What is a Passive Annual Heat Storage (PAHS) System?

John Hait's book Passive Annual Heat Storage, Improving the Design of Earth Shelters provides a detailed description of a PAHS system,  illustrates how to design and build one, and includes numerous warnings about how to avoid mistakes. For a summary overview of PAHS, see Umbrella Homes.

To better understand how a PAHS system works and what it does, it is helpful to elaborate on each of the four words, Passive Annual Heat Storage. For those without a scientific background, some of the theory behind PAHS may seem somewhat complex, so I've tried to simplify the explanations. John Hait devoted several chapters to the subject, and I will use only a few paragraphs, so please bear with me.

Passive: A properly functioning PAHS system should require a minimal amount of fossil fuels for heating and cooling, such as gas, oil, or coal. Warmth can enter the living area when the sun radiates energy in through the windows, when the sun radiates energy onto the doors, windows, and brick siding installed over the exterior walls, when heat energy is conducted in through the walls, doors, and windows that are in contact with warmer outside air, when warmer air leaks in through cracks or flows in through ventilation systems, and when heat energy is conducted in from warmer thermal mass surrounding the living area. Thermal mass is the large volume of concrete, steel, water, and soil surrounding the living area, which, in turn, is surrounded by an insulated umbrella. Warmth can leave the living area when interior objects radiate energy out through the windows, when interior objects radiate energy onto the exterior walls, doors, and windows, when heat energy is conducted out through the walls, doors, and windows by contact with colder outside air, when warmer air leaks out through cracks or flows out through ventilation systems, and when heat energy is conducted into the surrounding cooler thermal mass. The large volume of thermal mass enclosed within the insulated umbrella can store huge amounts of heat energy from the living space (warm up) and can release similar amounts of heat energy to the living space (cool down) as needed to moderate the inside temperatures.

Annual: A PAHS heating and cooling system is influenced by the annual climatic conditions surrounding the house and thus never quite reaches a steady year-round operating state. However, if an adequate amount of insulation is used in the umbrella and in the exterior walls, if a sufficient number of windows are properly placed to admit sunlight when needed, and if adjustable awnings and insulated drapes or curtains are used to block sunlight and heat transfer when not needed, a comfortable mean annual temperature within the living space can be obtained, which will vary by only a few degrees Fahrenheit (°F) over the year—a little cooler in the winter months and a little warmer in the summer months. On a daily basis, insulated drapes or curtains can make a big difference. Windows have lower resistance to heat energy flow or heat conduction (R-value) than do walls and doors, and they allow various types and amounts of radiant heat energy to more easily pass through in both directions. Cold windows absorb more radiant heat energy from people than they give back, so a person will feel colder when near cold windows. Insulated drapes will substantially reduce heat transfer through the windows and increase their effective R-values, thereby making the living space feel more comfortable. Drapes can also be used to temporarily block unwanted sunlight from occupied areas for increased comfort.

Heat: Heat energy is a mysterious entity. Each atom of everything around us—solid, liquid, or gas—has a specific amount of vibrational energy. Temperature is a measure of atomic vibrational energy level; the higher the vibrational energy in a solid, liquid, or gas, the higher its temperature and the warmer it will usually feel. Heat energy always wants to flow from warmer to cooler atoms or from warmer to cooler materials in general. Heat transfer processes are very complex and consist mainly of conduction, convection, and radiation. They are illustrated in Hait's book. Conducted heat flows between adjacent atoms and materials in contact, such as the warming or cooling sensations one feels when touching warmer or cooler objects. Convected heat energy moves as heat energy is carried in moving gasses or liquids passing over surfaces or intermixing to transfer by conduction, such as when a fan blows warm or cool air past your body and you warm up or cool down. Radiated heat energy is emitted from all atoms on the surfaces of all objects and is absorbed by all atoms on the surfaces of all objects, similar to the way antennas emit and absorb electromagnetic energy. You feel the warmth of the sun, because it radiates more energy to you than you radiate back to it. And you feel cold standing next to a cold wall or window, because you radiate more energy to them than they radiate back to you. Hait also describes a fourth method of heat transfer, important to the PAHS systems, which he calls "heat transport." In heat transport, heat energy may be carried into or out of a porous solid, liquid, or gas by the flow of another liquid or gas through it. It is similar to convective heat transfer. The biggest danger to a PAHS system is uncontrolled groundwater flowing through the soil under the insulated umbrella and adding or removing heat energy when it is not wanted.

Storage: As noted above, everything around us—solids, liquids, and gasses—is composed of atoms, which constitute mass. Atoms come in different sizes and glob together to form millions of different molecule types. Atoms and molecules may be packed together very loosely to very densely,  where mass or weight per unit volume describes a material's degree of density. Generally the denser a solid, liquid, or gas, the more atoms it will have per unit volume and the more heat energy it can store and release per degree of temperature change. The amount of heat that can be stored or released per unit volume and per degree of temperature change is called specific heat. Water, concrete, iron, and soil are high specific heat materials and can store and release large amounts of heat energy per degree of temperature change. These materials are said to have high thermal mass. Air and insulation are low specific heat materials and can store and release only small amounts of energy per degree of temperature change. These materials have low thermal mass. Each solid, liquid, or gas also has a thermal conductivity rate, which is a measure of how fast heat energy is transferred from warmer atoms or molecules to adjacent cooler ones. It takes a finite amount of time for a warmer atom or molecule to transfer some of its vibrational heat energy to an adjacent cooler atom or molecule. Scientists have determined that it takes about six months for heat to travel about twenty feet through soil, and this delay is the primary mechanism behind the PAHS system, which will be described later. A PAHS-based earth sheltered house contains thousands of tons of high thermal mass material in and under the floor, in and behind the walls, and in and over the roof. This material can store huge amounts of thermal energy from the living area and give it back when needed. The thermal mass of the surrounding water, concrete, iron, and soil acts like a giant flywheel, slowly absorbing and releasing vibrational heat energy from and into the living area to keep it comfortable.

Every place on Earth has an average annual temperature, which may range from more than 80 °F near the equator to well below zero °F near the poles. The average annual temperature could also be taken as the average of the average daily, weekly, or monthly temperatures at a location over an entire year. In the Peoria, Illinois area the average annual temperature is near 51 °F, which means that the soil temperature about 20 feet below the surface will remain close to 51 °F year-round. Temperatures in the soil surrounding a typical earth-sheltered house in the Peoria area would fluctuate up and down around 51 °F, falling in the colder months and rising in the warmer months. If the temperature in the living area were allowed to follow the fluctuating soil temperature, it wouldn't be very comfortable for the occupants. That is why insulation is usually placed on the inside or outside walls, roof, and floor to isolate the interior from the soil and why auxiliary heating must be provided to moderate the inside temperatures.

Covered Insulated Umbrella Along the South Side of the Earth Sheltered House

Insulated Umbrella Along East Side of Earth Sheltered House Ready for Dirt

Part of Insulated Umbrella Covering the Earth Sheltered House

More of the Insulated Umbrella Covering the Earth Sheltered House

In an earth sheltered PAHS house the insulation is moved out from the walls, roof, and floor into the surrounding soil to trap a large volume of thermal mass between these surfaces and the insulation. Two layers of insulation are typically sandwiched between three layers of 6 mil or 0.006 inch thick plastic vinyl. The above photos show part of the insulated umbrella placed around and over the earth sheltered house. Insulation is placed above the roof about two feet underground and extends about 20 feet out beyond the walls of the structure. It is also spread out from the exposed walls, two feet or so below ground level, to about 20 feet. The insulation layer is thickest over and near the house, and gets thinner farther out. In this application the insulation thickness varies from 5 inches down to 1.5 inches. The thicker insulation over and around the structure prevents most of the heat from passing through it, where the temperature differential is the largest, and the thinner insulation farther out also prevents most of the heat from passing through it, where the temperature differential is much smaller. Soil temperatures near the outside edges of the insulation will be about the same as temperatures in the ground farther out, but the temperatures under the insulated umbrella progressively closer to the walls, roof, and floor will approach a nearly constant and more comfortable value, which is controlled by the occupants. Temperatures remain stable near and inside the structure because it takes about six months for heat to travel 20 feet through soil, so the effects of cyclic annual temperature changes in the soil at the outer edges of the insulation never appear at the shell of the house, which is 20 feet away from the uninsulated soil at the edges of the insulated umbrella.

Brick Walls Going Up Around Windows and Patio Door on South Side of House

Windows Surrounding the Front Door on East Side of House

Our House In The Hill uses several methods to adjust the average inside temperature. The primary heating method allows the sun's radiant energy to pass in through the south and east facing windows shown above and uses insulated drapes to keep the accumulated energy from escaping at night and on cold, cloudy days. Adjustable insulated drapes help to control the flow of radiated, convected, and conducted energy through the windows in either direction. In the warmer months awnings are swung out over the south-facing windows to block direct sunlight, and they are retracted in the colder months to allow the sunlight in.

The sun's radiant energy entering through the windows is the primary source of heat, but its availability is controlled by several factors. Thus the sun's energy may not always be sufficient to maintain a comfortable living space. Cloudy days may hinder heating when it is most needed. Our house has about 200 square feet of effective glazing oriented 15 degrees east of south and about 86 square feet of effective glazing oriented to the east. Effective glazing is taken as 80% of gross window glass area to account for light blockage around the edges. The house sits on a southeast-facing hillside, so it receives full sun in the morning. Hills and trees block the sun after 3:00 p.m. or so in the colder months. Orienting the south-facing wall 15 degrees east allows more sunlight in through the windows in the earlier hours before it gets blocked by trees in the afternoon.

Masonry Stove in Entryway With Plenum Above Left and Lots of Thermal Mass

Part of the Air Distribution System Above the Kitchen and Away Room

Sunroom Between House and Garage Awaiting Polycarbonate Roof

A masonry stove, shown above, is located in the entryway between the house and garage. Its several tons of thermal mass and the many more tons of thermal mass in the walls, floor, and ceiling around it, will store heat from the stove and from the sun for later use. A plenum is located in the brick wall adjacent to and above the stove, and ductwork from the plenum (see the second photo above) runs throughout the house to distribute heated air to all the rooms as needed. Also shown above is the 16 foot wide by 12 foot deep sunroom that will have a 16 foot wide by 12 foot deep sloping clear polycarbonate roof, oriented 20 degrees south of east. The sunroom and the entryway directly behind it sit between the house and garage.

View From Sunroom Through Windows Into House

View From Sunroom Into Entryway Through 12 Foot Wide Double Patio Door Opening

During cooler months, the rising sun shines directly into the sunroom and reaches back into the entryway through ample doors and windows. See the above photos. In the last photo above, plywood scaffolding rests on the horizontal beams and obscures an additional 12 foot wide by 2 foot high transom window opening above the doors. Opening the two windows between the house and sunroom and the two sliding doors between the sunroom and entryway (not yet installed in the photos above) will allow air to circulate from the house through the sunroom and on into the entryway and plenum, so that warm air can be distributed throughout the house. The only cost for this auxiliary heat will be the electricity required to run the air distribution fan.

Top View of House Showing Air Tube Layout, Except the Tubs Entering the House

The exterior walls of the house are insulated to R40 and covered with bricks on the outside. The walls are airtight, so uncontrolled heat loss or gain by air leakage is minimized. Fresh air and additional heating and cooling within the living area is controlled by a set of eight 6-inch diameter air-tubes buried in the ground. The above image shows the approximate tube layout except that the tubes running into the house are not shown. This air-circulation system will be described in more detail in another post. The R40 insulation determines the steady resistance to heat transfer through the walls when a constant temperature difference exists between the outside and inside. But the bricks' ability to absorb, store, transfer, and release heat energy in response to the sun's radiant energy, air movement, and changing outside temperatures, gives the walls an apparent higher dynamic resistance to heat flow. Thus the dynamic R value may be somewhat higher than the steady R40 value, and this can save additional heating and cooling energy throughout the year and make the house feel more comfortable. Increasing R values keeps the inside wall surface temperatures closer to room temperature, so we feel more comfortable near them.

Reducing humidity to comfortable levels in the warmer months without the aid of auxiliary energy is a challenge for a PAHS system. Primary dehumidification is accomplished using the eight air tubes configured in heat exchanger fashion. To form a heat exchanger, the tubes are divided into two groups of four each, with the upper four placed directly above and in contact with the lower four over much of their average 200 foot lengths. Air always flows in opposite directions through the two sets of tubes, and there is usually a temperature difference between them along much of their length. So if one set of tubes is bringing in warm, moist air and the other set is taking out cool, dry air, the warm air will give up some of its heat to the cool air along the way by conduction. If the temperature drop in the warm air is large enough, some water might condense out in the process, but the humidity level will be near 100% if it wasn't there already. This air entering the house would feel cold and clammy, not a good prospect at all.

As suggested in John Hait's book, we used a large cold storage area to further cool the incoming air so more moisture could be condensed out and the humidity reduced to comfortable levels. In the image above, the north arrow points up and to the right at about a 45° angle. On the north side of the attached garage, buried deep in the hillside, is a large root cellar with a separate 40 foot by 40 foot insulated umbrella embedded in the ground above it. The root cellar can be seen to the right of the garage. The various tubes terminate in two 4' by 4' pits, one located in the garage and the other is in the root cellar. Two sets of four tubes each come in from the south (the curved set of tubes outside the house) to the pit in the garage. From there, two sets of four tubes each go (north) to the other pit in the root cellar. Some 4-inch diameter tubes are buried deep around the root cellar (not shown in the image) to circulate cold winter air through the large thermal mass surrounding it. This cooling system will remove large amounts of heat energy from the thermal mass surrounding the root cellar, and the insulated umbrella will prevent heat from flowing back into the mass from above when the outside temperatures moderate.

When additional moisture is to be removed from the incoming air, it is first diverted through one set of four 35 foot long tubes that form one half of the heat exchanger connecting the two pits. From the pit located in the root cellar, the incoming moist air then circulates through four 6-inch diameter tubes (the rectangular loop from and back into the pit in the image) passing through the cold thermal mass and is further cooled, losing even more moisture. These four tubes reenter the pit and deliver the cooled air, now containing much less moisture, back to the four returning heat exchanger tubes to be reheated by the incoming warm air and delivered into the living space. The condensed moisture flows from the tubes into the pit and exits through a drain to the outside.

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.

In the Beginning There was a Hill

We purchased the property in May 2008 and shortly thereafter had a rough idea where our earth sheltered house would be located.

Part of the Hillside Where the Earth Sheltered House Would be Located

Before we could finalize our house design, we needed to know how the land was laid out so the house would fit into the hillside and not interfere with the surrounding trees. Our daughter Jennifer and I took our rod and level out to the hillside in June and July 2008. Then we waded through weeds and brush over our heads, and swatted mosquitoes and bees to get an elevation profile on a 10' by 10' grid.

Elevation Profile with Top View of Earth Sheltered House Superimposed

This image shows the hill's elevation profile obtained from the rod and level measurements. North is to the right and about 42.5° above horizontal. We took the elevation measurements from right to left, starting at the top of the surveyed area, and then we worked our way down the hill. We assigned zero elevation to the upper-right corner. The hill drops about 37' to the lower-right corner. Each line represents a one foot change in elevation. The elevation numbers are negative, so the hill slopes downward from top to bottom in the image.

The elevation profile shows that the hill slopes downward roughly toward the southeast. We wanted a relatively narrow, rectangular house with its long side facing south so that we could take maximum advantage of solar heating. But this meant that the house would not fit very well into the hillside. As a compromise we split the house into two overlapping rectangular pieces and rotated the second part counter clockwise 45°. When we laid the modified drawing out on the surveyed profile, it wouldn't fit within the trees that we wanted to keep. In the end we had to rotate the second part of the house counter clockwise 55° as shown in the drawing above. The modified house fit better into the hillside as well.

The above drawing also shows a bunch of tubes and two junction boxes. They are part of the Passive Annual Heat Storage (PAHS) air movement system, which I will describe in another post.

Patricia and I at the Base of the Hill the Day Before Excavation Begins

Here we are, standing at the base of the hill that will never be the same starting tomorrow. Excavation for the earth sheltered house began September 8, 2009. The weather was clear and dry for the few days it took to move the many, many cubic yards of dirt. And then it rained and/or snowed for the next ten months (well, just about every day, it seems.)

This Sad-Looking Hillside is Ready for Our House

This is what the hillside looked like on September 11, 2009. I took the photo from about 30' east of where the northeast corner of the garage will be. The view is roughly to the west. On the far right of the image is where the root cellar will be located. To the left of that will be the garage and entryway portions of the house. And beyond the dozer and around the 55° corner will be the rest of the house.

Connecting a PAHS Air Tube

Today I hooked up one of the upper air tubes for the Passive Annual Heat Storage system. The PAHS system uses eight 6" diameter tubes to bring in fresh air and remove stale air. Four lower tubes enter through the back walls at floor level, and four upper tubes enter through the roof.

March 19, 2010, Shortly After the Roof Was Poured.

This image shows all the tubes protruding through the roof. The four smallest and shortest vertical green tubes protruding through the roof (some are hard to see) are part of the upper PAHS air tube system. Each of the short vertical tubes will be connected by a horizontal tube to one of the long green vertical tubes coming up from the ground behind the back walls. The two sets of vertical tubes could not be connected by horizontal tubes until after the backfill was put in place and settled, because the moving soil might distort and break them. (In the photo only three long tubes are visible; two of the tubes are adjacent to each other at the interior corner.) For example, the long tube closest to the viewer will be connected to the short tube to the right of the closest large tube. These are the two tubes I connected today.

Cleaning Wet Clay Off the Tubes and Elbows

Finally we've had no rain for the past ten days or so, and the top three-inch layer of backfill clay is dry. But below that, it is a wet muck from the continuous rains we've had all spring and summer. A couple hours of digging around the two tubes and between them with a round shovel, and I was ready to measure and fit the tubes and elbows together. The last step was to clean all the connections and glue them together.

Connected Tubes Ready to be Covered

Now the air-tube circuit is complete. The closer vertical tube is coming up through the roof. The bottom of the horizontal tube is about 18" above the top of the roof. (It's hard to imagine that there's an office below this tube.) The outside of the back wall lies just to this side of the back connecting elbow and the long buried vertical tube.

The PAHS insulated umbrella will run about two to four inches above the top of the horizontal tubes. Over the roof of the house, the insulated umbrella will consist of two layers of 2.5" styrofoam sandwiched between three layers of 6 mil clear plastic vinyl.

Welcome to the House in the Hill

Dear Visitors,

As I find the time, I will post descriptions of our earth sheltered house and keep you updated as the construction progresses.

I designed the house using VectorWorks 2008 and its predecessor.

Approximate Top View of the Earth Sheltered House

Here is a rough top view of the 2400 square foot earth sheltered house. The north arrow points to the right at about 20° above horizontal. The structure's reinforced concrete ceiling is 11'-0" high. Suspended 10'-6" and 9'-6" ceilings will be installed in most of the rooms as indicated in the drawing.

Southeast View of the Earth Sheltered House Under Construction

The earth sheltered house is nestled into the southeast-facing hillside. A Mansard-type roof hides a 5' high parapet wall along the south and east sides of the house that retains the soil covering the roof. The final graded terrain will be about a foot below the top edge of the 6' tall Mansard roof.

North View of the Earth Sheltered House Under Construction

Numerous tubes and a masonry stove chimney can be seen protruding through the soil in this top view of the earth sheltered house.  An insulated umbrella and two additional feet of soil must be added to this image before the final grading is completed. Except for the tubes and narrow Mansard roof, the house will have a green footprint of grass, flowers, and garden.

Rough-In Window Openings in the South Side of the Earth Sheltered House

Taken in April, 2010, this image shows the rough-in window openings along the south side of the earth sheltered house. The net window glazing area will be more than 20% of the floor space. Marvin windows, with low U value and high solar heat gain, will maximize solar heating in the colder months. Deployable awnings will block direct sunlight from the windows in the warmer months. These windows do not qualify for an energy efficiency tax rebate, even though they will save more energy than those that do; this earth sheltered house has no conventional furnace, but uses a masonry stove for supplemental heat.

View of the Kitchen and Away Room From the Southeast

This photo was taken from the southeast corner of the Great Room (standing in the Living Room area.) An 18" high curved soffit divides the Kitchen and Away Room from the Great Room. Suspended ceilings in the Great Room will be 10'-6" to accommodate the high transom windows. Suspended ceilings in the Kitchen and Away Room will be 9'-6" to allow for ductwork. The glow of a light tube can be seen in the middle of the kitchen, even though the sun was already below the tree-line. A 10" by 10" support column stands at the end of the Kitchen/Away Room wall. One of two 14" diameter support columns, defining the corners of the Foyer, can be seen in the right edge of the image.

Temp-Cast Corner Masonry Stove with Bake Oven

The Masonry Stove is located in the Entry Room at the north end of the earth sheltered house. It is primarily a backup to the Passive Annual Heat Storage (PAHS) system. A plenum, containing a 3-ton air conditioning coil, is installed behind the brick wall to the left of the stove. Ductwork, running the full length of the house, will evenly distribute heat from the stove when needed. The air conditioning coil, although not connected to anything at the moment, may be used as a backup to the PAHS system, which will have its own natural air conditioning capability. The PAHS air conditioning feature, based on a suggestion in John Hait's book, "Passive Annual Heat Storage, Improving the Design of Earth Shelters," will be described in a later post.