Achieving net positive energy starts with passive systems.
The building envelope is tight and well-insulated.
- R-values are 68 for the roof, 48 for the above-grade walls and 30 for the slab and basement walls.
- Air-changes per hour (ACH), indicating air leakage through windows, doors or other cracks in the building envelope, is 0.45. (ACH for a typical American home is 15; Passive House standard is 0.6 ACH.)
Direct gain window glazing covers two thirds of the long southern wall. Ceramic tile on concrete floors provides further direct gain heating. Trombe walls make up one third of the southern wall, storing heat during daylight hours for passive distribution into the living space through the winter nights.
Roof overhangs are sized to expose the southern wall to the sun during the winter, and to shade the south wall during the summer.
A 37-foot (11.3 meters) passive cooling tower maximizes the benefits of the prevailing winds, using both the stack and Venturi effects to draw warm air up and out. Generously-sized windows throughout the house, and especially along the long southern wall, “feed” the tower airflow and provide essentially 100% of house cooling during the summer. (Forced air cooling is needed only in extreme hot spells – for about a week total each summer.)
Active systems augment the passive systems. Sixty solar panels cover the south plane of the barn roof as part of a grid-tied 16.8-kilowatt photovoltaic system. The solar power system generates 20% more electricity in the course of each year than is used to supply the house.
Electricity loads come from (in descending load amount):
- Hydron Module Combination closed loop geothermal system, providing radiant floor heat, forced air heating and cooling, and potable water pre-heating;
- electric vehicle battery charger;
- high-efficiency water heater;
- kitchen appliances;
- house lighting
- barn meeting room mini-split heating/cooling system.
A heat recovery ventilator maintains the flow of fresh air through the house when operable windows are closed.
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