Editing Passive solar building design (section)

===Direct solar system===
In a '''''direct-gain passive solar system''''', the indoor space acts as a solar collector, heat absorber, and distribution system. South-facing glass in the northern hemisphere(north-facing in the southern hemisphere) admits solar energy into the building interior where it directly heats (radiant energy absorption) or indirectly heats (through convection) thermal mass in the building such as concrete or masonry floors and walls. The floors and walls acting as thermal mass are incorporated as functional parts of the building and temper the intensity of heating during the day. At night, the heated thermal mass radiates heat into the indoor space.{{Sfn|Wujek|2010}}

In cold climates, a '''''sun-tempered building''''' is the most basic type of direct gain passive solar configuration that simply involves increasing (slightly) the south-facing glazing area, without adding additional thermal mass. It is a type of direct-gain system in which the building envelope is well insulated, is elongated in an east–west direction, and has a large fraction (~80% or more) of the windows on the south side. It has little added thermal mass beyond what is already in the building (i.e., just framing, wall board, and so forth). In a sun-tempered building, the south-facing window area should be limited to about 5 to 7% of the total floor area, less in a sunny climate, to prevent overheating. Additional south-facing glazing can be included only if more thermal mass is added. Energy savings are modest with this system, and sun tempering is very low cost.{{Sfn|Wujek|2010}}

In genuine '''''direct gain passive solar systems''''', sufficient thermal mass is required to prevent large temperature fluctuations in indoor air; more thermal mass is required than in a sun tempered building. Overheating of the building interior can result with insufficient or poorly designed thermal mass. About one-half to two-thirds of the interior surface area of the floors, walls and ceilings must be constructed of thermal storage materials. Thermal storage materials can be concrete, adobe, brick, and water. Thermal mass in floors and walls should be kept as bare as is functionally and aesthetically possible; thermal mass needs to be exposed to direct sunlight. Wall-to-wall carpeting, large throw rugs, expansive furniture, and large wall hangings should be avoided.

Typically, for about every 1&nbsp;ft<sup>2</sup> of south-facing glass, about 5 to 10&nbsp;ft<sup>3</sup> of thermal mass is required for thermal mass (1 m<sup>3</sup> per 5 to 10 m<sup>2</sup>). When accounting for minimal-to-average wall and floor coverings and furniture, this typically equates to about 5 to 10&nbsp;ft<sup>2</sup> per ft<sup>2</sup> (5 to 10 m<sup>2</sup> per m<sup>2</sup>) of south-facing glass, depending upon whether the sunlight strikes the surface directly. The simplest rule of thumb is that thermal mass area should have an area of 5 to 10 times the surface area of the direct-gain collector (glass) area.{{Sfn|Wujek|2010}}

Solid thermal mass (e.g., concrete, masonry, stone, etc.) should be relatively thin, no more than about 4 in (100&nbsp;mm) thick. Thermal masses with large exposed areas and those in direct sunlight for at least part of the day (2 hour minimum) perform best. Medium-to-dark, colors with high absorptivity, should be used on surfaces of thermal mass elements that will be in direct sunlight. Thermal mass that is not in contact with sunlight can be any color. Lightweight elements (e.g., drywall walls and ceilings) can be any color. Covering the glazing with tight-fitting, moveable insulation panels during dark, cloudy periods and nighttime hours will greatly enhance performance of a direct-gain system. Water contained within plastic or metal containment and placed in direct sunlight heats more rapidly and more evenly than solid mass due to natural convection heat transfer. The convection process also prevents surface temperatures from becoming too extreme as they sometimes do when dark colored solid mass surfaces receive direct sunlight.

Depending on climate and with adequate thermal mass, south-facing glass area in a direct gain system should be limited to about 10 to 20% of the floor area (e.g., 10 to 20&nbsp;ft<sup>2</sup> of glass for a 100&nbsp;ft<sup>2</sup> floor area). This should be based on the net glass or glazing area. Note that most windows have a net glass/glazing area that is 75 to 85% of the overall window unit area. Above this level, problems with overheating, glare and fading of fabrics are likely.{{Sfn|Wujek|2010}}