The outer shell tempers the inner shell (living space) by creating a false climate based on the outer shell's connection to the earth (moderate temperature) and input from the sun (greenhouse effect)
The outer shell can be vented in summer, while the inner shell, which remains sealed, can be air-conditioned. Often the AC is only dehumidifying as the earth-contact keeps the living space at a comfortable temperature. In low humidity climates, like the western United States, no AC is even needed.
Redundancy: If the outer windows or roof are damaged in a storm, the inner shell still is a layer of protection- you can still live there, and your possessions are safe until you get the outer shell repaired.
Strength: The double north wall is interlaced with steel ties, so looking from the top down is is like a bridge truss laid flat. The strength of that wall is more than the sum of the strength of each single wall. With the correct tie-downs (which can be hidden in the cavity part, a double wall can be engineered for up to 240 mph winds- more than a category 5 hurricane. [photo of double wall under construction with ties]
Control: By opening and closing internal windows and doors you are in control, and can tweak the inner shell conditions. Some of our homeowners have likened it to sailing a sailboat. [photo of north wall windows]
Lower and healthier living space air temperatures. The problem with a forced-air heating system is that air must be heated 10-15 degrees warmer for the body to feel comfortable, than with radiant heating. This means you are breathing this hot air, which leads to the exhausted and sleepy feeling we are familiar with in winter. With radiant heating as the only heating in the inner shell living space (all convection is confined to the outer shell) you are breathing cooler air- you feel invigorated, not sleepy.
Sound Control: Inside the inner shell it is incredibly quiet. Street noise is adsorbed by, and contained by the outer shell. The superior acoustics of the solid wooden walls have been noted by musicians who have built our homes.