Design Considerations for Earthsheltering
Paul Shippee is director of Colorado Sunworks and is a solar designer and builder. He was the founding President of the Colorado Solar Energy Association, and a teacher. Paul holds a degree in Civil Engineering, with a major in Structural Engineering from the University of Connecticut. He helped plan housing experiments in energy conservation with the U.S. Department of Housing and Urban Development, and designed and built the SunEarth House, which was the best-rated energy conservation, earth-sheltered home in a HUD-sponsored study. He holds a U.S. patent on a solar water heating system. Paul is currently living in a rammed-earth/strawbale home that he has designed as a personal residence in Colorado. His book, THE LANGUAGE OF SOLAR ENERGY: Heat Loss & Solar Gain for Buildings, is available from his website: crestonesolarschool.com.
Q: My husband and I are looking into building an underground home here in Northern KY. We don't know where to start. Can you point in the direction I need to go? What's the best land? What about draining issues?
A: Usually the best place to start is with what Architects call the Program phase. It’s about planning. You
Q: We are planning to build a home in northern Wisconsin and are trying to decide between single story earth sheltered or earth sheltered basement plus one story. Do you know of any studies which have been done comparing energy costs and building costs between structures (in areas with very cold winters)?
A: (Leonard Jones) The best analysis of energy use in severely cold climates that you might find useful is in James Kachadorian's book, "The Passive Solar House." This book walks through several examples of estimating the energy use for houses located in cold climates and discusses some construction ideas that you may find interesting I can tell you with some assurance that a house with basement will cost more than one without, but the additional space has a much lower cost per square foot than the primary floor. However, building costs in general vary all over the place from one season to the next and from one part of the country to the next. If you are planning to contract for the construction, I'd suggest that you develop a couple of plan alternatives and approach a local contractor for his estimate of cost. If you are going to do the owner-builder thing, you'll have to estimate the quantities of materials and obtain costs from local suppliers.
A: (Mike Oehler) I know of no such studies. Why in the world go above ground when you can get all the wonderful benefits of earth sheltering and still have an abundance of light, air and great views?
Q: I am looking to build a house in Oregon, and would like to make it so it doesn't take away from the environment, or hurt the surrounding forest. Can you suggest some material we can use and if the saturated ground could withstand having a house under-ground.
A: (Kelly) If you are looking to build an earth-sheltered home in a wet climate, this can be done as long as the site isn't a perpetual swamp. Care must be made to design the house to mitigate against possible moisture problems. The best materials to build with underground are those that could not be adversely affected by moisture, such as earthbags, stones, or even ICFs (insulated concrete forms).
Q: Is it possible to have 2 stories underground, and what would you, if you could, build it out of?
A: It is possible to have two stories underground, but it is not feasible (ie, too expensive) for a residence. The structural, excavation, and waterproofing costs would be way too high. This kind of construction is done only for large commercial buildings. They are always made from heavily reinforced thick concrete.
Q: I'm doing some research into building a small earth sheltered home in the Southeastern US. The space will be about 25 - 30' wide and a bit less deep. I am thinking of pouring walls about 12" thick and backfilling them on three sides, southern front, etc. Then for the roof, I would like to make a form, temporarily supported from below that is the shape of an acute arch, maybe how a bridge is arched over a waterway to resist collapse or maybe even a bit more. I have done some rough calculations with this and I am in the ballpark, but would like to see some info from a CE or an architect as well. I am thinking the roof will need to be 12" - 14" thick to support 3' of earth on top and allow for a safety margin for water, etc.. The roof can be permanently supported in the middle from underneath if necessary, but I would prefer to keep the design free span. I am pretty good with trig and calc if you can point me in the right direction or give me some formulas to work with so I can calculate for live load, dead load, span, thickness, etc.
A: You seem to be on the right track with an arch-shaped roof. However, the dead loads are enormous: 3 ft dir t= 300psf, 1 cu ft water = 62 psf, 12" concrete = 140 psf, TOTAL = 500 Lb/sq ft, this is 5 times a normal roof loading. I suggest you hire a structural engineer, P.E. licensed for liability and have him design the rebar and concrete recipe, and most especially the structural "connections" between wall and roof.. all this trouble is well worth it when considering the extreme safety concerns of such a building.
Q and A (Kelly): I am the father of two young daughters and we are getting evicted from our N.Y.C. apartment. I have a small patch of land situated on the top of a shale mountain near Albany, N.Y. To get the material for a road to the site a semi-circular pit was mined into the S.W. side of the slope. Because stone is such a good conductor will I still get the benefits of a "real earth" bermed design: especially heat in the winter!
A: You certainly will get the advantage of berming, even into solid stone. I once visited a home in Utah that had been excavated from a solid rock cliff face, and was told that the indoor temperature varied little from about 65 degrees F. all year round. You will likely want to have some insulation between the shell of your house and the backfilled material to be able to better control the exact indoor temperature.
Also: Since the rock wall is pitched back can I get 2 stories underground (without the sagging weight of earth against the structure)?
Without knowing more about the exact conditions of the site, it is hard for me say, but I would expect that if the excavated stone face is fairly stable, then you would really only have the weight and pressure of the backfill material to withstand. I would advise that you consult with a local engineer about the specifics of the design.
Would a curved foundation wall in the back help direct water away from the structure?
This might be true, depending on the details of how the site is drained. Perhaps equally importantly, a curved wall will much better withstand the pressure from the berm than a straight one would.
Q: My wife and I live in Matamoros, Tamaulipas just across the border from Brownsville, Texas. We pastor a church and are ready to start building a larger building to seat 500 or so, with classrooms, offices, etc. As you can imagine heat is the main factor here. Stifling, humid heat! We only have maybe 15 days a year when it gets cold. We are 20 miles or so from the beach so we do get a nice sea breeze that we want to take advantage of but not sure how to do that. Our water table is high so in-ground building is probably out of the question. Although we don't know much about construction other than the typical "block kiln" type that is so prevalent around here, we are certainly thinking outside the box. One thing we have in surplus are many willing hands to work. Are there any suggestions you might have for us or possibly any book you might refer us to?
A: Even with a high water table, it is possible to take advantage of earth-sheltered building, by simply berming with soil that is brought to the site. So any of the books listed at on the keep cool page might be helpful. Also, there are some concepts used in Middle Eastern areas that capture prevailing breezes with "wind catchers" that channel the breeze past pools of water to help cool the house. I know I have seen them described, but I can't remember where at the moment.
Q: Do you think that Energy-10 would be a good program to evaluate a small earth sheltered home? I have tried Equest but the wizards are too limited to model anything involving earth sheltered elements. I want to know what I can achieve before I build.
A: I am not familiar with energy-10 but if it contains steady state heat transfer assumptions, then it would not be adequate to analyze heat flows in an earth sheltered building --unless some pretty educated modifications are applied to the equations. This is because of the thermal storage dynamics of the earth materials and how this impacts the diurnal temperatures driving the heat losses of the building.
Q: I have acreage in Little Switzerland, NC and am considering the feasibility of building an earth-sheltered home there someday. Is that an area where you have experts or familiarity with the terrain or have you seen any built there?
A: (Kelly) While I am not specifically knowledgeable about your area, I would say that an earth-sheltered home can be built almost anywhere that potential water problems can be dealt with, such as high ground water or saturated soils.
Q: Years ago I came up with an idea for a rather large underground home. The basic shape is like a large baseball diamond. On the bases are large silos constructed of cement (home plate) other for bases. My question is... what sort of construction materials would be strong enough to withstand the soil and a New England winter snow load on the roof. Is this a viable layout? The size of the interior I see as nearly 2 acres. Now I am off to buy a lottery ticket.
A: Reinforced concrete in a dome or other curved shape is best for the structural loads you describe.
Q: My project is an underground museum. My teachers are asking me only one question: How will I provide ventilation to the museum?
There are many ways to provide fresh air ventilation in an underground building. Air ducts can be provided, windows and skylights can be operable, the design can include an atrium that is either open-air or has ventilation built into it...
Q: I am planning on eventually building a house in the lower hills of the Cascade mountains, an hour or so outside of Portland OR. I would like to utilize an earthen design, along with solar power for the house. It would be a two story log cabin style reinforced with concrete with a berm on the back end of the house which would face uphill. Ideally I would like the top of the house to be earthen as well and try to flow rather naturally into the upward sloping forest of the mountain. The three remaining sides will be open and the front would face downhill to the south. With these designs in mind it will require that quite a bit of earth removal be done and an almost 25 ft. retaining wall be built for the back or bermed end of the house. I am concerned with issues of having to dig this deep and running into bedrock, and also whether the retaining wall would be able to withstand pressures of so much earth behind it. I have considered creating a more "stepped" design with the lower level protruding about another 10-15 ft. downhill and the upper level being recessed further into the mountain so as to follow the natural gradient of the mountain and require less excavation. Do you feel that this design is practical at all? If so do you have any suggestions that would help?
A: (Mike Oehler) Sounds to me like the disastrous "First Thought House" which is what I have been trying to get people NOT to do since 1972. This is the design which has kept underground housing from becoming the raging success that it deserves to be. I advise you to read "The $50 & Up Underground House Book" and to view "The Low Cost Underground House Workshop and Shelter Seminar" videos.
Q: I have been a home builder in Georgia for many years and have decided to look into building a vacation home in the North Georgia mountains. The idea of building an earth sheltered home intrigues me. However I do have some questions about the "roof" systems that are used. What types are there? I want to use a precast concrete type (to keep the basic flat ceiling look). In my research I cannot find anyone who uses this as a method. Why? If I am to place earth on top how much? How is it spread? I ask because of the weight factors of light grading equipment such as a "bobcat"; what type of roof will support that machine weight and the weight of the earth too?
A: When I built the SunEarth House-earth covered passive solar 100% heated home (visit http://www.crestonesolarschool.com ) --I evaluated several roof systems to hold up the 12 inches of soil placed on the roof with a bucket drag line (like steam shovel) with an adequate reach out over the roof. The most economical roof structure at that time, in my area of the country, turned out to be steel bar joists, with a modest concrete slab on top of steel decking, then hot mopped with asphalt, 3 layers, then the earth cover. I wouldn't run a bobcat on the roof. Precast concrete will also work, like twin T's, when properly designed, but cost comparisons with other systems are in order.
Q: I live in Phoenix Arizona. There are periods in the summer here where the nighttime temperature does not go below 100 degrees F.! This nighttime temperature is well above the comfort zone. One curious problem that I have had with my research, is concerning the relative stable earth temperature below ground. This relates to questions about earth sheltering, cooling tubes, and geothermal heat pumps. There is not consistent agreement with what depth the earth's temperature should be stable and what this temperature might be. On one hand if I reference Mike Reynolds or your website under the "Keep your Cool" link, the earth's temperature is a nearly stable 50 to 60 degrees 4 to 6 feet all over the world. The other school of thought relates to the "average yearly temperature" or the "mean annual air temperature". The most official link to this is energy.gov which is from the Energy Efficiency and Renewable Energy website. In this link it is quoted "The temperature of the earth at depths of 20-100 feet (6.1-30.5 meters) remains about two to three degrees higher than the mean annual air temperature. At depths less than 10-12 feet (3.1-3.7 meters), earth temperatures may be strongly influenced by air temperatures and may vary during the year, depending on the locale. Near the surface, earth temperatures closely correspond to air temperatures." I have seen many variations, if not hundreds, on these two schools of thought. I am hoping that 8 feet deep is enough. Got any ideas? The average yearly temperature in phoenix is 73 degrees F. and the summers are quite warm with many days over 110 degrees F.
A: There is no way to cool a house in your climate except by using earth tubes, or night sky radiation with roof ponds. The earth temps you should measure yourself --right now in late summer for best reliability. You are not interested in Phoenix mean temps; only a temp profile at 6, 8, 10 and 12 feet down at your site that you measure yourself.
Q: I am in something called Lego League, and for our presentation, we have to come up with a problem, solution, and presentation, for CLIMATE CHANGE, in our area--aka. Houston--, and the problem I (and some of my team) came up with is wasting energy because it is hot and we have to crank up the air conditioning! So, I came up with this idea to either build houses underground (that won't work--you dig two feet and ya hit water!), in hills (not many hills here), in caves (not many caves here, either!), or cover the houses with dirt (might possibly work!), so in the summer, the dirt (covered in grass to make it look nice) would keep us nice and cool, like Super Insulation, and we wouldn't have to crank up the air conditioning and waste energy; thus saving money and energy! Would it work?
A: Yes, it would work. In fact several earth-covered homes have been build already in Texas. You might be able to research and find some of these homes and read about some evaluations of their performance as part of your project. Earth cover is more than just super insulation; it is earth that stays cool when connected to the ground. Another thing to look into is bringing cool air into the home via earth cooling tubes. These are large metal or plastic pipes 8 ft down in the earth which bring outdoor air into the home, cooling it by the natural deep earth cool temperatures.
Q: I'm looking at a piece of property in a canyon that I love. I want to dig back into a steep hillside so that my home will be 2 stories tall on the south side but with the majority of the roof and 3 sides underground. Is this feasible with earth packed tires? Pretty tall! Thinking timber inside.
Don't use tires! I have designed a two story home for a client in Nebraska that seems similar to what you are planning.
Q: Home for me is an off-grid cabin atop a hill in the wilderness of Ontario, Canada. We have a cold and snowy 5-month winter and a hot 3-month summer. I dream of building underground, a south-facing home at the peak of the hill. I love the idea of building with tires, but am having a hard time finding a reference to a place where this has been done. Is there a reason? Are tires a poor choice for building underground?
A: Tires work fine underground. Most Earthships are basically underground. Earthbags also work well underground, and are easier to work with; there is more information about them at www.earthbagbuilding.com .
Q: I am in the very early planning stage of having an earth sheltered home built on my 160 acre farm...I do not have a hill to dig into. I would like to have passive solar, and I would like only 1 level, as my 90 yr old mother will live with me and I want it to be accessible for all. Any hints or advice you can give would be so very appreciated.
A: Well, I designed and built just such a building, the SunEarth home, in Colorado on flat ground some years ago. This home is famous as the best performing solar home in the USA (as documented by the US Dept of Energy -you can access this data). You could purchase the as-built plans for this home, attend my CRESTONE SOAR SCHOOL to learn the details of how it is done, or ask me to consult with you on your venture into earth covered passive solar. Whatever you choose, I wish you luck in achieving sustainable & renewable energy independence in your domestic life!
Q: I was interested in an underground home, but my idea for one is sort of different I think than what you are offering. I was thinking of totally underground, maybe 6 feet under, 2 story, 4 bed 2 bath, with dining and kitchen. Totally concrete and a garage on top of it. Could you tell me how much it would be? I imagine it would be a lot. I'm just looking for an estimate...not going to actually do it...just interested.
A: (Kelly) It is hard to say how much this might cost, but it certainly would be more than the going rate per square foot for conventional building in your area. A two-story underground home with a garage on top would have to be engineered with a lot of steel reinforcement and concrete...and that is expensive!
Q: Are you aware of any clear economies to be had by building a cut/cover "domed" or "tunnel-shaped" structure (in lieu of a rectangular solid of some kind)? My thinking here is that earth-pressures would more easily be borne by these quasi-arched shapes, and also that traditional formwork might be virtually dispensed with using this approach, in favor of (say) polyethylene-sheet 'forms', rebar curved to fit the shape...and lots and lots of gunite?
A: (Kelly) I think that you are right the a curved roof of some sort would likely require less steel and concrete to support an earth-sheltered roof. One idea that I have had to accomplish this that could be quite economical and effective would be to bury a prefabricated steel vaulted shell (like a quonset). Some of these are actually designed for direct burial (see http://www.americansheltertechnologies.com/ ). This could be insulated on the outside with the volcanic stone if required...
Q: I have searched the net till my eyeballs are falling out trying to find information on earth-berming the concrete block side walls of an existing house. I think I have adequate information on water-proofing & insulating the walls, but do you know of any publications on what kind of reinforcing could be done to in-place walls so they could withstand the lateral stress of the retrofitted earth berm?
A: (Kelly) This sort of engineering question often best answered by an expert who can actually examine the specific situation. In general, it is possible to buttress walls against lateral stresses with either other interior walls placed at right angles to the existing wall, or by other shorter buttresses. To some extent this can also be achieved with the top plate or bond beam or ceiling/roof attached to the wall...if the wall is sufficiently reinforced with concrete and steel. But this is where professional inspection may be needed. It would take some considerable study of the engineering of concrete structures to figure out yourself...so look for such a text book if you want to get into it.
Q: I have a site with two hillside excavations dug out years ago for an old dam on a nearby creek. Each hole is about 20x40 x 8-12 feet deep, sloping. The holes are open (no roof) and are separated by a 6-10 feet of un-excavated "rib" from the hill. I dug a 4ft. (un-roofed) connecting passage at the back between the two excavations. What would you suggest I build for roof support? Straight flat beam or arch? I have seen a historical bunker website for curved buried bunker iron beams that could be 6 feet underground! I don't know if anyone could even get this type today. I got several 15" telephone poles a few years ago for central supporting pillars. My original plan was to use these with some kind of straight beam roof support. I am also interested in the benefits of arch support, maybe even quonset metal arch. I know that arches can be much better load bearing than mere beams.
A: (Kelly) Certainly curved or arched shaped buildings are stronger in many ways than more linear or rectilinear structures; nature has demonstrated this abundantly. Any building needs to be well thought out and engineered, and this is particularly true with underground structures. I personally would consider arched or curved forms for direct burial over other shapes that rely on the tensile strength of overhead beams. One approach that might work for you, which you suggest, is to use a steel prefabricated quonset-style building as the basis for your structure. There is at least one company that advertises that they engineer their steel building for this purpose: www.americansheltertechnologies.com/
Q: My husband and I have lived in our earth home for three years. The roof is a flat roof that had just rotted out. I was wondering if you would possibly have any suggestion for a new roof?
A: Apparently you have (had) a wood roof, since it rotted. Is the structure that is holding up the earthen roof intact or is there a structural hazard? Sounds like you may have to dig up the dirt on the roof to see the condition of the waterproofing. Better remove all the dirt and check every square foot. Then add 6-8 inches of foam board and leave the dirt on the ground, unless you really want an earthen roof; then you have to pay to remedy the leaks, put in new waterproofing, as certain if the structural supports are safe, etc... In other words, think about why you want an earthen roof? Is it worth the expense? Good foam board (not the bead board kind) will insulate your roof better. Then you can leave the earth berming at the walls in place.
Q: My wife and I are designing a 1000 square foot house in the Cascade mountains just south of the Canadian border. The winters are cold (average in the 20's with 3 feet of snow) and the summers are warm (high 80's to 90's). Our property is sloped and lends itself very well to berming. As such we were considering setting the structure back into the hill so that dirt comes up about 6 feet on three of the sides. At this point we are wanting to use a conventional, non-dirt roof. The only problem is that to sink the structure back into the hill we have to face the building to the southwest (with a greater emphasis on the west). Many neighbors have used this approach with great success. On the other hand we can build above ground and face true south, but the resulting structure would be 1200 square feet split over two levels. Additionally we are concerned that the above ground design would stick out like a sore thumb on the hillside. I realize that there are many variables, but in general which approach will yield the best result? I have not found anything discussing the trade-offs of direct gain passive solar vs berming. Most likely this is due to the fact that they should be used together. Any help would be greatly appreciated.
A: I have not been to your building site, but it seems worth while to design a house that can soak up winter sunshine directly from the south. Is there no way to accomplish this in your hillside? The berming on three sides cuts heat loss for sure, but similar results are achieved with enough insulation. I have designed angled saw tooth type windows to face south on some building projects, for example. There are many benefits to getting south noon winter sun to enter a building. The trade off with berming on a heat loss vs solar gain basis only is about 50/50, but morning sunshine coming in is hard to give up. I would try to bend and shape the house to get as much of each. It is a design challenge. I say this without seeing your land...
Q: I have a farm in KY and I'm thinking of building a house that is between an earthbag and subterranean. I was thinking the walls would be earthbag. I honestly don't know how I'm going to be doing the roof at this point but I want the roof to be dirt covered (any suggestions on how I can do this would be appreciated). I've built with wood before but I don't think that wood joists would be the best thing to put there and have the dirt on top of them. Number 1, What are possible things I need to watch for with Kentucky for building codes? Number 2, Does anyone have any ideal how I can build the roof so that I can plant grass on top of the house. Number 3, Flooring. I saw something about tamped earth. Is this possible using only materials I would find on my farm. I have 13 acres. I am hopping to build this with as little money spent as possible.
A: #1 Get familiar with, understand, and follow the Kentucky building codes. They are there for your safety mostly. You will need a structural analysis by a structural engineer to support any amount of dirt on your roof. This is for safety reasons so it does not fall in on you when it rains.
#2 Grass grows in most any dirt, best to plant the kind that needs the least moisture.
#3 Tamped earth works for a floor only if the earth contains around 20% clay and then has coatings of warm linseed oil and mineral spirits 50/50 applied to the surface. Good luck, and watch your structural assumptions!! Safety first!
Q: I would like to build a pre-engineered metal building but with earth bermed around three walls. The exposed wall will face south. Any special considerations related to shielding the metal from the earth and the moisture that it will contain?
There is one company that manufactures metal quonset buildings that are specifically designed for direct burial: http://www.americansheltertechnologies.com/s_c_earth_covered.html . In some personal communication with this company, they revealed the following: You are correct that your earth covered building will use Galvanized steel. We suggest that the earth covered buildings be grouted into a keyway or slot foundation for water proofing reasons. Galvalume cannot be used with a keyway/slot foundation since it would be in contact with uncured concrete. Additionally, earth covered buildings over 10' wide use a minimum of 16 gauge steel which is not available from US steel mills in Galvalume. Earth covered Wonder Buildings use Grade 40 steel which has the right combination of strength, formability, and ductility for a self-supporting arch building application. This is especially important with an extreme load buildings. Our earth covered Wonder Buildings use either our 9-1/4" deep sheet or our 14" deep True-Arch panel.
Q: We had a look at some land today and were advised it has a very high water table for 1/2 the year. We had originally planned to have our dwelling partially below ground level and we would still like to do so. We have some ideas as to how that may be possible, possibly by having a two water proof membranes separated by about a yard all around and under the build so as to keep a thick dry thermal layer.
A: (Kelly) I think that the best way to deal with a high water table when you want to go underground is to basically build on the surface but then berm, or push soil around the sides (and even overhead, if you want). This gives you most of the advantages of going underground without the danger of water intrusion.
Q: How deep do you usually recommend having the earth above your home for optimum energy efficiency?
A: The depth of earth on the roof is a trade-off between heat/cool protection and the structural cost to hold it up. One foot deep is minimum, two feet maximun.
Q: We live in North Texas about one hour Northwest of Fort Wroth. My wife and two children have sold our current house and we are wanting to build a earthbag home. The location we are building our earthbag home on has no trees. We will have to plant. Also in Texas it can be very hot in the summer and cold in the winter. What direction would be best to face our house to maximize the energy of the sun. Also, what would be the best way to heat and cool in the North Texas Climate.
A: (Kelly) You are in an excellent area to benefit from underground construction because the average temperature about 6 ft. down is 68 degrees...quite comfortable. In other words, with an underground (or earth bermed) home all you would need is a small amount of sunlight coming in from the south to bring the temperature up to about 72 degrees. I think that you could build such a home using earthbags filled with your local soil and do fine.
Q: I am strongly interested in building an earth sheltered home but need direction as to design and construction. Can you help? I am in north Texas and am wanting to get the process started and building soon.
A: The biggest challenge is structural safety in earth sheltered homes. This means working locally with a structural engineer after you have soils analysis done at your site.
Q: I wish you could look at (and comment on) the Sami houses, called GAMME, built by the Laps/Same people in the north of Norway and Sweden. I have heard that they can keep cool inside in warm summers, and warm inside in mid winters with just a small wood burner, even in temperatures lower than minus 40 degrees Centigrades.
A: Yes, these are wonderful examples of the basic principle of taking advantage of the stable underground temperature by digging into the ground and surrounding the house with thick walls of turf. I know somebody in Patagonia who has emulated this style and is living very happily in a similar house.
Q: I'm a final year student of architecture currently working on my thesis project. As part of this project a large span gallery of sorts is underground and the roof is accessible, allowing people to walk over the structure at ground level. So I was wondering what kind of structural systems I should be using for the entire structure and also what would be the best way to span the structure. I'm not sure if I can use a space frame roof structure.
A: Steel bar joists would be worth looking into for that, or for larger spans, concrete double T beam/slab commonly used in parking garages.