Introduction: Tiny Underground Solar House

I started building this tiny home in 1984 and I've been living in it continuously since 2009. I never built a house or anything this big before. i did all the labor except heavy earth moving. The house itself cost about $7000 (some prices from the 1980's) and if I include a 600 foot road, utility connections and furnishings the total construction cost was about $10,000.

Step 1: View From the Path, View From a Ladder

I wanted to start building before I had a clear vision of what it would look like. I enjoy the technique of doing a step I think I know how to do, and expecting that having that much done will make it easier to figure out the next step. So the first step was buying land where I would not need to consult building inspectors (Smoky Mountains, far east Tennessee.)

Step 2: Original Site Grade

I cut the trees on a spot about 40 feet square, and hired a bulldozer operator to scrape the top soil uphill so I could put it on the roof later. Then he excavated a flat platform about 6 feet below original ground level where dog Sophie sits. He pushed dirt downhill where it could be retrieved for later use in the berms.

Step 3: Foundation and Floor Insulation

I rented a ditch digging machine and cut a 2 foot deep ditch around the future floor. That's a rectangle 20 feet East-West and 24 feet N-S. In the ditch is vertical styrofoam insulation, 2x8 foot sheets one inch thick. You can see the top edges of two layers of blue styrofoam. Don't try this with any insulation material that absorbs water. The purpose of insulating the top 2 feet of the floor is so sunlight and room heat will be absorbed in that volume.

Outside the insulation I filled the trench with coarse crushed rock. There is an extension of the trench downhill from the SouthEast corner. That drains the whole trench and keeps the floor slab dry.

The foundation beam you see lying North-South is 8 inches wide, 6 inches thick, and 24 feet long. As you can see in the end of the beam, it is a sandwich made of 1x8 inch planks on the outside of two 4x4's. There will be another beam like that under the west wall, and 20 foot beams across the N and S ends. The 20 foot beams will lie flat on the ground. The 24 foot beams are tilted inward to be square with the bottoms of the arches. Before putting earth on top of the structure I hammered the crushed rock under the tilted beams to pack it tight.

Step 4: Thermistors Monitor Ground Temperature

There are 16 thermistors buried at various depths attached to the styrofoam. Their wires come to screwhead terminals in the SE corner.

Some day I will analyze the thousands of resistance measurements I've made and see if the temperatures are really warmer inside the insulation, on top of the floor, compared to 1 foot below the surface outside the insulation.

Step 5: Raising Arches

re photos 2-5 above, sorry about the low resolution of those stills taken from VHS camcorder in 1986. (Now the movie is below, in the comments from woodlandgrace.).

I assembled each arch in place flat on the ground then lifted its center joint onto a temporary prop. I pulled it up to vertical using a cable and pulley which is not visible, attached to a tree at the far end of the house.

Angle iron bolted to the 24 foot beams keeps the bases of the arches in line. Arches are spaced 2 feet apart.

I wanted the inside layer of planks to slope at 60 degrees so I had to cut a few feet off the top ends of the 12 foot planks (photo 1). The outer layer of planks was sloped at an angle to minimize waste. LOTS of nails anchor the planks to the 4x4s. I pounded a nail thru every outer plank where it crosses every inner plank. All those nails are driven from the outside and don't penetrate the inner planks so you see no nails from inside.

Step 6: Patterns and Fasteners

Each arch is made of four 8 foot 4"x4"s and two 8 foot 2"x4"s. Six ends of the 4x4s are cut at 67.5 degrees. The bottom ends of 2 of the 4x4s are left square. This makes half an octagon. Photo 1 shows two steel jigs to guide my circular saw at the angle for the miter. I needed two jigs because a 7" circular saw blade will not cut all the way thru a 4x4, so another cut must be made from the other side with the other jig. Also many drill bits will not go all the way thru a 4x4. Maybe it would have been easier to use 2x4s.

Holes thru the 4x4s are drilled near the cut ends and the ends are held together with a U-bolt (photo 2) 4 inches wide, with threads on both 5 inch legs. The bolt is 1/2" thick, shown with a nut on one leg.

The 2x4s are cut into 32" pieces and bolted as paired gussets across the joints of the 4x4s. Each joint needs two bolts 7" long and 3/8" diameter. Why is this 7" bolt bent? (Suspense.) Photo 3 is a piece of white plastic I used as a pattern to cut and drill each 32" gusset. Two corners of the 2x4 must be cut to keep them from sticking up above the 4x4 where they would interfere with the flat surface where planks will be nailed. The bolt holes in each 32" gusset are 28" apart.

Step 7: Cover First Half With Earth, Watch It Settle

9 arches have been erected in photo 1, and planking completed on them. The roof planks extend 2 feet North of the first arch. The planks make a ragged edge on the south end because I intend to add several more arches. I decide to test the strength of the structure before building the rest.

I covered the planks with two layers of styrofoam, using the 2'x8'x1" sheets, and several layers of plastic. The thickest plastic I used was 6 mil, I should have used thicker. The outer covering is a slick blue plastic tarp.

I was not sure when I threw the first shovel of dirt (photo 2) onto the roof whether it might slide off, but it stayed.

You see deflection measuring gauges set up inside the house. There were 3 gauges, each with 4 boards nailed to each other at angles so one end of each board rests on the ground and the other end comes close to the middle of a 4x4. The gauge does not touch the house. The upper end of each gauge board is marked parallel to the 4x4 which it almost touches. If the 4x4 bends while earth is loaded on the walls and roof I will be able to tell by comparing the new 4x4 position to the gauge marks.

I hired backhoe operators to pile dirt on the roof (photo 3) and berms while I watched the deflection gauges. The 4x4s did not bend perceptibly while 8" of earth was added on top. It was done carefully keeping in mind that if too much weight was added to the berms, the roof might break upwards; or if too much weight was on the roof the walls might deform outward. To assure that the bottom 24 foot beams under the East and West walls did not spread apart, I had two 1" steel cables running E-W across the floor temporarily tying those beams together.

With 8" of earth on the roof (maybe 10 tons?) (photo 4) and the berms high enough to keep it there, all looked good. I urged grass to grow, wanting roots to keep the roof dirt from washing down.

But what happens when the earth is soaked with rain, and a foot of snow? I let it age in place in the 1990s while I did other things.

Photo 7 is an aerial photo looking down, and photo 8 is looking up from inside.

when I jump on the roof peak (photo 9) it doesn't wobble or boom. It feels and sounds like jumping on solid ground.

September 2023 update -- earth moving by critters

Scritch, scritch, scritch Who is digging in my roof? I thumped the ceiling with a broom handle. It was a dark night and hand light was inadequate so when I investigated I only glimpsed a big skunk (I hope) leaving into the forest. In daylight I could see skunk had dug out a paper wasp nest and left behind some empty cells (photos 10,11). Skunk didn't damage waterproofing but dug a tunnel a foot long. I'm glad to see lots of root development and that the soil depth is not decreasing, I didn't know.

Step 8: Make It Stronger

So, after 10 years of weather and weight, does it look like the first 9 arches (made of 4x4s at 2 foot spacing) are strong enough?


I used the same technique to add 4 more arches and I reinforced the first 9.

Here is a parts list for the major lumber:

13 arches : #52 4"x4"x8' and #26 2"x4"x8' and #39 U-bolts (4"x5"x1/2") and #78 hex bolts (7"x3/8").

4 foundation beams : #22 4"x4"x8' and #16 1"x8"x12'

2 layers planking : #106 1"x8"x12'

reinforcement for the arches : #20 4"x4"x8' and #12 2"x4"x16' and #3 1"x12"x12' and 8 steel stanchions.

That total includes 94 4x4s and 122 1x8s. All the lumber is pressure treated for ground contact without rotting or insect damage.

That bent 7" bolt in step 6 was one of a dozen gusset bolts that I needed to replace after 10 years of settling. I pounded out the bent bolt and drilled new holes in the gussets to match the existing hole in the 4x4 and put in a new bolt. The new gusset holes are maybe 27" apart instead of 28", to fit the new angle between the 4x4s. It is less than 135 degrees (twice the 67.5 miter) because the 4x4s are bowing inward.

Some wall 4x4s were obviously bowed, and one was partly cracked. I added a second 4x4 to the bending ones.

In one case (photo 1) I pushed hard on the east wall to shift as much load as possible from the original 4x4 to the reinforcements. Here I am testing my escape route before I expand the hydraulic jack behind my left knee. It is backed up with 2 axle stands, all lifting a 6x6 timber anchored against the opposite wall. As it rises it pushes a 4x4 with great leverage against the middle of the east wall. There are other beams pushing against the joints between the roof panels and the west wall, because I did not want those joints to move while I pushed the east wall.

I added a reinforcing 4x4 between the midpoints of stressed 4x4s of several arches. The arch 4th from the south end (photo 2) got a complete set of these reinforcements, adding a second half-octagon rotated 22.5 degrees. (See the ancient Rainbow Bridge in Jilin China for a structure with rotated 3/8 octahedrons.)

I was at the dumpsters at the right time to find 10 steel stanchions with 10 adjustable shelf supports, the kind that support a grocery aisle of canned goods. I added 8 of those as verticals further supporting the new reinforcements. The sink (photo 3) was a dumpster find too, trivially easy to install on two of the stanchions and shelf supports . I was guided where to place it by roof leaks. I collect the leaks into the sink.

I added 12 rafters, each a 16 foot 2x4 (photo 4). These are high enough to walk under, low enough to use as an overhead shelf.

With hardboard panels screwed to them (photo 5) this shelf does not have enough headroom to be called a loft but I can comfortably crawl under the top gussets and sit up between them.

Step 9: Keeping Earth on the Roof

It was a challenge to get the south ends of the berms high enough to keep the roof soil up. For the SW corner
I tied 7 horizontal logs in a pattern like a spiral staircase.

The SE corner was the highest berm and most difficult. At the recycling store I found a nylon net about 7' x 35' and I stretched that over the south end of the roof, folded up the ends, and filled the folds with dirt. At other places I filled nylon sacks (like for 10 pounds of potatoes) with dirt and tied them up near the top of the berm. Each sack was tied by nylon string over the roof to another sack hanging on the other side of the house, like a pair of saddlebags. The hope is to keep earth stable on the steep edges of the roof long enough for roots and ground cover to take hold.

There was a bad landslide on the roof that I repaired by adding timbers lying on top of the roof on both sides. Earth piled along those timbers can not slide down because the timbers are tied together over the roof peak with strong wires.

Step 10: South Windows

By the time all the reinforcing had been done and another 10 tons of earth added to the south end, I had windows on hand but didn't know how they would fit. They were just such a great bargain that it was obvious I should design the walls around them. My strategy of "build first - plan as you go" had lucked out, because I had avoided a major expense for a passive solar house: the big south window.

I had looked in multiple places for big windows and failed in finding anyone with 4WD able to deliver them intact. While I completed the load bearing vault I still had no plans for windows and waited for inspiration.

I guess the city replaced the glass wall around the community swimming pool and that's how a load of 4'x8' frames with single pane tempered glass came up in a local auction. I got 12, including matching aluminum frames for screens, delivered, for $180. I doubled them and used 6 in the south wall and 4 in the north wall.

I found a double pane glass door at the dumpster which was exactly the width needed to complete the south wall. It had no hardware so I built an overhead track (photo 2) and lower track (photo 3) and put my childhood roller skates (photo 4) under it.

In photo 3 you get a good look at the yellow fiberglass 2x4 material I found so useful. I've seen it used most often in the rails of ladders. The recycling shop had enough for a small house, $.50 for an 8 foot piece. It has the strength and dimensions of a 2x4 but it is a C-channel shape, and lighter than a wood 2x4. It can be drilled and sawed, but expect nasty sharp particulates in the dust.

Step 11: North Windows and Filling the Gaps

Seeing how well the 3 panels of 4x8 glass plus the random sliding door fit in the half-octagon shape, you might think I had planned that.

Nope, the octagon is the size it is because I started with 8 foot 4x4s. I made scale drawings of actual house and window dimensions and slid them around in image manipulating software using layers until I found the obvious fit.

I got a load of grocery store freezer doors (double pane, magnetic gaskets, $50)) to fill the north wall (photo 2) and make opening transom windows for north and south walls (photo 3).

At local noon on the winter solstice, sunlight reaches a little more than halfway thru the house (photo 4). At the equinoxes at noon sunlight only reaches the southernmost 5 feet of the floor. At the summer solstice the south windows are entirely shaded by leaves on big trees to the south. Even without trees the extension of the roof 2 feet south of the window would shade the window at noon. So in winter I get the most sunlight warming up the inside; and summer is underground in forest shade.

To fill in the non-load-bearing holes above the windows I used Durock cement board. It's intended for other purposes. This 1/2" thick fiberglass reinforced cement comes in 3x5 foot panels that can be cut and screwed like fireproof waterproof plywood. If you want a straight line you can score it with a razor knife and break it. I needed a slight curve to match a stressed roof so I used a circular saw, the dusty way (photo 5,6). Special self-tapping screws make installation easy with a power driver.

Step 12: Grate Grace, Retaining Walls

More grace - I found two pieces of heavy expanded iron grate in the ashes of a demolished grain mill.

I got the idea for a triangular air vent by noticing how exactly that heavy grate fit without cutting it if I put it where it obviously belonged (photo 1).

The other piece I only bent a little to get the ramp I needed from outside ground level over the south threshold. It is an excellent mud-scraper ramp in front of the door (photo 2).

Looking west along the north wall (photo 3) you see the vertical retaining wall that holds up the north end of the west berm. Why don't those vertical posts lean over? They are tied together with custom-bent pieces of 3/8" steel reinforcing rods usually used for embedding in concrete. They are also tied with rods to a timber buried 10 feet deep in the berm. All the scrap from cutting planks was used in these retaining walls.

You can see (photo 4) that lycopodium flabelliforme (aka diphasiastrum digitatum aka fan clubmoss), looks like green spiders) has crept in from the forest and is growing up the berm, onto the roof, and draping down the retaining wall at the north end of the east berm. Photo 3 shows the north bank is bare, but by now lycopodium has covered it and is climbing the north window. Unlike invasive kudzu, it is native, older than pine trees, and it won't climb more than a foot up other vegetation. It stays green all winter. I encourage it.

Step 13: Level Floor, Free Furnishings

It was hard to get the floor level. I measured the distance from dirt to a laser beam from a laser level in the center, and marked the discrepancy with 1 bottle cap per centimeter. Red caps mark areas below level.

I put 6 mil plastic on the ground, then many short boards which I got for pennies from the recycle store. Smooth 4'x8'sheets of hardboard on top makes a floor that is easy to sweep (photo 2).

The future bathroom (photo 3) is assembled for < $50 in a 5 foot square. That's a christmas tree storage box that will become a shower stall.

They don't let anybody take metal from the dumpsters any more, but if you find an abandoned washing machine or dryer you will often find the side panels come off easily and can be used for an enameled table top, like the one east of the front door outside. There's another panel-table inside (photo 4). The upside-down gutted housing of a 4 foot fluorescent light fixture hangs from the wall as a CD rack. A very heavy shelf of books hangs by wires from the gussets above.

And if you see a refrigerator, take the plastic drawers (photo 5) and the removable frame that supports them. When your walls slope inward it is very easy to hang the frame. Typically I have 10 pounds of flour in one drawer and pounds of onions in the other, and the flat glass surface on top could be loaded as heavily as it was in the refrigerator. Don't hang the drawer frame from vertical wires, or it will swing like a pendulum. Instead make the wires cross at angles, that will hold the frame rigidly. I can open a drawer without the egg rolling off. I have 10 drawers mounted so far, they are sturdy (photo 6,7).

It is easy to fit small shelves with cubby holes against the wall between the 4x4s. Guys, don't forget to add a urinal pipe so you don't have to get out of bed (photo 8).

Step 14: Light Inside, Invisible Outside

It is nicely bright inside since the north wall is glass too. When there is snow on the bank north of the house, sunlight reflects in from the north (from the right in photo 1.) I didn't expect that.

My 8 year average electrical consumption is 414 KWH/month. I don't use my electric and kerosene portable heaters because I can tolerate 55 degree temperature inside. I run a dehumidifier in winter to keep 75% humidity. Without any heating except sunlight and waste electricity, the maximum temperature difference has been 6 degrees F outside and 47 degrees inside. In summer it's been 96 outside and 79 inside, so I don't need air conditioning. I do feel breezes inside with doors closed and transoms open. The inside temperature at the peak of the roof is the same as at 4 feet above the floor.

The house is practically invisible from 30 feet away. Photo 2 contains an identical photo pair (looking SW) with blue lines on the right side indicating the shape of the buried house.

... We paused before a House that seemed A Swelling of the Ground -- The Roof was scarcely visible -- The Cornice -- in the Ground -- ... -- Emily Dickinson

Photo 3 is looking East, showing the house's west wall and roof outlined in blue.

Photo 4 is another photo from the NorthEast like photo 2. The bright spot in the middle is the floor inside, seen thru the north windows.

The ramp to the north peak of the roof is visible in 2,3,4 and more clearly seen in photo 5 looking south over the roof. The blue handrail goes down to the north door. The south door is at original ground level. The north peak of the roof is about a foot higher than the original ground level 24 feet North (uphill). So the ramp starts from the undisturbed ground north of the house above the excavated bank. It goes slightly up to reach the north peak of the roof, about 12 feet above the floor level.

Photo 6 is looking in the south window at night. Photo 7 shows the view out the south window while a deer grazes.

That's all I have time for before the Tiny Homes contest deadline!

I hope it gave you some ideas.

"I learned this, at least, by my experiment: that if one advances confidently in the direction of his dreams, and endeavors to live the life which he has imagined, he will meet with a success unexpected in common hours. He will put some things behind, will pass an invisible boundary; new, universal, and more liberal laws will begin to establish themselves around and within him; or the old laws be expanded, and interpreted in his favor in a more liberal sense, and he will live with the license of a higher order of beings. In proportion as he simplifies his life, the laws of the universe will appear less complex, and solitude will not be solitude, nor poverty poverty, nor weakness weakness. If you have built castles in the air, your work need not be lost; that is where they should be. Now put the foundations under them." -- Thoreau, Walden

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