A Little Granola

Passive solar heating is one of the most efficient uses of the sun’s energy possible. Once the building is designed it requires little to no energy input and the heat it produces is basically free. In traditional passive solar homes the sun heats a thermal mass during the day and the radiates nice even heat throughout the night. In places like the American south and midwest this works great but in areas like the beautiful pacific northwest what do you on days when the sun doesn’t shine (and it frequently doesn’t)? In situations like this Annualized Geo-Solar (AGS) design is a good bet.

Basically heat from the abundant summer sunshine is stored in a large thermal mass. Original Passive Annual Heat Storage (PAHS) designs involved either an underground home or a home with a green (dirt) roof. Don Stephens developed the AGS technique to apply passive solar heating to areas with limited winter sun without the need for earth-sheltered houses.

AGS basically involves collecting heat from an isolated source such as a solar collector or greenhouse and transferring the heat from the air to the earth under the house via ducts. The system requires (at its most basic): 1) a well insulated structure, 2) dry earth under the structure for storage, 3) an isolated solar heat collector, 4) a method of controlling when the collected heat is dispersed into the structure, 5) a radiant floor in contact with the soil.

#1 can be accomplished by superinsulating your building envelope, such as is found in a straw bale house.
#2 is accomplished by surrounding the house with a insulated water repelling apron.
#3 can be accomplished in a number of ways. Hot air can be collected from a sunspace, attic, or even under metal roofing. Dedicated solar collectors can also be built and ideally situated downhill from the structure, then the hot air will naturally siphon through ducts under the house without the need for fans.
#4 is important because you don’t want the house to begin receiving heat until approximately 6 months after you begin collecting it. One way is to deposit the heat deep enough under the structure that it takes 6 months reach the level of the floor (about 9-10′ deep). Another way is to deposit the heat under insulated floor 9-10′ from uninsulated floor and use the horizontal distance to control the when the heat begins to be released.
#5 This design obviously requires a slab type floor. The floor must be in contact with the soil underneath in order to radiate the stored heat into the house.