Deep Earth Thermal Conduit

I live in Tucson, AZ. I have been rather obsessively studying passive solar design, rammed earth, earth sheltered housing, etc. In the not-so-distant future I will have the opportunity to build a house and have decided on rammed earth.

From all the information I could gather it seems that the ground temperature here at around 30 feet down maintains a steady 75 degrees. I would like to take advantage of this by vertically insulating around the foundation/slab to a depth of 15 feet, in effect making a large thermal conduit between the slab and the deep earth. I would combine this with a super-insulated roof and 30 inch thick rammed earth interior and exterior walls and some passive solar heating for winter. It seems to me this design would maintain a fairly steady, comfortable interior temperature close to the average annual temperature.

Thoughts, suggestions, criticism?

Thats an interesting idea Dusty. I was skeptical that your area had ground temps that high as most US locations are much lower but it seems to be true. I think the problem is, it seems only one side of your planned structure would be exposed to the ground temp while the other "5" sides of your structure would be exposed to the air temps. For much of the year, those air temps are going to influence the envelope's performance more than the low Delta T of the ground couple.

The higher your R values on the air-exposed sides of the home, the more likely the ground will help condition the structure. I think a 30" mass wall of R8 to R11 would be much too low even with a super insulated roof and high performance windows. Its likely that air-sealing and window performance will have bigger impacts on the overall performance even with super high levels of insulation. Personally, I might consider a light layer of insulation directly below the slab and put the resources into the above grade portions of the building envelope and mechanical systems.

Just to clarify, I think a mass wall might be an ok choice for a home in your climate but I dont think it would be enough to help appropriately condition the structure youre proposing.

As I was reading this a thought struck( yes it hurt! ), the heat pipes that Ernie talks about in the latest podcast might work to move the heat into the house??

Thank you, Brian, for your input. I should have included all my means of maintaining thermal comfort. Here is a list of the systems that would be in place along with other conservation strategies.

-R-90 roof with summer ventilation and reflective coating to control "attic" temperature.
-30" rammed earth interior/exterior walls to average out diurnal wall temps and provide lots of thermal mass to average out temps over longer periods, say 3-5 days. Most extreme heat and cold spells only last a few days around here. I'm considering increasing internal(isolated) wall thickness to 36"-48".
-Thermally isolated floor/deep earth thermal conduit. The average floor temperature should stay around 75 degrees and would effectively provide a huge thermal mass.
-Night purging to further bring down average interior summer temps. I am thinking of making use of a thermal chimney with temperature actuated shutters.
-Ceiling fans all tied to a thermostat which will activate in the hopefully rare event that temps exceed 80 degrees in summer. Based on my estimates this would only be necessary in the late afternoon/evening for less than a month each year.
-Solar gain to raise wintertime average temps. Slightly over-sized windows to store extra energy to maintain thermal comfort in cold spells. In the event of overheating the ventilation system would activate in order to maintain thermal comfort.
-Efficient appliances and lighting to reduce internal heat loads and electricity use.
-Efficient multi-paned windows to reduce heat loss and gain. Minimize north/east/west glazing.
-Minimal air infiltration with controlled and appropriately timed fresh air ventilation.
-Exterior shading using deciduous trees to minimize unwanted solar gain on the building envelope.
-Skylighting to provide natural light and further reduce internal heat loads due to electric lighting. According to studies on existing structures electrical lighting adds to internal heat loads much more than skylighting.
-Low maintenance structure/systems. I was hoping not to have to insulate the exterior walls. I like the looks of rammed earth and want to reduce the construction workload and eliminate exterior maintenance. Insulating and plastering the exterior would be expensive and time-consuming.
-No energy intensive thermal control systems.
-PV system.
-Water harvesting.
-Grey water.
-Xeriscape landscaping.
-Gardens, orchards, permaculture. Growing ones own food saves lots of money and resources and enhances health.
-Solar hot water combined with point-of-use on-demand water heaters.


Average high/low/median mid-summer air temps are 100/76/88. So average wall temp should be about 88 degrees. Then if you average wall temp with floor temp(about 75 degrees) you come up with 81.5 degrees. Wall area is 1200ft2 and floor area is 1000ft2 so it should average out fairly well. Then you take that average and consider the night purging you should wind up slightly under 80 degrees. I think with such a high level of insulation combined with attic ventilation the roof can almost be taken out of the equation. So add a few degrees for minimal external solar gain and internal heat loads it seems you might wind up around 82 degrees average during the peak of summer, probably for less than 30 days. That should be comfortable with a slight breeze from the ceiling fans. Also, there is an interesting phenomenon in breathable earthen walls where they release more moisture with increasing temps, resulting in a slight evaporative cooling effect.

Here is a very interesting and informative page about human thermal comfort by the way: http://sustainabilityworkshop.autodesk.com/buildings/human-thermal-comfort.

Feel free to give me a reality check if you still feel I'm not taking enough into consideration. Any other ideas you have for reducing general resource usage would be welcome. I wish ground temps here were lower than they are, it would make everything much less complicated.

Joe, I have been thinking lately about using multiple small diameter vent pipes through the thermal mass in a mass heater to more efficiently transfer the heat.

I am not sure how Ernie's idea could be put to use in my design however. Can you articulate?

Not a well formed idea, more of a ragged concept. The copper pipes are sealed with a vacuum and a small amount of low boiling point liquid. The pipe is drilled? down to depth and the liquid is boiled by the heat in the ground. The vapor rises to the top and condenses releasing the heat to the house. The liquid falls down the pipe and is continuously recycled to move more heat energy. As I said, just a ragged concept on my part....

Distributing and circulating heat through plumbing is fun but tends to be expensive and trouble prone. Its hard to match the packaged nature, efficiency and reliability of heat pumps. Youve got some great plans going Dusty. I dont find much to improve on other than some minor gripes and based on your level of research Iam sure youve thought about what I will add.

R90 is an enviable roof level but beware the skylights. Punching sizable R2-R6 holes with associated thermal bridging will dramatically effect total area R value in a powerful location when heating while introducing more unwanted solar heat gain when cooling. Lighting energy is rendered mute by LED and other energy efficient options. There are better natural lighting strategies than skylights, especially when designing from scratch as Iam sure you know going passive solar.

Thermal Mass TM is a usual topic of discussion here and I love to challenge people on its relative importance. Most energy modeling and high performance building certifications point to air leakage, insulation and fenestration as the main variables when evaluating the performance of building envelopes and how well they keep occupants comfortable while making efficient use of available energy.

Yes, TM blunts interior temperature swings, but youre not going to get much more usefulness out of more than about 4" worth of thermal mass directly coupled to the indoor air. TM influence is easily overshadowed by many other things like windows and skylights, efficiency and CFM rate of the fresh air introduction system, low R values, higher ACH50 etc...........

Even going with above grade earth walls that are 4' thick will still just barely get you to the wood framing level of insulation called for by international codes in your location as the cheapest performance allowed by law. However! I dont want to limit your ideas of earthen walls and you live in one of the very few climates in this country where Iam personally at peace with people's choice of mass-only walls for the building envelope.

I think your plans are sound but you may be overestimating the effects of deep TM and putting all that insulation underground when the resources could be better spent somewhere else like air-sealing, better windows, more efficient HVAC, and renewables.