Earth Sheltered Basics
“No house should ever be on a hill or on anything. It should be of the hill. Belonging to it. Hill and house should live together each the happier for the other.” ~ Frank Llyod Wright
I am sure Frank wasn’t actually talking about earth sheltering the house, but I think he would have liked the idea. During Frank’s time, the technology wasn’t right yet. Even during the 1970’s, when it really took off, many people made lots of basic mistakes.
This website will try to cover the basics of earth sheltered home design and construction. In this section on Earth Sheltered basics, we will look at the properties of the earth, passive solar, insulation, earth tubes, etc…
Earth Sheltered Basics
When you build a home on the surface, you set the temperature on the inside to around about 70 degrees Fahrenheit that humans find comfortable (note, you can find whole books on the complex relationship between temperature, humidity, emissivity, etc. and human comfort, so this is simplified). Meanwhile, the temperature outside our homes varies considerably, even within the period of a day or an hour. The rate of heat transferred thru the envelope of your house (heat flux) is proportional to ΔT/R, where ΔT is the difference in temperature between the inside and outside of your home. This is known as Fourier’s Law of heat transfer. If we assume that two homes are are constructed the same way (the same wall cross section with the same R value), then you are left with ΔT as the primary driver of heat loss or gain. At this point, we are not yet discussing passive solar or earth tube cooling, so lets assume that you paid for that heat or cool. We are also saving “infiltration”, the main cause of heat loss or gain for another section.
Summer heat is transferred into the home, and winter cold draws heat out of the home. Assuming a well sealed building envelope, the majority of the winter heat is lost thru the conventional roof. Walls and windows also lose a lot of heat, proportional to their area and inversely proportional to their R values. This heat exits the home and is lost forever; some thermal energy is carried away by the wind, some is radiated out into space or contributes to the green house affect. However, under the floor, something different (and not generally well understood) is happening… Heat is leaking out and saturating the earth under the middle of the floor. As the ground absorbs the heat and warms up, the temperature gradient levels off and the heat flux slows. Near the edge of the floor, heat is still conducted to the outside, so the temperature is closer to the outside temperature and more heat is lost. This is why many builders, even in cold places like Sweden, only insulate the perimeter of the floors. Insulating the outside of the wall or even using “Frost Protected Shallow Foundations” can help retain some of the perimeter heat that would have been lost from the outside of the foundation. But there is much less need to insulate the center of the floors.
This is why many Home Heat Loss Calculators (such as this excellent one), used for calculating the heating requirements of above ground homes, ask for area and R value for the roof, walls, windows and floors, but only want the perimeter for slab on grade floors… “Heat loss from slab on grade floors is primarily dependent on the length of the perimeter and not the area of the floor.” ~ Gary Reysa. Imagine what happens when you bury the walls and roof?
Home in the Earth
If we take that same building, with the same R values, and move it under ground (an earth sheltered home), it is like moving it to a more moderate and stable environment. This means a significantly reduced ΔT between the inside and outside of the building envelope, and therefore a significantly reduced heat flux for a given R value and area. When the air outside is freezing, the soil on the other side of the wall is well above freezing, and when the air outside is sweltering hot, the soil temperature is still mild. Above the roof, where the soil depth is less for practical structural reasons, transpiration from plants helps provide cooler temperatures. Along the walls, the temperature becomes more stable the further down you go (research suggests you need much less insulation near the bottom of the wall). The time lag effect of the soil is another benefit. As we mentioned earlier, the soil outside the walls lags behind the air temperature by 3 to 6 months providing the coolest earth in the middle of summer and the warmest earth in the dead of winter.
Some refer to the earth as a “heat sink”, which is true to some extent, but at least it conducts the heat away slowly. The air around an above ground home can actually transport the heat away much more quickly, without any potential to store or return it.
Of course, the performance of the system is highly dependent on a number of factors including the rate of heat transfer thru the walls and windows and the rate of heat conduction away from the earth sheltered home, deeper into the earth, as well as more dynamic factors like solar heat gain, external temperatures, etc. Achieving an ideal balance is the challenge for an earth sheltered home designer, and it likely requires some controls that can be tweaked over time…
Keys to Earth Sheltered Success
Ancient cave paintings show that humans spent quite a bit of time (historically) in caves, most likely for protection from the elements. Those caves were likely too cold or damp to be comfortable at the start (but still better than the freezing cold or sweltering heat outside), but over time, as the inhabitants lit fires for heat or cooking, some of that heat would have been stored in the rock and the caves would have warmed up and become more comfortable. I am sure south facing caves with solar gain were the most in demand… location location location. With few exceptions, those caves were part of massive thermally-conductive rock formations and the heat probably leaked away from the relatively small fires rather quickly. But we have some technology they didn’t have;
A way to heat the home efficiently with passive solar gain.
A way to isolate our portion of the earth, our micro climate, away from the rest of the environment (heat sink and infiltration) with modern construction and insulating materials.
The hardest part about designing an ideal passive solar home is to balance the amount of free solar energy that you allow into the home with the amount of thermal energy the home can store. Too little mass and the home will overheat, too much mass and its temperature will respond too slowly. Above ground passive solar designs work to ensure they have enough mass, usually in the form of cement floors and maybe some thick masonry walls or even trombe walls, to store some low-angle winter solar energy for about a winters day. In Michigan, in January, only about 1 day in 5 has sunshine, so needing a daily recharge won’t work for us. These same homes must be careful to keep out the sunshine in summer. They take advantage of the earths tilt and design awnings to keep out the hotter sun which would certainly cause overheating.
Earth sheltered homes are surrounded by a lot more thermal mass. And unlike the expensive thermal mass often used by above ground solar homes, dirt is, well, cheap. Unfortunately, most earth sheltered homes are designed with the insulation attached to the outside of the cement walls. I won’t even discuss those who put the insulation on the inside of the walls! This insulation (aka out-sulation) does its job and isolates the home from the thermal mass of the surrounding earth. The passive solar storage is limited to the cement or other thermal mass the home owner built into the design. Since these homes are carrying an earth load, they are typically more substantial than the above ground passive solar home and can store more energy, but they are still missing out on a lot of “free” thermal mass.
John Hait came up with his idea for Passive Annual Heat Storage. He found that positioning the insulation further from the home incorporated many tons of earth into his thermal mass and allowed him to store thermal energy for much longer periods of time (theoretically for the whole year). He had large un-shaded windows that let passive solar energy into his solarium during the summer. His design was reasonably successful, except that the heat transfer rates from the living space into the earth were not high enough and his living area tended to be a bit too warm in summer (energy couldn’t get into the earth fast enough). His book suggests improvements to his design including extending the umbrella out to 20ft around the home (his only extended ~10ft.)
I plan to take the next theoretical step. My home will be massive (hundreds of thousands of pounds of cement, which, surprisingly, doesn’t actually even cost that much at about $100/3240lbs + instalation) and I will use an insulation umbrella to increase my thermal mass with many times more earth. But instead of letting the sunlight into the home directly where it can make living conditions uncomfortably warm, I plan to “by-pass” the home and collect heat all summer thru solar collectors which should heat up the earth under my home… The idea is to heat up my earth tubes with very hot air (well over 120°F) and let the energy slowly conduct thru the earth, timed to reach my home just about when I need it, 6 months later. I will still augment this with conventional passive solar in winter.
I plan to provide many of the details for how to setup this passive system over the next few months.