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Old Main          Mani Shop          The ARC          SPARC          TWARC           Permaculture Garden

Energy Independence in an Affordable Single-Family Home

SPARC and TWARC are both similar in that they were the culminating projects after years of research and experimentation. The ARC was too big and experimental to be practical. So we built a Small Practical ARC or SPARC. SPARC was our first attempt at constructing a more standard single-family residence that can be both energy efficient and cost effective. 

Following up on thermal mass experiments in The ARC, this building sits on a “slab of concrete floating on a bed of sand in a highly-insulated box.” This time the box includes 10 inches of foam insulation on the sides and 10 inches on the bottom. The ARC had no air flow through the sand; Old Main did not have enough; the Mani Shop had more than was needed. We took lessons from all those buildings to develop the thermal mass under this building. Instead of the six layers of air flow tubes found in the Mani Shop, we used a single layer for this building, topped by 30 inches of sand.


The walls are made of Structural Insulated Panels (SIPS), offering R-80 insulation, and like the ARC, windows are a foot thick. Two double-pane windows separated by the width of the SIPS makes for a great space to display items or keep some heat-tolerant plants. In-floor heat is used as backup warmth for the tougher weeks in winter.

Underground air exchanger

keeps things warm

This diagram shows the simple air flow chart for the building. From the peak of the passive solar greenhouse, hot air is forced down through vents into 4-inch corrugated piping that extends the length of the floor and back again. The heat traveling through the pipes is transferred to the sand, which then radiates through the floor. The cooled air returns to the greenhouse floor to be warmed by the sun and then recirculated to the sand. Note that this design includes a hot air chimney that helps vent the greenhouse during the hot summer months.

Capturing the solar energy



SPARC construction begins


Square feet
of living space.


Average monthly cost includes all computers, lights, appliances,
and heat.


Insulation value of the walls/

3 / 2

Bedrooms / Baths

Heating & Cooling Costs

Scaled to compare heating and cooling costs per square foot against a standard new home.

Old Main          Mani Shop          The ARC          SPARC          TWARC           Permaculture Garden
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