Berming dirt around the chain wall helps to thermally ground the house to get closer to those more comfortable temperatures, and provides for good drainage away from the house to keep the plenum dry. Visqueen is laid on the ground with an 8 inch overlap at the joints to prevent moisture vapor migration. To switch from one to the other is accomplished with a damper at the HVAC unit. It is possible to be comfortable during much of year by just moving unconditioned air through the plenum. This is done by turning just the fan from the HVAC unit on which moves the comfortable temperatures from the plenum to the rest of the house.
Underground temperatures can be very beneficial in balancing the thermal comfort of the home.
An energy efficient design means having a large mechanical system. Underground temperatures become constant at about 30 feet below the surface of the ground. The constant underground temperature is the same as the average yearly temperature of an area. A plenum is used to supply or return air. The underfloor plenum is a crawlspace open to the outside.