Curved walls are not very affordable for wood construction, but they are a somewhat natural choice if you are working with shotcrete, particularly in an earth sheltered application.   But even if the curved walls are easy, many other aspects, from surveying to carpeting, are much trickier.  Tricky translates into expensive.  I will keep track of curve related issues as I build and put them here.

Damns are curved to better hold back the enormous pressure of the water behind them

If you try to balance a playing card on its edge, it will fall over, but curve that card and it will stand.   When building a concrete wall, a curved wall is self bracing.   Damns are curved because it allows them to hold back much more water pressure.   Earth sheltered homes with flat roofs need at least 12 inches of concrete, but curved roofs can be as thin as 4 inches.

Curves also look and feel very appealing.  Humanity has only been living in wooden boxes for a few hundred years.

“The idea of putting a square home under the earth made no sense.  Caves are not boxes, and the box is not a shape that lends itself to the immense load of earth above.”  ~William Lishman

On the other hand, curves bring lots of issues.   While laying out our curvy house, we had lots of issues making things work.  It is just not as easy to lay out a wedge shaped room as it is to lay out a square one.  It also added lots of difficulty for the architect.  In fact, if my house were rectilinear  I might not even have needed an architect.

Looking ahead to construction, I am sure that siting the house will be much more difficult due to the curved walls.  I can’t just string a rope to make a straight line.  Digging a curved trench will be more difficult than digging a straight one.  Framing will require curved track which costs more per ft than straight track costs.  Placing floor joists is more difficult because each joist is a different length and angle from the wall.  Some constructions systems (like pre-stressed concrete planks) are out of the question, and others (like curved ICFs) just get a lot more expensive.

Rectilinear homes rely on the principles of the post and beam, but when the beam between two posts is curved horizontally, it results in torsion.  the weight above the beam doesn’t just push the beam down, it applies a rotational force to twist it off its posts.

In a curved wall, window lintels are not just a simple beam.  They need to be specially fastened to the posts to resist this torsion.   I only have windows in the curved front wall of the house.  Here the curve is really just to be continuous with the other earth sheltered walls.  To reduce this torsion effect, I have kept the wall above the windows as light as possible.  It is the only section of wall made of steel studs without shotcrete.  I also put beams between each 40 inch wide window so that the max deviation at the mid point between supports is only about an inch.

The Formworks earth sheltered homes all use curved back walls to support the earth.  But most have very simple layouts and simple flat front walls.  I am treading into dangerous territory with my curves, but at least I am aware of it.

Earth Tube Concerns

A variety of concerns are often stated as reasons why earth tubes should not be used or may even be dangerous.  These range from concerns about cost effectiveness to serious health concerns related to mold or radon.  In general, these concerns are based on anecdotes or experience improperly installed earth tubes.

Earth Tubes and Radon:

One earth sheltered home owner I spoke with told me that after many years of living in his earth sheltered home and loving his earth tubes (his wife told me how much she liked the natural humidity control), an air quality test showed that they were bringing in high levels of radon gas. Radon is an inert “noble gas”, 8 times more dense than air, that also happens to be the radioactive offspring of Uranium decay.   The inspector told them that the level found in their home was the cancer causing equivalent of smoking several packs of cigarettes a day.  They immediately plugged up their earth tubes.  But what had gone wrong?

While the danger of cigarette smoke in now almost universally accepted (well paid tobacco lawyers and tobacco executives excepted), there is actually still some controversy about how dangerous naturally occurring Radon actually is.  Controversy or not, it makes sense to assume that radioactive R86 is probably something you want to avoid if you have a choice.

Radon works its way out of the ground thru little gaps in the soil.  This is a natural process that happens all the time, but the radon is usually dissipated quickly.  The problem starts when earth tubes can provide a sort of Radon collection system, a path of least resistance that draws in all the radon from the soil around the home.  Once the radon is channeled inside an airtight home, the heavier-than-air gas can become concentrated.

Many earth tubes are made from flexible drainage pipe with weep holes along every inch of it.  This is great for humidity control as it lets moisture in an out as needed, but it also makes it easy for radon to enter the pipe.  Sloping the tubes toward the home also makes it a bit easier for the 8x-denser-than-air radon (and water) to enter the home.

Using sealed solid wall earth tubes (such as HDPE double wall pipe) and sloping down away from the house to daylight, prevent both of these problems.

Even if radon (which only has a radioactive half life of less than 4 days) does enter the home, it is really only a concern if it is trapped in the home and becomes concentrated.  Actually, this can even happen in “normal” homes with poorly sealed basements and poor ventilation.

Homes that allow for a proper number of air exchanges each day, should not have Radon problems. In fact, a good earth tube system that provides ample air exchanges can actually be the solution to a radon problem.

Earth Tubes and Mold

Every now and then an earth sheltered home or PassivHaus gets diagnosed with “Sick House Syndrome”.  Here is an example,  In this case, the problem was really in the walls, but the earth tubes were not sloped to drain and were actually collecting rain water directly...  Generally, these houses are unhealthy because they literally can’t breathe, and the levels of internal pollutants (chemical and biological) build up (become concentrated) to the point where they irritate the occupants and may even cause serious health problems.  In extreme cases, the home may even be condemned.

A properly designed and installed earth tube system could provide for healthy air exchanges, so it is ironic that the headlines often blame the earth tubes, specifically, mold growing in the earth tubes, for all the problems.

We all know that mold likes to feed on organic material in warm, moist dark places.  No one wants to breathe air that just passed thru a mold filled earth tube.   The idea that earth tubes provide this environment scares many people away from the whole idea…    Properly designed earth tubes should not provide the right environment for mold.  They should be warm and dark, but without the right level of moisture or anything organic to eat.

Keep in mind that even prairie dogs can engineer an earth tube system with enough flow to keep their dry grass bedding from molding.  The keys to a successful earth tube design are slope and airflow.  And choosing a nice smooth inorganic material (like HDPE plastic pipe) helps too.

Water can get into earth tubes by seepage thru joints or holes, but mostly it is by condensation on the inner surface as warm moist summer air flows thru a cool tube.  In some particularly poor designs, rain water can enter directly.  These problems can be avoided with careful design.

Condensation is unavoidable and happens when warm moisture laden air comes into contact with the cool pipe walls.  If this moisture sits there, it could make a suitable environment for mold spores that happen to pass thru.  So don’t let it sit there!

A stagnant earth-tube just sits there stewing… but good airflow can either bring in dry air that will absorb the moisture, or it can bring in more humid air that will condense further until the weight of the droplets exceeds the surface tension and friction holding it in place so that it will run down to the lowest point in the pipe…  A proper design anticipates this water and directs it out to daylight or into a drainage pit.

In some cases, builders have used HDPE pipe with an expensive antimicrobial coating on the inner surface.  I suspect the expensive coating is overkill.  However, the smooth walls and seams of HDPE pipe promote greater airflow and allow water to drain away more easily (provided the pipe is sloped), which probably does make a difference.

The real kicker is that scientific studies of microbial and fungal growth in earth tubes actually show a net benefit as these two quotes explain.

“The Earth-Air Tunnel is found not to support the growth of bacteria and fungi; rather it is found to reduce the quantity of bacteria and fungi thus making the air safer for humans to inhale. It is therefore clear that the use of earth-air heat exchangers [earth tubes] not only helps save the energy but also helps reduce the air pollution by reducing bacteria and fungi.”                                           ~~~Rabindra Nath B, Shailendra Kumar M, and Pawan Basnyat; Use of Earth Air Tunnel HVAC system in minimizing indoor air pollution; Air Quality Monitoring and Management, proceedings of Better Air Quality 2004.

“This study [of 12 different EAHX] was performed because of concerns of potential microbial growth in the buried pipes of ‘ground-coupled’ air systems. The results however demonstrate, that no harmful growth occurs and that the airborne concentrations of viable spores and bacteria, with few exceptions, even decreases after passage through the pipe-system”       ~~~Flueckiger B, Monn C; Microbial investigations and allergen measurements in ground-coupled earth-to-air heat exchangers; AIVC 20th Conference and Indoor Air 99, the 8th International Conference on Indoor Air Quality and Climate, UK, Garston, BRE, 1999, proceedings, Edinburgh, Scotland, 8–13 August 1999, Volume 5.

Earth Tube System Saturation

Thermal saturation is a very real concern that is often underestimated.  Many books and articles on the subject repeat the false idea that the temperature of the earth at some magical depth (5, 10 or 20 feet depending on the writer) is stable at some fixed temperature all year long.  These articles overly simplify the situation and make the soil around the earth tubes seem like an inexhaustible thermal reservoir.  Of course, the laws of physics, including laws about transfers of energy, still apply 10 ft below the ground.  Please see the results of my own experiment to get an idea of an actual underground temperature profile…

The moment you bury an earth tube in that “stable” environment and start to run air thru it, you start to change the temperature profile of the environment.   The original temperature profile was somewhat stable because it was separated from the air by 10 or 20 ft of compact earth.  If you now bring the air down into the earth for the purpose of exchanging heat with it (Ground – Air heat exchange), you should expect the temperature of the earth to vary according to that heat exchange.   If the soil warms up air in the earth tube on a cold day, that thermal energy must come from somewhere.  Of course it comes from the soil, and in the exchange, the soil temperature will be reduced proportionally to its higher mass and heat capacity.   If the air is cold and is warmed by the earth, the earth will cool down during the exchange.  This is why it is called an “exchange”.  The rate that the soil is warmed or cooled is affected by things like the exchange surface area and conductivity, the volume flow rate of the air, the temperature difference (which drives the conduction and is constantly changing during the day), the mass of the earth in proximity to the tube, how quickly the heat from the surrounding earth is conducted to the earth next to the tube, etc.

Since this is a dynamic conjugate heat transfer problem, it is very difficult to predict the exact behavior.  However, if your tube is too short or your air flow is too high, or your ΔT is too great, you could quickly exhaust the thermal difference between the air and the soil near the tube.  As the temperature difference drops, the heat exchange will slow to the point where the earth tube becomes thermally saturated and ineffective.    In a cooling situation, it is possible that by 10 or 11 in the morning, the warm air could increase the temperature of the soil to above the desired room temperature.  For the rest of the day, hot air flowing thru the pipes would just keep increasing the temperature of the soil around the pipes with the theoretical maximum being the outdoor air temperature.  (I plan to use this to my advantage by solar air heating the soil during the summer, see the section on “by-passive” solar).  

In a truly successful earth tube design, the pipes are long enough and the flow speed is low enough that the exchange between the earth and the air happens at a rate the earth around the tubes can absorb.  Since the external air temperatures fluctuate on a daily cycle, it won’t be a truly stable system, but at least it shouldn’t run out of ΔT.  In an even better design, this air flow can be driven by completely passive processes.

In a closed loop system, air is cycled thru the system over and over again.  You don’t get fresh air, but the ΔT is lower, so the earth air exchange system can handle more airflow.

Earth Tube Cost

It is very possible for the install of an earth tube system to cost thousands of dollars.  Many have performed cost benefit analysis and decided against installing them (opting for an air to air heat exchanger instead). I myself have spent many hours agonizing over how to reduce the cost of an earth tube system (while also reducing other risks) in order to make it viable.   Costs can be broken up into 3 main areas; Materials, Installation and Operation/Maintenance.  Of course, the benefits of the system is mostly related to the effectiveness of the system, which is dependent on good design.  Take a look at the Earth Tube Design section of this site.

One way that I planed to reduce the cost of my earth tubes was by reusing the trenches already planned for drainage and sewerage.   Less additional trenching meant less additional cost.  I also needed to compromise on my depth to keep costs down, but I still ended up with depths of up to 15 ft (20′ would be better).

The cost effective depth depends on length of the arm on the excavator, so greater depth may just be a matter hiring an excavator with better equipment.  In my case, the contractor I hired had several large excavators to choose from and came with one that made short work of the trench for the septic line.  It went from the basement of the house (10 ft deep) and cut thru a bit of a hill (up to 15 ft deep) on its way to the septic field nearly 200 ft away (the location at the bottom of the hill was specified by the county health department).  I manually put back a foot or so of earth over the septic lines before placing my double wall earth tubes, that whole manual process took my wife and I most of a Saturday.   When the excavator back filled the hole, he probably took a couple extra hours because he had to be a bit more careful regarding my tubes during the process.

Earth Tubes in hot & humid environments:

If you have 80% humidity at 35°C entering your earth tube, and it cools to 25°C degrees, it will drop a lot of water as condensation on the earth tube walls, but this drop in temperature only increases the level of humidity.  In addition to manageable concerns about where all that condensate goes, the “cooled” air may enter your space at 100% humidity.

This is a real concern and many experts simply recommend against using earth tubes in these environments.

However, I have a two stage arguments for earth tubes in hot and humid environments.

1) The humidity level within the earth tube may increase to 100%, but then it enters the larger habitable space which probably has a higher temperature, so the humidity will automatically drop again. The total moisture in your habitable space will be less than without the earth tubes because some of it was left behind in the tubes.  The cooler your earth/tubes are, relative to your habitable space, the better the natural de-humidification.

2) Assuming the earth is cooler than your habitable space, the earth tubes will remove at least some of the moisture from the air. If you need to remove more with a dehumidification system, you are still better off than without the earth tube. Perhaps the earth tube will enable you to choose a smaller system or use a desiccant wheel or something like that.

After that, it becomes a cost benefit analysis to see if you can put in earth tubes economically enough to make it worth your while.  Often, these hot and humid areas have lower labor rates that make earth tubes more affordable than importing expensive dehumidification equipment.

I am in Michigan, which has cold and relatively dry winters.  However, the summer and fall seasons can be hot and humid.  Hot enough that I also expect my earth tubes to reach 100% humidity and condense out gallons of water per day.  This is why I used smooth wall HDPE earth tubes and sloped them carefully.  It is also why I plan to pass my earth tube air thru an efficient dehumidification system before being distributed thru my home.  My neighbors will simply draw in the hot humid air directly.  The difference will be how much harder their system has to work to dehumidify the air.