Understanding the ups and downs of building underground
Has the idea of living in an underground home tempted you?
If so, you’re part of a growing minority. More and more people, worldwide, have already or plan to build an earth-sheltered or earth-bermed home. Earth-sheltered homes usually have their tops and sides completely covered with earth, while earth-bermed homes usually have an exposed side and roof.
Many underground enthusiasts join local and international organizations for support, ideas and information. Most of these enthusiasts and their groups can readily spiel off what they see as the advantages of an underground lifestyle, and, surprisingly to some, our own U.S. Department of Energy (DOE) agrees.
Conservative energy use tops just about everyone’s list. The DOE says, “An earth-sheltered home is less susceptible to the impact of extreme outdoor air temperatures, so you won’t feel the effects of adverse weather as much as in a conventional house. Temperatures inside the house are more stable than in conventional homes, and with less temperature variability, interior rooms seem more comfortable.”
Other advantages cited by the DOE include protection against the extremes of Mother Nature, such as high winds, hailstorms, tornadoes, hurricanes and earthquakes; less susceptibility to fire; lower insurance premiums; less maintenance; natural soundproofing; conservative use of land and natural resources.
To these, non-governmental groups usually add a few more advantages. Most say that a buried house provides maximum protection from not only natural disasters but man-made ones, such as explosions, nuclear accidents, burglaries and break-ins. Many claim that earth-sheltered homes are the only way to gain total privacy. Still others like having the ability to grow your food on top of your house.
Monolithic Dome vs. Underground
“But,” says MDI President David B. South, "a Monolithic Dome home – by its very nature – already has most of those advantages without being buried. Think about what the DOE says about earth-sheltered homes. They could be describing any unburied Monolithic Dome.
“On the other hand, some people want the added security and privacy of an earth covering. There are folks who like the idea of a house that does not change the natural landscape or deplete our tree supply. Still others might want a really real roof garden.
An underground Monolithic Dome
“Fortunately, for those who want to, it’s very possible to build a Monolithic Dome underground,” South adds. “It’s already been done. We have a number of buried domes. Some have an earth covering of 15 feet. Monolithic Domes have the strength to bear this added weight.”
According to South, Monolithic Domes generally need very little increase in thickness and rebar size to be buried. He says, “Usually another inch or two of concrete and a size larger rebar will take care of all the structural considerations. But the footing has to be quite a bit larger since the weight of the dome plus the earth cover will try to push it into the ground. Consequently, burying a Monolithic Dome usually increases its construction cost by about 20 percent.”
Condensation and Insulation
While the added weight of an earth covering may not seriously challenge a buried structure, condensation will. South claims that when any underground house fails it’s usually due to condensation on the inside, since such moisture becomes feed stock for mold and mildew.
American pioneers built storages and shelters underground. First they built them with rocks, logs, and straw. Later they started building them with reinforced concrete, but they all suffered from same problem. They were always dank and damp and had mildew growing inside them.
But what causes the condensation? South blames insufficient insulation. The earth’s temperature tends to stay about 55°. If you take warm humid air, bring it inside, and run it up against a wall surface that is 55° it will condense.
He says, “Any home that’s buried has to be super-well insulated. We have two reasons for insulating. Primarily, we want to contain heat by keeping it out or in. But in an underground home, we also want to keep the surface temperature of interior walls and ceiling approximately equal to the temperature of the air inside the structure. That takes a lot of insulation, but not insulating properly will invite condensation.”
A glass of ice water
So the most common cause for failure of underground houses is not gross heat escaping the structure but an interior surface temperature that allows condensation.
South uses a simple demonstration to prove his point: a glass of ice water sitting on a table. “The ice water is obviously taking on heat from the room,” he says, “and if there were millions of glasses of ice water they would cool the room. But there’s only one. Still that one proves the axiom: opposites attract. The heat in the room is attracted to that glass of ice water, and since the surface temperature of the glass is far below that of the room, moisture condenses and begins running down the glass.”
According to South, an underground house can be compared to that glass of ice water. He says, "Moisture in the air condenses when it contacts cool outside walls. Though there may not be enough moisture to run, it will be enough to attract mold and mildew.
“The only answer is to have enough insulation so that the interior surface temperature of the walls equals the temperature of the air inside the house. Three inches of urethane or six inches of Styrofoam should be used. And even with super-insulated walls, it’s sometimes necessary to dehumidify.”
Obviously a Monolithic Dome can be sufficiently insulated to prevent condensation. But before you begin looking for your shovel, you might consider a few other factors. Even the most enthusiastic proponents of underground construction say that getting financing is a problem and resale is almost impossible.
No code stops you from burying a house, but there is a code that makes it quite miserable: All sleeping spaces must have a window to the outside.
That window must have a clear opening of 5.7 square feet, a minimum dimension of 20 inches, and be no more than 40 inches above the floor. Since every bedroom must have a good-size window that opens to the outside, it’s virtually impossible to bury a bedroom.
Some have been built with large window wells outside the bedroom to allow egress, but in reality that is not very practical.
The DOE says that soil type is another critical consideration. They say the best are granular, such as sand and gravel, since they compact well but are permeable and allow water to drain quickly. Cohesive soils, such as clay, and permafrost areas are least suitable for underground construction.
Other factors cited by the DOE include radon, an invisible, odorless radioactive gas produced naturally when uranium in rock decomposes, the groundwater level at a chosen building site, and the selection of an adequate air exchange system.
For all these considerations, the DOE recommends appropriate testing and consultations with professionals.
And David South recommends that anyone interested in building an underground Monolithic Dome take “ample advantage of the information and resources we have here at MDI.”
Note: Reprinted from the Spring 2001 Roundup.