The Gist of This Earth Sheltered House

Not all earth sheltered houses are created equal. Just as with the three little pigs, one can find earth sheltered houses made of straw, sticks, and bricks. Build a house of straw and be prepared to repair it often. Build it of sticks and repair it occasionally. Or build it of concrete and rebar and know that it will last a long time.

I wanted a house that would require minimal maintenance and repair in our golden years, so I reasoned along these lines: Spend money up front for an energy efficient and reliable house and save on energy and maintenance in the future. With the cost of energy and materials guaranteed to skyrocket in the future, I think this was a better choice. Hopefully I'll be enjoying my later years in a more comfortable environment.

It is well known that the temperature of soil twenty or so feet below ground level is nearly constant at the average annual temperature for that location. And the closer to the surface one gets, the more variation around the average annual temperature will be found. These phenomena occur because it takes, on average, about six months for energy to travel twenty feet through soil. As explained by Professor John Hait in Passive Annual Heat Storage, Improving the Design of Earth Shelters, the change in average daily temperature of soil as one goes farther below ground tends to lag the average daily temperature of the ground at the surface. At the surface, the phase difference is clearly zero days. Just above twenty feet below the surface, the phase difference approaches six months.

This is what makes Passive Annual Heat Storage (PAHS) work. Insulate the soil around and over an earth sheltered house out about twenty feet in all directions, and the average annual temperature inside the house will settle in at around the average annual temperature in the area. In Peoria County that would be about 50.7°. But not many people in Peoria County would like living in a house year-round that has a constant temperature of 50.7°. Brrr.

So what can be done to raise that average temperature to something more agreeable, such as 68° or 72° or whatever temperature you and your special ones agree upon? What about a furnace or heat pump? Just burn enough fossil fuel to pump enough energy into the thermal mass trapped under the insulated umbrella to bring the average annual temperature up to 68° or 72° or whatever you like. Once you pay the cost of getting the temperature up there, the amount of energy needed to keep it there will decrease significantly, depending on how well the exterior-facing part of the house has been insulated and how effective your insulated umbrella and other parts of the PAHS system are.

Is there a more energy-efficient or greener method to do this? What about burning wood? If one plants trees to replace those that are consumed, then one is using a renewable energy resource. We installed a masonry stove in our house that burns wood at a high temperature and quite efficiently at that. Our timbered property has many dead and undesirable trees such as honey locusts and hedge apple or osage orange. The primary energy consumption is cutting the wood with a chainsaw and hauling it to the house.

But we are also using the sun as a free energy source when we need it in the colder months. Conditions weren't ideal for placing our house on the southeast-facing hillside. We would have liked one long and narrow house with the long exposed side facing directly south to maximize solar heat gain in the colder months and minimize it in the warmer months. But we had to break the house into two parts, with the exposed side of the west half facing 15° east of south and the exposed side of the north half facing 20° south of east.

The south-facing walls have a net window glazing area of about 200 square feet, and the east-facing walls have a net window glazing area of about 320 square feet, which includes a fairly large sunroom. This is 520 square feet of effective window glazing in about 2500 square feet of floor space, or a little more than 20% glazing. That number is quite large by many standards and would overheat houses with small to moderate amounts of thermal mass, even on the coldest sunny January day. But our house is surrounded on nearly every side by hundreds of tons of thermal mass—all that concrete and soil trapped inside the insulated umbrella. Even the south and east-facing exterior walls of the house will be covered by 4" of masonry brick, as will be the north and south side walls of the east-facing sunroom.

Topography Map of the Southern Portion of Our Property

Aerial View of the Southern Portion of Our Property

The elevation on the above topo map runs from about 520 feet in the valley (grey) on the far right to about 640 feet on the plateau (light yellow) on the far left. The map shows a nearly level shelf (yellow) on the east side of our property, and this corresponds to the mostly treeless area inside our property lines in the aerial view. The dark, comma-shaped area is a 1/3 acre pond. The house sits about 150' west of the larger Morton building, which has a white roof, and this puts it about the same distance north-northwest of the smaller building, which also has a white roof.

These two images show that our property has a nearly flat, circular shelf of about 3 to 4 acres at about the 556' elevation, surrounded by hills on the north, west, and south sides. The earth sheltered house sits roughly on the northwest edge of this shelf at about the 576' elevation level. Thus we have a nice view in the south and east directions out over this shelf.

I did a solar profile from the south side of the house and found that it has nearly unlimited access to the sun from early morning, almost yearround. Trees do block some of the early morning sun in the warmer months, which is good for reducing unwanted solar heat gain on the east-facing walls. However, in the afternoon, trees on the hillside west and southwest of the house will block the sun as early as 2-4 p.m., depending on the time of year. Thus we tend to lose some direct sunlight in the afternoon, but that is good for the warmer months, since the sun swings more to the northwest and drops behind the treeline even sooner.

But there are some other accidental benefits associated with the way we laid out the house design and oriented it on the property. Since the exterior side of the west half of the house is oriented 15° east of south, the winter sun can enter the south-facing windows earlier to provide more heating in the morning hours. This makes up for part of the losses in the afternoon. In a similar manner, the exterior side of the north half of the house is oriented 20° south of east, so the rising winter sun shines almost perpendicular to these walls, providing maximum early-morning heating, especially in the sunroom, where it is most welcome.

Plenum and Masonry Stove in Entryway Between House, Garage, and Sunroom

A plenum, in the upper, left corner of the above image, is installed behind the brick wall to the left of the masonry stove in the entryway room. Ductwork, running the full length of the house, will evenly distribute heat from the stove or the sunroom and entryway when needed. Consider a cold winter day where the sun rises brightly over the east-southeast horizon. It shines directly into the sunroom through its roof and into the entryway through an equivalent of 80 square feet of window and door glazing between the two rooms. When heat is wanted from the sunroom and entryway, two sash windows are opened between the house and sunroom and the doors are opened between the sunroom and entryway. This creates a complete air circuit between the sunroom, entryway, and the rest of the house through the air distribution system. The plenum fan is turned on and heated air is drawn from the sunroom and entryway through the plenum and distributed to all the rooms in the house. Cooler air from the house then returns to the sunroom through the two open sash windows to be reheated by the sun.

Plumbing and Electrical Inspection

The house will soon be ready to spray insulation in the outer walls and install drywall. But before that can be done, it must receive the seal of approval from the plumbing and electrical inspectors. I invited them out on August 31. The day was quite warm and dry, so there was no mud to contend with. I was up on the roof connecting another pair of PAHS air tubes when they arrived around 11:30 a.m.

This was the third and maybe next-to-last plumbing inspection and the first electrical inspection. Let's see if I can find a photo of what the plumbing inspector saw the first time he visited last year.

December 17, 2009—Rod Egli Installing Drains for Bathrooms 1 and 2

Here is Rod Egli, our plumber, installing underfloor drains for bathrooms 1 and 2 next to the office. It isn't apparent from the photo that the entire area had been covered by several inches of snow just a few days earlier. Note the rolled up plastic and straw along the back wall, the groups of vertical rods protruding from the ground, the green tube sticking out of the wall on the right, and the square column with a ladder leaning against it in the upper-left corner. The green tube is one of the eight air-movement tubes forming the PAHS system. The square column is composed of some concrete forms surrounding a rebar backbone that will be braced vertically and eventually filled with concrete.

December 8, 2009—Recently Poured Concrete Covered by Insulation and Plastic

Right after Davis Caves had poured a lot of concrete, the weather turned nasty and cold. The ridge along the left side of this photo is a tent assembled from 4' x 8' sheets of 2.5" styrofoam board and covered with 6 mil plastic. Under that ridge are footings running along the south side of the house. In the foreground is a similar flat-roofed tent covering several 7' by 7' by 2' reinforced concrete pads that will support some of the vertical columns to be poured in a few weeks. There is at least a 1' deep airspace under these covers, and even though the tents are covered with snow, the temperature below them stayed around 50° due to the heat generated by the chemical reaction of curing concrete.

The area covered by straw in the background is where the plumbing will be installed the following week. Under the straw is a layer of 6 mil plastic to keep moisture out of the gravel where the pipes will be laid. The weather got really cold, and even with the straw and plastic, some of the ground froze where the plumbing was to be installed. Rod used a pickaxe and auger to help break up the frozen ground where needed.

The vertical rods protruding through the straw in the above photo identify three locations where support columns will be poured later. Under each cluster of rods is another 7' by 7' by 2' reinforced concrete pad that had been poured a couple weeks earlier.

Things weren't looking much better when the plumbing inspector was called out the second time about a month later.

January 19, 2010—Second Installation of Plumbing

There had been a lot of snow and freezing rain and then just plain cold rain before an insulated blanket and straw cover was rolled back so that Rod Egli could install the plumbing for bathroom 3, utility room, and kitchen. And then it turned cold and miserable. Everything was wet, and our waterlogged gloves were virtually worthless. The plumbing inspector didn't complain. Note the water line next to the wall at the left edge of the photo and the two green PAHS air tubes protruding through the wall near Rod.

All PEX Hot and Cold Water Lines are Insulated Below and Above the Floor

High grade PEX tubing was used for all water lines below and above the floor. The lines were insulated for three reasons. First, because the entire volume under, over, around, and inside the house is part of the tempered PAHS system, the insulation will minimize heat transfer between the lines and environment, potentially conserving energy and minimizing disturbances to the living area. Second, we will have an 80 gallon electric hot water tank with a 4500 watt heating element and an internal heat exchanger installed in the utility room next to the incoming water line. On the roof will be a 52 square foot solar collector connected through a 6" diameter tube to the heat exchanger. Our goal is that the energy expensive heating element will be used only on the rare occasions of extended cloudy days. Since bathrooms 1 and 2, and the kitchen sink and dishwasher are some distance from the hot water tank, insulating the water lines means that less energy will be wasted when drawing hot and cold water on an occasional basis. Third, some of the cold water lines run above the ceilings, and the insulation will minimize the potential for condensation forming on the tubes and dripping onto the drywall and suspended ceiling panels.

September 2, 2010—View of Bathrooms 1 and 2 From the Office

Here is a sample of what the pluming inspector saw on his third visit. An enclosed acrylic tub has been installed in one bathroom and an enclosed acrylic shower in the other. Insulated lines have been run to the showers, stools, and lavatories. Stool and lavatory instalation will be completed after the drywall is installed, finished, and painted and the tile floors are laid.

September 2, 2010—Water Central in the Utility Room

This image of water central was taken inside the utiliy room. The incoming water line through the floor is closest to the corner. Next to the incoming water line in the floor are three insulated water lines running under the floor to the kitchen island—hot and cold lines to the sink and a hot line to the dishwasher. In the next large tube in the floor to the right are another three insulated water lines running under the floor to water faucets installed in the walls—hot and cold lines to the sun room, and a cold line each to exterior south and east walls. Most of the other red and blue water lines run to the bathrooms.

Just to the right of the tubes coming up through the floor will stand the 80 gallon hot water tank. Above the tank is a 6" diameter tube running through the roof for bringing in the plumbing to connect the 52 square foot solar collector above the house. And just to the left of the tubes coming up through the floor, way back in the corner, will stand iron filter and water softener units. On the left wall, just this side of the white drain and clean-out pipes in the floor, visible in the lower, left corner of the photo, will be a utility cabinet and sink. The insulated hot and cold lines to the sink run through the 2 x 4's in the wall.

The plumbing inspector was mostly happy with what he saw and required only one modification.

September 2, 2010—200 Amp Panel Awaiting Wires

Electrical is another story. I hired a professional electrician to install the electrical panel and the underground cable running up it. And to save a bundle of money, I decided to do all the wiring myself. Although I'd obtained a degree in Electrical Engineering some 43 years ago, that didn't qualify me to be an electrician. So I purchased a few books on wiring a house to meet the 2008 National Electrical Code, because Peoria County, Illinois follows it very closely. Then Patricia and I spent some weeks figuring and arguing over what we wanted inside and outside the house. We put everything down on drawings and shopped for electrical supplies. It took me a couple months of climbing up and down latters a few thousand times to get all the boxes and wires run. At the rate I worked, I'm sure that no one would pay me more than $2 an hour to wire their house. The above photo shows about two dozen circuit wires waiting to be hooked up by the electrician.

September 2, 2010—Planting Switches on a Cement Column is a Challenge

When all was said and done, the electrical inspector liked my work. He wrote up only a few minor corrections and omissions I needed to take care of. He was especially pleased that I'd labeled everything and that all cables were organized and layed out neatly. When he asked how I was able to do the wiring, I replied that I'd purchased some books on the 2008 NEC code. He commented that he wished more electrical contractors would do that.

First Line of Defense—Drain the Water

In the last post I showed the gash in the hillside where the house will sit.

View of the Excavation Site From the Southwest Corner Looking Northeast

Here is a second photo of the excavation site showing the pure clay and vertical banks formed by the excavator. On this dry, sunny September morning, the area looks so inviting, but that can change so quickly when the rains come. Uncontrolled water infiltration into and through the surrounding area may be the number one enemy of earth sheltered houses and Passive Annual Heat Storage (PAHS) systems. My biggest concern before the excavation began was that we would encounter a rock ledge or discover an underground spring. Fortunately the largest rock I saw was about 6" in diameter, and I didn't see any gravel or wet spots. Just a lot of clay as this image shows.

The problem with pure clay is that it's about the worst type of material to build in or on or sometimes even near, because its extremely small particle size and smooth surfaces yield low shear friction angles when wet. This means that wet clay will typically support less ground pressure than other types of soil, so foundations may more easily slide or sink when the soil is saturated with water. And the steep, exposed banks along the backside of the house may break free and slide down when they get wet. Some of the exposed banks are over 20' high, so the excavator stepped them back to reduce the severity of mud slides. Concern for worker safety was paramount.

Orange Stakes and Lines Locate the Walls and Outside Perimeter of Footings

Before the house footings could be poured, water and electrical had to be run underground from a Morton building about 150' away. In addition, a 4' deep French drain had to be installed around the perimeter walls that would face into the hillside. Another French drain would be installed down the center of the structure. And before all that could be done, the house profile had to be staked out and the outside perimeter of the footings marked with orange paint. An orange stake and part of the perimeter is shown in the above image. The water and electrical lines had already been installed when this photo was taken. Part of the trench for the French drain had already been dug as can be seen at the left edge of the photo.

The 2" PVC Electrical Conduit and Insulated Waterline

This image shows part of the 2" PVC electrical conduit and insulated waterline run from the Morton building in the distance up to the construction site. Part of the waterline was insulated to minimize heat transfer between it and the tempered soil beneath the floor and on the east side of the building that will be part of the PAHS system.

Laying a 4" Drain Tile 4' Deep Around the Perimeter of the Structure

The first task was to run water and electrical lines underground to the house as shown in the previous photo. This image shows the end of the waterline coming up vertically near the inside corner of the house where the utility room will be located. The waterline was enclosed in a box formed of 2.5" thick styrofoam insulation. To the right of the mini-excavator's blade can be seen a 2" PVC tube that will eventually hold three heavy copper wires for a 200 amp service. An electrical cable with four outlet plugs was initially threaded through the tube to supply power for construction. Part of the orange electrical cable is coiled around the end of the PCV tube.

The mini-excavator dug 4' deep trenches as close to the banks as possible so that they wouldn't interfere with the house footings that would go in later. After the drain tubes were laid, the trenches were backfilled with one-inch gravel to insure adequate drainage and support for the foundation. These outer drain tubes are the first line of defense for the footings and poured concrete floor. Any water that finds its way through the soil to the footings area will descend to four feet below floor level, keeping the immediate volume of soil under the footings and near floor level much drier.

Backfilling the 4' Drain Trench that Runs Down the Center of the House

As shown in this image, a second 4' deep trench was dug down the centerline of the house, running from one end to the other. A four inch drain tile was laid in this trench, and a second trench and tile were teed off it and run to daylight. This extra drain under the house will ensure that over time the soil beneath the concrete floors stays very dry.

During the incessant rains over the following months, thousands of gallons of water have flowed out of these drain tiles. They have done their job admirably.

September 30, 2009—The Rains Came and the Clay Banks Gave Way

This photo shows what a lot of rain in a short period of time can do to steep clay banks. I had draped 6 mil plastic over the banks and installed silt fence in a vain attempt to protect them from the water. But it got in anyway, and down they came. This collapse occurred right next to the waterline and electrical conduit shown in earlier photos

September 30, 2009—First Footings for the Back Wall Were Poured in the Mud

This view shows the first back wall footings. The edge of the waterline's insulation box can be seen in the lower, right corner, and the collapsed bank area in the previous photo is off to the right.

October 13, 2009—A Few More Footings Poured and a Lot of Mud

Almost two weeks have gone by and it has been too wet to get much done. That lovely first picture has been transformed to muck, and this is what we can expect right on up to mid summer 2010.