Earth-Sheltered Houses: How to Build an Affordable Underground Home

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?

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. Wooly-minded thinking in this matter can cause great problems in the home at the design stage, even before a shovel of earth is moved. So, I’m going to say it one more time, in a slightly different way, with the hope that something twinks with readers who are still unclear on this point: If we do not insulate properly between the home’s mass and the earth’s mass, we lose our right to easily control the home’s temperature. The home would perform thermally like a giant rock set in the earth. Placing the insulation on the interior of the block or concrete wall is a mistake and doesn’t help at all with regard to using the home’s mass storage potential to our advantage. In fact, an interior insulation would further decrease the temperature of the mass fabric, and if moisture finds its way through this interior insulation, there will be condensation and mold problems which will not be readily accessible, a real catastrophe.

And, too, by insulating on the wrong side of the wall, earth can freeze up against the building and cause structural damage. This occurrence happened to us at the concrete block basement of our first owner-built home, Log End Cottage, when we erroneously insulated on the interior of the block wall. In all of my books, I have made a point of sharing both successes and mistakes. I’m not entirely stupid, so, for the most part, I’ve learned from my mistakes. But the reader can be more than smart. You can be wise, and learn from the mistakes of others, including mine.

INSULATION IN THE SOUTH

As we build further south, insulation strategy should be adjusted, for two reasons. Firstly, at the depths that we set earth-sheltered homes, earth temperatures in the South will be warmer in both summer and winter. With the higher earth temperatures, dew point in the form of sweating is less likely. Second, the further south we go, the more important cooling the home becomes in terms of both comfort and energy cost. By lowering the amount of insulation, the moderating effect of the earth’s temperature performs more effectively as a means of natural cooling.

I am from the north and have no personal experience with earth sheltering in the South, but based upon the experiences of people I’ve spoken with, case studies, and other research, I would suggest an adjustment to insulation placement as per the following two paragraphs. I do not think of myself as the definitive authority on the subject by any means, but am confident that the recommendations below will be an improvement on the insulation recommendations given for northern climes above.

In areas with climates of 4,000 to 7,000 degree days, I would insulate with two inches (R- 10) of extruded polystyrene down to the footings, one inch adjacent to and under the footings, and one inch under the floor for a distance of four feet in from the footing. Leave the insulation out from under the rest of the floor to promote natural cooling. If in-floor radiant heat is to be used in a concrete floor, get advice from a local radiant heat installer, who might suggest an inch of insulation under the entire floor, or even two.

Fig. 1.3: Degree-day map of the United States.

In areas of less than 4,000 degree days, insulate with an inch of extruded polystyrene down to the footings, and leave it out around the footings and floor.

Finally, in states bordering the Gulf of Mexico, atmospheric humidity levels can be extremely high, approaching 100 percent a good part of the time. In areas like this, it is almost certain that some form of dehumidification system will be required in the earth-sheltered home. The good news is that earth-sheltering will save a lot of energy on the operating costs of air conditioning.

Figure 1.3 gives a degree-day map of North America (the United States). North of the border area, in Canada, it wouldn’t hurt to go with 2 inches (R-10) around the footing and under the floor, if the building budget allows. It’ll pay for itself in a few years.

THE EARTH ROOF FACTOR

It is not absolutely necessary to put an earth roof or lightweight living roof on an earth-sheltered house, but not to do so, it seems to me, is a great opportunity lost.

I share seven different advantages to earth roofs with my students, and pull out a timeworn placard to illustrate the points, which are:

1. Insulation. I can hear it now: Hold on there, mate, you just told us earth is poor insulation! Well, yes, it is. Author and earth-shelter owner-builder Dan Chiras reckons earth is worth about a quarter of an R (.25-R) per inch of thickness, and that rings true with me, particularly for earth a few inches down. But the first three or four inches of earth, where the plant roots aerate the soil, is considerably less dense and, therefore, has some insulation value. The grass or wildflowers – don’t mow ‘em – also flop down in the autumn and add more insulation. And, finally, the earth roof holds snow better than any other roof surface, and light fluffy snow is worth a good R-1 per inch of thickness. We notice that our home is even cozier and requires less fuel to heat with a cap of two feet of snow overhead.

Fig. 1.4: Author Rob Roy and his timeworn placard.

2. Drainage. With non-earth roof systems, you need some sort of drainage system to remove a lot of water quickly from the roof during a downpour: gutters, downspouts, storm drains, etc. The earth roof drainage – particularly where the roof drains at a single pitch directly onto berms, such as the Log End Cave design – is slow and natural. Even a freestanding earth roof, like the one at Earthwood, must fully saturate before runoff must be attended to.

3. Aesthetics. The earth roof is hands-down the most beautiful roof you can put overhead, particularly one of natural wildflowers.

4. Cooling.The sun beating down on most roofing causes high surface temperatures. You can literally fry an egg on some of them. The living roof, however, stays nice and cool because of the shading effect of plants, the mass of the earth, and the evaporative cooling effect of stored rainwater. Stick your finger into the living roof and you can feel the coolth.

5. Longevity. Built properly, as described in Chapters 7 and 8, the roof will require very little maintenance. We don’t even mow ours anymore. All other roofs are subject to deterioration from the ultraviolet (UV) rays of the sun, from wind and water erosion, and from something called freeze-thaw cycling. In our climate near Montreal, most roofs are subjected to between 30 and 35 freeze-thaw cycles each winter, and each occurrence breaks the roofing down on the molecular level. Sun, wind, and frost never get to the roofing surface, so, protected by the earth from these adverse conditions, the waterproofing membrane is virtually non-biodegrad-able. It should last 100 years or forever, whichever comes first.

6. Ecology. While not the right place to grow shrubs, trees or root vegetables, the earth roof can support all sorts of plants and microbial life. Instead of killing off – say – 1,500 square feet of the planet’s surface to yet more hot, lifeless black tarscape, we can return the home’s footprint to cool green oxygenating living production. We’ll discuss vegetation options in detail in Chapter 8.

7. Protection. Just a few inches of earth afford all sorts of protections not found with other roofing surfaces: fire, radiation, and sound, just to name three. In combination with a Log End Cave-type berm, the earth roof can also contribute to tornado, hurricane, and earthquake protection, as well.

Fig. 1.5: Wildflowers in bloom atop the Earthwood office.

The seven advantages to an earth roof all occur with just a few inches of earth on the roof. Doubling the thickness, from – say – 6 inches to 12 inches does not double the value of the advantage. With fire and sound protection, for example, extra earth beyond six inches adds little advantage; you’ve still got windows, doors and some portion of above grade walls influencing these considerations. But doubling the earth does double the potential saturated load of the earth component of the roof. And this extra load greatly increases the structural cost of the home. I accent timber framing (also called plank and beam roofing) as the most suitable roof structural system for the inexperienced owner-builder. Other options, such as pre-stressed concrete planks (which are very expensive and must be installed with a crane), or poured-in-place reinforced concrete roofing (which should be professionally installed), not only add greatly to the structural cost, but also don’t look nearly so nice overhead as a ceiling.

Roy’s General Theory on Earth or Living Roofs

This is as good a time as any to tell you about the realities of the earth roof with regard to its weight, or load. Calculating the desired load is the first step towards designing.

But, first, a clarification of terms might be in order. I use the term earth roof to describe a roof system that relies primarily on a certain thickness of earth or topsoil to nurture the desired vegetation or ground cover. A living roof might have earth on it, or it might have some other growing medium for the plants, such as straw. Or it might combine earth with another medium, as we have done on the roof of our straw bale guesthouse. Lots of work has been done in the past ten years in both Europe and North America on these alternatives that eliminate or considerably diminish the need for placing heavy earth overhead. And the reason is usually an effort to keep the structural cost down. I will share some of the methodologies in use in Chapters 7 and 8.

Because of the heavy weight of saturated earth, my theory for 25 years has always been – and still remains – that we want to use enough earth to maintain the green cover, and not a whole lot more. The reason, as we will soon see, is that wet earth is very heavy, and a great depth of it – while technically possible – adds unacceptably to the structural cost for owner-builders who want to own the home themselves, and not have a bank own it for them.

At Log End Cave and at Earthwood, we had good success with maintaining a green cover (mowed grass at the Cave, wild at Earthwood) with an earth roof with a final compacted depth of about six inches of soil. A couple of times at Earthwood, during its so-far 23-year life, the roof almost died off during drought. We never watered it. But always, after some compensating rain, the roof would come back and flourish once again.

Back in the 1970s, several builders were placing from 18 inches to 3 feet of earth on the roof, and, yes, they engineered the structure properly to support that kind of load. But these homes were very expensive, with a good part of the expense caught up in the roof support system. Why they did it remains a little unclear to me. After you’ve got an honest 6 inches of earth on the roof, the seven advantages listed above are present. If additional insulation is desired, earth is a poor choice. An extra inch of extruded polystyrene, which weighs practically nothing, will yield as much additional R-value as an extra foot of earth. And neither do the other advantages listed increase proportionally to the use of greater amounts of earth.

HOW MUCH DOES IT WEIGH?

Now, let’s do the numbers as they say on a popular National Public Radio program. Essentially, there are two different loads that need to be added together to arrive at the grand humungous total for which the building must be engineered: the dead load and the live load. The dead load is sometimes called the structural load, and refers to the fabric of the building itself: the rafters, the planking, the waterproofing membrane, insulation, the drainage layer, and the like. Firstly, any building we design has to be able to support itself. Over the next few paragraphs, I’ll be referring to Table 1 (p. 17), which lists the weight or load of various common materials.

The Earthwood dead load consists of 5-inch-by- 10-inch red pine rafters, 2-inch-by-6-inch tongue-in-groove planking, the Bituthene® 3000 waterproofing membrane, and four inches of Dow Blueboard™ insulation (an extruded polystyrene). Everything above the Styrofoam® we’ll put under the classification of live load, as its weight is subject to change. The dead load calculations are on page 16.

I’ve put everything else, even the crushed stone and earth, under the classification of live load (also on p. 16), because all of these components vary with conditions.

This is kind of a worse case scenario, and that is what we have to engineer for. Note that most of the load is live. If you have difficulty picturing 150 psf, think of 1,400 people packed in on a 1,400 square foot roof (Earthwood with overhang). Each person averages 150 pounds in weight and each occupies a square foot. People are live loads, too, but it is unlikely that they will be up on the roof at the same time as the maximum snow load.

Now, a couple of points must be made here. The first is that in life’s reality, just when you think that things can’t get any worse, they do. I have been told by a reliable source that the ski resort of Whistler, British Columbia, must engineer roofs for a snow and ice load of 350 pounds per square foot. They get several feet of snow, then rain and a freeze, then more snow, then more rain and freeze, and several feet of very dense snow and ice can accumulate on the roof. And these are not even earth roofs. So Plattsburgh’s 70 pound snow load, while rare, could, under extreme conditions, be even worse. This is why people physically (and wisely) shovel excess snow off their roofs during extreme snowstorms, especially anyone living in a mobile home. Note also that with the 6-inch-thick earth roof at Earthwood, almost half of the total maximum structural load is the snow load, so find out what this load is for your area, and keep in mind the earth roof holds snow better than any other kind. This is a positive characteristic in terms of insulation, but a possible negative with regard to load.

The second point is that when engineers do their stress load calculations carefully and accurately, using the right formulas and the right unit stress numbers for the grade and species of wooden rafters and girders (or metal beams or pre-stressed concrete planks or whatever), they will have built in a safety factor of approximately six. This is not overbuilt. This is the kind of redundancy that is considered to be good engineering.

Dead Loads

Rafters: A 5-inch-by-10-inch red pine rafter weighs about 8.4 pounds per linear foot. Although it is a radial rafter system, it is fair, for purposes of load calculation, to figure an average spacing of 24 inches center to center, designated as 24 inches o.c. If rafters were 12 inches o.c., a linear foot of rafter would support a square foot of roof, and the rafters themselves would add 8.4 pounds per square foot (8.4 psf). However, at 24 inches o.c, each rafter supports 2 square feet, so the load per square foot would be exactly half, or 4.2 psf.

4.2 psf

Planking: We used 2-inch-by-6-inch tongue-in-groove planking, which weighs six pounds