¶ … passive solar architecture is nothing new. Socrates observed, more than 2400 years ago, that when a house is facing to the south, the sun's rays penetrate through the windows of the house in the winter, giving heat to the inside, and shines right above the house in the summer, giving shade and cooling on the inside. At that time, this house probably lost heat as fast as it was collected, because of the convective and radiation losses. But the idea was there, which the Romans built upon, by using windows, covered with glass, to trap the solar energy. This caused the internal temperature to stay constant throughout the night. Passive solar architecture has come a long way since the time of Socrates. Most passive solar homes have the same characteristics in each that make them more energy efficient than they were back then. In the Northern Hemisphere, the windows are facing south, and in the Southern Hemisphere, the windows face north, in order to get the most efficient use of the sun.

Solar radiation passes through solar-oriented glass windows, or solar spaces, and is absorbed by surfaces of the materials inside the insulated envelope of the building. These heated surfaces will re-radiate energy into the interior of the house; the air temperature rises, but the heat does not get rerouted back outside, nor can it escape, resulting in trapped thermal energy.

Light strikes the interior surfaces that are made from high density materials, such as concrete, brick, stone, or adobe. These materials can absorb energy and re-radiate it over time, which results in a very smooth temperature swing...

When a passive solar house is well-designed and well-insulated, air leakage rates are reduced and solar heat stays inside the building envelope. Open floor plans facilitate movement of the solar heat from the south side to the rest of the house. If a mechanical device is needed during extended cloudy periods, smaller, direct vented units or a woodstove work nicely.
There are two types of solar heat used in passive solar architecture design: Direct Gain and Indirect Gain. Direct gain is the simplest type as they rely on receiving solar heat directly from south facing windows, called solar windows. Some of this heat is used immediately, while the rest of the heat is stored in the walls, floors, ceilings, and furniture. This is then radiated through the house during the day, and into the night.

Performance and comfort levels will be increased if the thermal mass, such as concrete, concrete block, brick, or adobe is increased within the space. This is so that the thermal energy can be absorbed and spread out evenly. Recommended mass surface-to-glass area is 6:1. These surfaces should be covered with carpet, cork, wallboard, or any other materials with R-values greater than 0.5, to effectively insulate the mass from the solar energy being collected.

Indirect gain collects and stores energy in one part of the house, and uses natural heat movement to warm the rest of the house, which is done by placing a thermal storage wall 3-4 inches inside an expanse of the south facing glass. This wall is…