Thursday, September 2, 2010

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.


  1. Can you tell us where you got that topological map? I have tried several services (all seem to be based on USGS data), but none seem to be anywhere near this quality.

    Oh yea, I should mention that I am planning to start construction on my earth sheltered home next spring (if all goes according to plan).

  2. Simon said, "Can you tell us where you got that topological map?"

    Unfortunately I don't remember. The original file from which I extracted this portion seems to have vanished, and I can't locate the website from which I downloaded it.

    What I do know about this topo map is that it is part of Flood Hazard Information. I tried Google with various word combinations and came up empty. Sorry.

    Simon said... "Oh yea, I should mention that I am planning to start construction on my earth sheltered home next spring (if all goes according to plan)"

    Where are you building and what are the specifics? Are you planning a blog site?