterns. Moreover, the need to conserve energy has never been greater and we are running out of time before these changes become irreversible. However, in this paper, we do not include the moral and environmental aspect in our calculations. Rather, we approach it from a pure economical view. The purpose of this research is to address the advantages and disadvantages of underground living, discussing the tangible benefits and challenges as well as the cost difference between above ground and underground structures as a residential option. This is not meant to be a broad and comprehensive research, which would cover this option in many different angles. In this research, however, we approach the comparison only from structural and economical angles, although the environmental aspect is an overall concern that initially motivated this research. Underground living isn’t a new idea. Living in sub-terrain structures might have been the most popular habitable living in the past, hence “caveman” refers to primitive

and ancient humans. Over four thousand years ago, the Egyptian city of Luxor was a sprawling underground city with very few above ground structures. The town of Kandovan in northern Iran is built in the side of a mountain and consists of a series of rock caves, and the recent discovery of a whole city with tunnels, houses and churches underground in Cappadocia, Turkey to name a few. In this paper, we compare the cost of building an average size underground structure with an above-ground one, and the reason we approach it from a cost perspective is that, as any engineer will tell you, we can build almost anything and make it almost as strong as we want to if we have an unlimited amount of money. So, the governing factor here is cost. Is it economically viable to build underground living space, and will it meet the minimum standards expected in a home? In several recent surveys, between 56% and 89% of Americans, especially millennials, said they are willing to pay a little bit more for eco-friendly products and services, but the surveys did not ask how much more are they willing to pay for “green” products, so in this research we will ignore the “green” factor, or at least leave it up to the reader to decide that markup value. The criteria for this comparison is based on an average size house in the United State in 2017, and the cost of labor, equipment and materials is based on the mean average cost to get a rough estimate, since labor cost varies widely between states and even within each state. Underground structures can save hundresd of gallons of heating fuel in winter, and electricity used to cool houses during the summer, which deeply impacts the environment and our pocket books every month. Moreover, aesthetically speaking, an underground structure doesn’t stick out like a sore thumb but becomes part of the natural terrain. Horizontal living is very common in the USA, much more popular than most countries, which provides us reasonable motivation to explore this option. According to a recent study, 71% of Americans live in houses, and more than half of the other 29% wish

they could. Eco-friendly architecture and sustainable living have been gaining momentum recently, with nearly 38% of Americans expressing their willingness to pay a little more for environmentally friendly products and services. In this project, the methods, materials as well as the challenges of sub-terrain construction will be addressed, and proposed solutions will be presented, including discussing previous science-based research on the feasibility and sustainability of this old yet new type of construction. The figures, statistics, costs, and most other numbers are broad averages and are only meant to provide a very rough estimate for nothing more than a general comparison between the two options; above ground and underground structures. The comparison is based on the average size home in the USA in 2016, which was 2640 square feet (the average size home in the USA in 1973 was 1720 ft2). The average cost of building a home (labor, material, and equipment) varies widely between states and even within each state, so the numbers provided are for the averages only. Most of the methods and materials that go into building a house above ground will apply to building underground, so we will only focus on the differences and calculate the Δ cost between above and underground building, but first let us recognize that, just as we cannot build aboveground structures everywhere, the same applies to underground structures. Unlike traditional above-ground buildings, underground structures face a completely different set of challenges. While building a traditional home requires leveling the lot, building an underground (or a partially underground) home works better in rolling, hilly or unleveled topography. Fortunately, the natural topography is rarely flat. Another factor is ground water. While the water table usually does not affect the construction of a residential home (light construction), it is a very important factor in determining whether we can build an underground house. Some areas in the USA have a very shallow water table which makes building an underground structure not economically feasible. According to the USGS, most of the land in the USA has a water

table depth that would not hinder building a 15-20 ft structure below the surface. The City of Fresno* in California’s central valley was chosen as an average representation of the depth of groundwater. It showed that the average ground water level (which slightly fluctuates throughout the year) was 44.7 ft in 2016 after measuring 10 wells within the city limits. Clearly in places like Florida and Louisiana, a good portion of the state cannot accommodate underground structures, but for the rest of the USA, this should not be an issue, except near bodies of water for obvious reasons.

The Malator, a house built by Future Systems in Druidston, Pembrokeshire, Wales in 1998. It is owned by Bob MarshallAndrews, a British Member of Parliament form 1997 till 2010. The house is partially underground, the façade is supported by an exterior glass wall and has an open flat layout.

Another factor that resembles the issue of groundwater table is the risk of flooding. It is extremely risky and not at all recommended to build underground residential structures in flood zones. There are several areas in California that are prone

to flooding According to GIS. Many areas in Central California are within the 100 Year Floodplains. Even though this risk eliminates the potential of building residential substructures in over 8% of the total area of the USA, it leaves more than 84% of the land in America that can safely accommodate underground houses without the risk of flooding or a shallow water table. A third factor is the type of soil, but for the most part, this last factor applies to all types of home and not specific to underground structures. Home Efficiency As far as energy efficiency and building design, a moderate-winter-zone 2600 ft2 traditional home requires a little over 5 million BTUs per month to heat and even more during the summer to cool it down in warmer areas. Here an underground home has a cler advantage. The temperature of the soil underground does not fluctuate as much as it does on the surface. The figure below shows the average temperature in Fahrenheit throughout the year versus the depth in feet below the ground level (X).

As the figure shows, the fluctuation of temperature (ΔT) decreases over the year as we go deeper underground. The figure also shows that the increase of time lag in temperature is directly proportional to the depth (X) below the surface (4 months @ X =

12 ft). At a depth of 12 feet, the temperature fluctuates between 58° F in early April and 66° F in late October, with the average temperature hovering around 62° F. Moreover, dirt in general has a high R-values, which measures the resistance of any material to heat flow or transfer (2.22 for dirt, 1.25 for wood, 3.17 for fiberglass). A simple calculation shows that one foot of dirt is equivalent to 8-inch thick fiberglass if used as an insulation to prevent ambient temperature from seeping into the walls and the surrounding dirt. It’s also worth noting that the thermal mass of dirt is high, which helps keeping the temperature close to constant. (Thermal mass considers the storage effects of the mass of soil or other material)

By the Numbers: Assuming the parcel is suitable for placing an underground structure, now going back to the cost and the savings of each structure. Below is the list of items and the average cost for a 2640 ft2 home. For simplicity, total cost (labor, material, and equipment) for each item is listed. The list is not comprehensive; only the large items are listed so please bear in mind that smaller items could add up to a significant mount of money that could tip the scales one way or another in favor or against earth structures and underground building.


Above-ground Cost

Underground Cost

Excavating 300 yd3 of dirt



Moisture Barrier 1



Gravel &Sand Drainage



Tar or Asphalt Barrier









Windows 3






Roof 4



Added Dead Load Cost



Total Cost



1. 2. 3. 4.

Cost of moisture barrier (Tyvek won’t cut it). In this case a 20 Mil silver pack was used to estimate the cost. For the underground structure, a small heater is assumed to be sufficient for heating to keep the house at a comfortable temperature during the winter. The number of windows is the only variable contributing to the difference in cost between above ground and underground structures. The above ground structure would have a tile roof, while the underground structure has felt and gravel.

These calculations show that the underground house would cost (on average) around $28,000 more to construct than a traditional house, but these figures exclude the savings on heating and cooling per year. According to the 2016 residential energy consumption figures, the average annual heating cost in winter for a 2640 ft2 is $2315 and the average annual cooling cost in the summer months is $981(using standard efficiency equipment). Using the table above, the estimated heating cost for an underground house during the winter months is approximately $217, also using standard efficiency heaters. This gives us an annual energy savings of a little over $3000 in favor of the underground structure. Using a simple calculation with the assumption of an annual interest rate of 4%, it would take the homeowner approximately 11 years to recoup the additional initial costs he or she will incur by choosing an underground design versus a traditional house.

In conclusion, an underground house costs on average $30,000 more to build and saves around $250 a month in heating and cooling costs. It provides more protection from the elements, but the homeowner will get less sun and possibly more humidity. It is an option that is viable only where the water table is deep enough and where there’s no risk of flooding. The dirt insulates the house better and cheaper than traditional insulation.


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2. Sarah B. Schindler. (2010). Following Industry's LEED : Municipal Adoption of Private Green Building Standards. Florida Law Review, 62, 285-1463.

3. Davies, R. (2003, January 21). Subterranean ideas resurface: TECHNOLOGY: Economic factors have brought underground living back into vogue, says Ross Davies:. Financial Times, p. 14.

4. Van Dronkelaar, Cóstola, Mangkuto, & Hensen. (2014). Heating and cooling energy demand in underground buildings: Potential for saving in various climates and functions. Energy & Buildings, 71, 129-136.

5. Lindblom, U., Green, L., & Manell, G. (1995). Sweden's national library goes underground. TUNNELLING UNDERGROUND SPACE TECHNOLOGY, 10(2), 149-154

6. Alkaff, Sim, & Ervina Efzan. (2016). A review of underground building towards thermal energy efficiency and sustainable development. Renewable and Sustainable Energy Reviews,60, 692-713.

7. Boyle, Mark (2010). How to Build Sustainable Homes Without Spending a Penny. THE GUARDIAN. 10 August 2010. Print.

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