than new, conventional buildings. In warm climates, those having less than two weeks of frosty nights per year, there is no cost impact. The basic requirement for passive solar heating is that the solar collectors must face the prevailing sunlight (south in the Northern Hemisphere, north in the Southern Hemisphere), and the building must incorporate thermal mass to keep it warm in the night. A recent, somewhat experimental solar heating system "Annualized geo solar heating" is practical even in regions that get little or no sunlight in winter. It uses the ground beneath a building for thermal mass. Precipitation can carry away the heat, so the ground is shielded with skirts of plastic insulation. The thermal mass of this system is sufficiently inexpensive and large that it can store enough summer heat to warm a building for the whole winter, and enough winter cold to cool the building in summer. In annualized geo solar systems, the solar collector is often separate from (and hotter or colder than) the living space. The building may actually be constructed from insulation, for example, straw-bale construction. Some buildings have been aerodynamically designed so that convection via ducts and interior spaces eliminates any need for electric fans. A more modest "daily solar" design is very practical. For example, for about a 15% premium in building costs, the Passivhaus building codes in Europe use high performance insulating windows, R-30 insulation, HRV ventilation, and a small thermal mass. With modest changes in the building's position, modern krypton- or argon-insulated windows permit normal-looking windows to provide passive solar heat without compromising insulation or structural strength. If a small heater is available for the coldest nights, a slab or basement cistern can inexpensively provide the required thermal mass. Passivhaus building codes, in particular, bring unusually good interior air quality, because the buildings change the air several times per hour, passing it through a heat exchanger to keep heat inside. In all systems, a small supplementary heater increases personal security and reduces lifestyle impacts for a small reduction of autonomy. The two most popular heaters for ultra-high-efficiency houses are a small heat pump, which also provides air conditioning, or a central hydronic (radiator) air heater with water recirculating from the water heater. Passivhaus designs usually integrate the heater with the ventilation system. Earth sheltering and windbreaks can also reduce the absolute amount of heat needed by a building. Several feet