Earth-Sheltered Houses

How to Build an Affordable...

Rob Roy

• Publisher: New Society Publishers
• Year: 2006
• ISBN:
  9781550923759
• Topic: Architecture
• Subtopic: Sustainability in Architecture

The only how-to manual on the subject directed to mainstream owner-builders

An earth-sheltered, earth-roofed home has the least impact upon the land of all housing styles, leaving almost zero footprint on the planet.

Earth-Sheltered Houses is a practical guide for those who want to build their own underground home at moderate cost. It describes the benefits of sheltering a home with earth, including the added comfort and energy efficiency from the moderating influence of the earth on the home's temperature-keeping it warm in the winter and cool in the summer-low maintenance, and the protection against fire, sound, earthquake and storm afforded by the earth. Extra benefits from adding an earth or other living roof option include greater longevity of the roof substrate, fine aesthetics, and environmental harmony.

The book covers all of the various construction techniques involved including details on planning, excavation, footings, floor, walls, framing, roofing, waterproofing, insulation and drainage. Specific methods appropriate for the inexperienced owner-builder are a particular focus and include:

• pouring one's own footings and/or floor
• the use of dry-stacked (surface-bonded) concrete block walls
• post-and-beam framing
• plank-and-beam roofing, and
• drainage methods and self-adhesive waterproofing membranes.

The time-tested, easy-to-learn construction techniques described in Earth-Sheltered Houses will enable readers to embark upon their own building projects with confidence, backed up by a comprehensive resources section that lists all the latest products such as waterproofing membranes, types of rigid insulation and drainage products that will protect the building against water damage and heat loss.

Chapter 1

EARTH-SHELTERED DESIGN PRINCIPLES

Any discussion of design must begin with an explanation of how earth sheltering and earth roofs can be made to work to our advantage.

There is a popular misconception that earth is a great insulator, and that is why we put houses underground, surround them with earthern berms, or cover them with grass roofs. The reality is that earth is not a very good insulation, with its best insulating characteristics to be found in the first few inches of the soil, where plant roots provide aeration. At depth, where the earth is densely packed, earth is very poor as insulation. The misunderstanding of earth as insulation leads to the danger of an equally erroneous view of how earth sheltering really works to provide thermal comfort in a home. If a designer-builder proceeds with an earth-sheltered project from a false understanding of earth’s thermal characteristics, the building may not perform well (at best) and could be damp and cold, like so many basements.

EARTH AS THERMAL MASS

Earth’s big advantage is as thermal mass. In electronics, a capacitor (or condenser) is a device for storing an electrical charge. I like to think of earth as a capacitor for heat storage. I also like to think of storing “coolth,” a word that my word processor tells me does not exist. Coolth is what I call heat at a low temperature. Until you get to absolute zero (defined as the absence of heat) any temperature has a degree of heat to it. To give an example of storing coolth, our 23-ton masonry stove at Earthwood stores heat when it is in use during the winter, but it is also an effective storage medium for storing coolth in the summer. I can still remember back in the 1950s when the iceman delivered blocks of ice around Webster, Massachusetts for use in people’s insulated iceboxes. The ice was a capacitor for “storing cold.”

So how can we take advantage of this great thermal mass, and not fall subject to potential disadvantage?

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Fig. 1.1: The thermal advantages of an earth-sheltered house, summer and winter.

The first ten feet or so below-grade space is a giant thermal mass, which is very slow to change temperature. So there is a real advantage in setting the entire home into this sub-surface climate, which is quite a bit different from the climate above grade. Typically, we are setting a house no more than six to ten feet deep, or “berming” an above grade structure with a similar amount of earth. Our Log End Cave, an “almost underground” home, was set about seven feet below grade, whereas our Earthwood home, built on the surface, is bermed with about 500 tons of earth to a depth of about 13 feet on the northern side, sloping to about 4 feet depth at the southeast and southwest parts of the cylinder.

Figure 1.1 shows summer and winter situations for a house above grade and also for one that is earth-sheltered. The air and earth temperatures given are typical of the range that we would expect to find in the northern United States and southern Canada, from 40 to 50 degrees north latitude. (Pacific coast temperatures might be a little higher, Rocky Mountain temps a little lower.) Our Earthwood home is in northern New York, at 45 degrees north.

The air temperatures range in the example’s climate typically varies from about 95 degrees to -20 degrees Fahrenheit, about a 115-degree range. (It can get warmer or colder than these parameters, but rarely.) Note that the temperature range below grade is only about 20 degrees, from about 40 degrees at the beginning of March to about 60 degrees towards the end of August. Temperatures change very slowly below grade, about 1/10 of a degree per day on average. Where we live, a 30- to 40-degree temperature shift from day to night (or day to day) is not uncommon, and we once experienced a change of 70 degrees in a 24-hour period.

Because of its great mass, the earth temperature is slow to respond to climatological changes. This characteristic – sometimes referred to as thermal lag – explains why the coldest earth temperature (in the depth range where we typically place earth-sheltered homes) lags about six weeks behind the surface climate, both in winter and in summer. This is why large lakes with huge water masses, like our Lake Champlain, reach their highest water temperatures towards the end of summer, the end of August. It takes all summer to bring the water temperature up to its highest reading. We can take advantage of this thermal lag, both for summertime cooling and wintertime heating.

I think of earth-sheltering a home as the same as building it in a steadier, more favorable climate. Think of how easy it would be to heat and cool a home in a climate that has a range of temperature of 40 to 60 degrees. This is precisely the advantage of earth sheltering. In terms of winter heating, our earth-sheltered home in northern New York performs as if it were built in the coastal plains of the Carolinas. A more favorable ambient temperature in summer yields a similar kind of energy advantage with regard to summertime cooling. In our wintertime situation, the interior of a home on the surface needs to be about 90 degrees warmer than the outside temperature. However, an earth-sheltered home needs to be only 30 degrees warmer than the 40-degree earth temperature on the other side of the wall.

In the summer situation, the surface home in the north needs to be cooled about 25 degrees to achieve comfort level, usually by some energy-expensive air conditioning system. The earth-sheltered home does not require cooling. Nor will it be too cool. The earth outside the walls may be at 60 degrees, but residual heat – cooking, sunlight, body temperature, refrigerators, and so forth – will keep the temperature up to comfort level.

Notice that in the commentary above, it is the earth’s mass that provides this favorable starting point from which we can begin to heat or cool. We have not begun to bring insulation into the equation. But we must, or we can make a big mistake.

THE IMPORTANCE OF INSULATION

We have spoken of the earth as a thermal mass. It is, in fact, a huge thermal mass, and it is not easy to influence its temperature, although it can be done using a rather labor-intensive method called the insulation/watershed umbrella, and described in John Hait’s Passive Annual Heat Storage, listed in the Bibliography. An insulation “umbrella” extends some distance from the home and encloses the earth near the home. In my view, it is easier and more practical to use the fabric of the building itself as a second and separate thermal mass, one over which we can exercise some control, through the proper placement of insulation. While the home’s thermal mass is tiny compared to the earth’s, it is still considerable and typically several times greater than the mass of a home built above grade. The Earthwood house, for example, has over 120 tons of thermal mass entirely wrapped inside an insulation barrier. It takes a long time to change the temperature of 120 tons of something, but it will happen, and, through insulation, we can control the rate of heat transfer, and, thus, the temperature of the mass fabric. Consider the homes in Figure 1.1, wintertime situation. The above-grade home relies on plenty of insulation to protect its inhabitants from the sub-zero outside temperature. The earth-sheltered home also needs insulation, or else the 40-degree earth temperature will actually wick the heat out of the home’s fabric through conduction. Without insulation – and properly placed insulation at that – the fabric of the building becomes one and the same with the earth’s mass. The 120 tons of mass at Earthwood would have the value of a 120-ton slab of stone, a part of the earth itself. Its temperature would be the same as the surrounding earth. Massive walls and floors at 40 degrees would be difficult to heat without insulation.

Fig. 1.2: At the original Log End Cave, the footings conducted heat to the earth, causing condensation on the interior. At Earthwood, extruded polystyrene around the footings keeps interior surface temperatures above dew point.

In order to control the mass fabric of the home itself, we must place the insulation between the home’s mass and the earth. In northern climes, we must completely wrap the below-grade portion of the home (concrete, concrete block, stone, etc.) with a layer of substantial insulation. By this method, we use whatever our internal heat source might be (wood, solar, fossil fuels) to “charge up” the mass fabric of the building itself to comfort level. This insulation must be continuous, and without gaps which would create thermal bridges in the mass. I like Mac Wells’ term for these: “energy nosebleeds.”

I myself made the mistake of not insulating under the footings at Log End Cave, creating a serious energy nosebleed. The left side of Figure 1.2 shows the situation at Log End Cave. Fearing the insulation would be crushed under the great weight of the footings, the 12-inch concrete block walls, and the heavy earth roof they support, I deliberately left out the insulation around the footings. The arrows simply indicate the transfer of heat. Or you can think of it as the transference of coolth; it’s all heat at different temperatures and, following the law of entropy, doing its best to be the same temperature.

Without insulation, conduction through the dense and massive concrete footings causes the inner wall and floor surface near the footings to be about the same temperature as the earth at this depth, say seven or eight feet. Each spring at Log End Cave, particularly in May, June, and early July, when the earth’s own mass temperature was still low, any warm moist air created in the home would condense on the cold surface temperature at the base of the external walls, causing condensation, also known as sweating. This is the same effect as you get on the inside of your car windows in the wintertime. Your hot breath condenses on the cold inner surface of the windows. It was late July before the temperature of the footings would get up above dew point, and the sweating would stop. I should have paid better attention to wise Uncle Mac.

At Earthwood, the example on the right, we have not had the problem, because we insulated right around the footings with extruded polystyrene. We have had zero condensation anywhere in the home, and the amount of earth sheltering varies from none on the very south side to four feet at the southeast and southwest parts of the cylinder to 13 feet at the base of the northern part of the earth-sheltered portion of the home. We will discuss the correct insulation to use in Chapter 3.

Any direct conduction, particularly with dense materials such as concrete, metal and stone, can be a serious energy nosebleed. The heat loss isn’t even the worst part; the unwanted condensation is the real problem. Always detail some form of thermal break between the house’s mass and the earth, or, for that matter, the outside air. We take for granted the importance of continuous insulation above grade. It is at least as important below grade in northern climates.

INSULATION IN THE NORTH

How much insulation should be installed at the various parts of the building’s fabric? I think that what we did at Earthwood is a very good pattern for northern climates of 7,000 to 10,000 degree-days. We placed 3 inches of R-5 extruded polystyrene – R-15 total – down to “maximum frost depth” (considered to be about 4 feet in northern New York), and 2 inches (R-10) down to the footings. We went with an inch (R-5) around the footings and under the floor. It should be noted that installers of in-slab radiant heat flooring specify 2 inches of extruded polystyrene – R-10 in all – under the floor.

The situation is a little different in the South, as will be seen below.

This rather lengthy discussion of mass, insulation, and the correct placement of insulation is one of the key concepts of earth sheltering. ...

Table of contents

• Praise
• Title Page
• Dedication
• Books for Wiser Living from Mother Earth News
• Acknowledgements
• I NTRODUCTION
• Chapter 1 - EARTH-SHELTERED DESIGN PRINCIPLES
• Chapter 2 - SITING &EXCAVATION
• Chapter 3 - FOUNDATIONS
• Chapter 4 - THE FLOOR
• Chapter 5 - EXTERNAL WALLS
• Chapter 6 - TIMBER FRAMING
• Chapter 7 - WATERPROOFING, INSULATION & DRAINAGE
• Chapter 8 - THE LIVING ROOF
• Chapter 9 - FINISHING THE EXTERIOR
• Chapter10 - INTERIOR FINISHING CONSIDERATIONS
• Chapter11 - PERFORMANCE
• Chapter12 - OUR EARTH-SHELTERED HOME – A CASE STUDY
• APPENDIX A : RADON
• APPENDIX B : RESOURCES
• APPENDIX C : STRESS LOAD CALCULATIONS
• APPENDIX D: METRIC CONVERSION TABLE
• ANNOTATED BIBLIOGRAPHY
• INDEX
• ABOUT THE AUTHOR
• Copyright Page