Editing Passive solar building design (section)

==Key passive solar building configurations==
There are three distinct passive solar energy configurations,{{Sfn|Wujek|2010}} and at least one noteworthy hybrid of these basic configurations:

*direct [[solar gain|solar system]]s
*indirect solar systems
*hybrid direct/indirect solar systems
*isolated solar systems

===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}}

===Indirect solar system===
In an '''''indirect-gain passive solar system''''', the thermal mass ([[concrete]], masonry, or water) is located directly behind the south-facing glass and in front of the heated indoor space and so there is no direct heating. The position of the mass prevents sunlight from entering the indoor space and can also obstruct the view through the glass. There are two types of indirect gain systems: thermal storage wall systems and roof pond systems.{{Sfn|Wujek|2010}}

====Thermal Storage (Trombe) Walls====
{{Main articles|Trombe wall}}
In a '''''thermal storage wall''''' system, often called a '''''Trombe wall''''', a massive wall is located directly behind south-facing glass, which absorbs solar energy and releases it selectively towards the building interior at night. The wall can be constructed of cast-in-place concrete, brick, adobe, stone, or solid (or filled) concrete masonry units. Sunlight enters through the glass and is immediately absorbed at the surface of the mass wall and either stored or conducted through the material mass to the inside space. The thermal mass cannot absorb solar energy as fast as it enters the space between the mass and the window area. Temperatures of the air in this space can easily exceed 120&nbsp;°F (49&nbsp;°C). This hot air can be introduced into interior spaces behind the wall by incorporating heat-distributing vents at the top of the wall. This wall system was first envisioned and patented in 1881 by its inventor, Edward Morse. Felix Trombe, for whom this system is sometimes named, was a French engineer who built several homes using this design in the French Pyrenees in the 1960s.

A thermal storage wall typically consists of a 4 to 16 in (100 to 400&nbsp;mm) thick masonry wall coated with a dark, heat-absorbing finish (or a selective surface) and covered with a single or double layer of high transmissivity glass. The glass is typically placed from {{frac|3|4}} in to 2 in from the wall to create a small airspace. In some designs, the mass is located 1 to 2&nbsp;ft (0.6 m) away from the glass, but the space is still not usable. The surface of the thermal mass absorbs the solar radiation that strikes it and stores it for nighttime use. Unlike a direct gain system, the thermal storage wall system provides passive solar heating without excessive window area and glare in interior spaces. However, the ability to take advantage of views and daylighting are eliminated. The performance of Trombe walls is diminished if the wall interior is not open to the interior spaces. Furniture, bookshelves and wall cabinets installed on the interior surface of the wall will reduce its performance.

A classical '''''Trombe wall''''', also generically called a '''''vented thermal storage wall''''', has operable vents near the ceiling and floor levels of the mass wall that allow indoor air to flow through them by natural convection. As solar radiation heats the air trapped between the glass and wall and it begins to rise. Air is drawn into the lower vent, then into the space between the glass and wall to get heated by solar radiation, increasing its temperature and causing it to rise, and then exit through the top (ceiling) vent back into the indoor space. This allows the wall to directly introduce heated air into the space; usually at a temperature of about 90&nbsp;°F (32&nbsp;°C).

If vents are left open at night (or on cloudy days), a reversal of convective airflow will occur, wasting heat by dissipating it outdoors. Vents must be closed at night so radiant heat from the interior surface of the storage wall heats the indoor space. Generally, vents are also closed during summer months when heat gain is not needed. During the summer, an exterior exhaust vent installed at the top of the wall can be opened to vent to the outside. Such venting makes the system act as a solar chimney driving air through the building during the day.

Vented thermal storage walls vented to the interior have proven somewhat ineffective, mostly because they deliver too much heat during the day in mild weather and during summer months; they simply overheat and create comfort issues. Most solar experts recommended that thermal storage walls should not be vented to the interior.

There are many variations of the Trombe wall system. An '''''unvented thermal storage wall''''' (technically not a Trombe wall) captures solar energy on the exterior surface, heats up, and conducts heat to the interior surface, where it radiates from the interior wall surface to the indoor space later in the day. A '''''water wall''''' uses a type of thermal mass that consists of tanks or tubes of water used as thermal mass.

A typical unvented thermal storage wall consists of a south facing masonry or concrete wall with a dark, heat-absorbing material on the exterior surface and faced with a single or double layer of glass. High transmission glass maximizes solar gains to the mass wall. The glass is placed from {{frac|3|4}} to 6 in. (20 to 150&nbsp;mm) from the wall to create a small airspace. Glass framing is typically metal (e.g., aluminum) because vinyl will soften and wood will become super dried at the 180&nbsp;°F (82&nbsp;°C) temperature that can exist behind the glass in the wall. Heat from sunlight passing through the glass is absorbed by the dark surface, stored in the wall, and conducted slowly inward through the masonry. As an architectural detail, patterned glass can limit the exterior visibility of the wall without sacrificing solar transmissivity.

A water wall uses containers of water for thermal mass instead of a solid mass wall. Water walls are typically slightly more efficient than solid mass walls because they absorb heat more efficiently due to the development of convective currents in the liquid water as it is heated. These currents cause rapid mixing and quicker transfer of heat into the building than can be provided by the solid mass walls.

Temperature variations between the exterior and interior wall surfaces drive heat through the mass wall. Inside the building, however, daytime heat gain is delayed, only becoming available at the interior surface of the thermal mass during the evening when it is needed because the sun has set. The time lag is the time difference between when sunlight first strikes the wall and when the heat enters the building interior. Time lag is contingent upon the type of material used in the wall and the wall thickness; a greater thickness yields a greater time lag. The time lag characteristic of thermal mass, combined with dampening of temperature fluctuations, allows the use of varying daytime solar energy as a more uniform night-time heat source. Windows can be placed in the wall for natural lighting or aesthetic reasons, but this tends to lower the efficiency somewhat.

The thickness of a thermal storage wall should be approximately 10 to 14 in (250 to 350&nbsp;mm) for brick, 12 to 18 in (300 to 450&nbsp;mm) for concrete, 8 to 12 in (200 to 300&nbsp;mm) for earth/adobe, and at least 6 in (150&nbsp;mm) for water. These thicknesses delay movement of heat such that indoor surface temperatures peak during late evening hours. Heat will take about 8 to 10 hours to reach the interior of the building (heat travels through a concrete wall at rate of about one inch per hour). A good thermal connection between the inside wall finishes (e.g., drywall) and the thermal mass wall is necessary to maximize heat transfer to the interior space.

Although the position of a thermal storage wall minimizes daytime overheating of the indoor space, a well-insulated building should be limited to approximately 0.2 to 0.3&nbsp;ft<sup>2</sup> of thermal mass wall surface per ft<sup>2</sup> of floor area being heated (0.2 to 0.3 m<sup>2</sup> per m<sup>2</sup> of floor area), depending upon climate. A water wall should have about 0.15 to 0.2&nbsp;ft<sup>2</sup> of water wall surface per ft<sup>2</sup> (0.15 to 0.2 m<sup>2</sup> per m<sup>2</sup>) of floor area.

Thermal mass walls are best-suited to sunny winter climates that have high diurnal (day-night) temperature swings (e.g., southwest, mountain-west). They do not perform as well in cloudy or extremely cold climates or in climates where there is not a large diurnal temperature swing. Nighttime thermal losses through the thermal mass of the wall can still be significant in cloudy and cold climates; the wall loses stored heat in less than a day, and then leak heat, which dramatically raises backup heating requirements. Covering the glazing with tight-fitting, moveable insulation panels during lengthy cloudy periods and nighttime hours will enhance performance of a thermal storage system.

The main drawback of thermal storage walls is their heat loss to the outside. Double glass (glass or any of the plastics) is necessary for reducing heat loss in most climates. In mild climates, single glass is acceptable. A selective surface (high-absorbing/low-emitting surface) applied to the exterior surface of the thermal storage wall improves performance by reducing the amount of infrared energy radiated back through the glass; typically, it achieves a similar improvement in performance without the need for daily installation and removal of insulating panels. A selective surface consists of a sheet of metal foil glued to the outside surface of the wall. It absorbs almost all the radiation in the visible portion of the solar spectrum and emits very little in the infrared range. High absorbency turns the light into heat at the wall's surface, and low emittance prevents the heat from radiating back towards the glass.{{Sfn|Wujek|2010}}

====Roof Pond System====

A '''''roof pond'' ''passive solar system''''', sometimes called a '''''solar roof''''', uses water stored on the roof to temper hot and cold internal temperatures, usually in desert environments. It typically is constructed of containers holding 6 to 12 in (150 to 300&nbsp;mm) of water on a flat roof. Water is stored in large plastic bags or fiberglass containers to maximize radiant emissions and minimize evaporation. It can be left unglazed or can be covered by glazing. Solar radiation heats the water, which acts as a thermal storage medium. At night or during cloudy weather, the containers can be covered with insulating panels. The indoor space below the roof pond is heated by thermal energy emitted by the roof pond storage above. These systems require good drainage systems, movable insulation, and an enhanced structural system to support a 35 to 70&nbsp;lb/ft<sup>2</sup> (1.7 to 3.3&nbsp;kN/m<sup>2</sup>) dead load.

With the angles of incidence of sunlight during the day, roof ponds are only effective for heating at lower and mid-latitudes, in hot to temperate climates. Roof pond systems perform better for cooling in hot, low humidity climates. Not many solar roofs have been built, and there is limited information on the design, cost, performance, and construction details of thermal storage roofs.{{Sfn|Wujek|2010}}

===Hybrid direct/indirect solar system===
Kachadorian demonstrated that the drawbacks of thermal storage walls can be overcome by orienting the Trombe wall horizontally instead of vertically.{{Sfn|Kachadorian|2006}} If the thermal storage mass is constructed as a ventilated concrete slab floor instead of as a wall, it does not block sunlight from entering the home (the Trombe wall's most obvious disadvantage) but it can still be exposed to direct sunlight through double-glazed equator-facing windows, which can be further insulated by thermal shutters or shades at night.{{Sfn|Shurcliff|1980}} The Trombe wall's problematic delay in daytime heat capture is eliminated, because heat does not have to be driven through the wall to reach the interior air space: some of it reflects or re-radiates immediately from the floor. Provided the slab has air channels like the Trombe wall, which run through it in the north-south direction and are vented to the interior air space through the concrete slab floor just inside the north and south walls, vigorous air thermosiphoning through the slab still occurs as in the vertical Trombe wall, distributing the impounded heat throughout the house (and cooling the house in summer by the reverse process).

The ventilated horizontal slab is less expensive to construct than vertical Trombe walls, as it forms the foundation of the house which is a necessary expense in any building. Slab-on-grade foundations are a common, well-understood and cost-effective building component (modified only slightly by the inclusion of a layer of concrete-brick air channels), rather than an exotic Trombe wall construct. The only remaining drawback to this kind of thermal mass [[solar architecture]] is the absence of a basement, as in any slab-on grade design.

The '''''Kachadorian floor''''' design is a ''direct-gain'' passive solar system, but its thermal mass also acts as an ''indirect'' heating (or cooling) element, giving up its heat at night. It is an alternating cycle hybrid energy system, like a [[hybrid electric vehicle]].

===Isolated solar system===
In an '''''isolated gain passive solar system''','' the components (e.g., collector and thermal storage) are isolated from the indoor area of the building.{{Sfn|Wujek|2010}}

An '''''attached sunspace''''', also sometimes called a '''''solar room''''' or '''''solarium''''', is a type of isolated gain solar system with a glazed interior space or room that is part of or attached to a building but which can be completely closed off from the main occupied areas. It functions like an attached greenhouse that makes use of a combination of direct-gain and indirect-gain system characteristics. A sunspace may be called and appear like a greenhouse, but a greenhouse is designed to grow plants whereas a sunspace is designed to provide heat and aesthetics to a building. Sunspaces are very popular passive design elements because they expand the living areas of a building and offer a room to grow plants and other vegetation. In moderate and cold climates, however, supplemental space heating is required to keep plants from freezing during extremely cold weather.

An attached sunspace's south-facing glass collects solar energy as in a direct-gain system. The simplest sunspace design is to install vertical windows with no overhead glazing. Sunspaces may experience high heat gain and high heat loss through their abundance of glazing. Although horizontal and sloped glazing collects more heat in the winter, it is minimized to prevent overheating during summer months. Although overhead glazing can be aesthetically pleasing, an insulated roof provides better thermal performance. Skylights can be used to provide some daylighting potential. Vertical glazing can maximize gain in winter, when the angle of the sun is low, and yield less heat gain during the summer. Vertical glass is less expensive, easier to install and insulate, and not as prone to leaking, fogging, breaking, and other glass failures. A combination of vertical glazing and some sloped glazing is acceptable if summer shading is provided. A well-designed overhang may be all that is necessary to shade the glazing in the summer.

The temperature variations caused by the heat losses and gains can be moderated by thermal mass and low-emissivity windows. Thermal mass can include a masonry floor, a masonry wall bordering the house, or water containers. Distribution of heat to the building can be accomplished through ceiling and floor level vents, windows, doors, or fans. In a common design, thermal mass wall situated on the back of the sunspace adjacent to the living space will function like an indirect-gain thermal mass wall. Solar energy entering the sunspace is retained in the thermal mass. Solar heat is conveyed into the building by conduction through the shared mass wall in the rear of the sunspace and by vents (like an unvented thermal storage wall) or through openings in the wall that permit airflow from the sunspace to the indoor space by convection (like a vented thermal storage wall).

In cold climates, double glazing should be used to reduce conductive losses through the glass to the outside. Night-time heat loss, although significant during winter months, is not as essential in the sunspace as with direct gain systems since the sunspace can be closed off from the rest of the building. In temperate and cold climates, thermally isolating the sunspace from the building at night is important. Large glass panels, French doors, or sliding glass doors between the building and attached sunspace will maintain an open feeling without the heat loss associated with an open space.

A sunspace with a masonry thermal wall will need approximately 0.3&nbsp;ft<sup>2</sup> of thermal mass wall surface per ft<sup>2</sup> of floor area being heated (0.3 m<sup>2</sup> per m<sup>2</sup> of floor area), depending on climate. Wall thicknesses should be similar to a thermal storage wall. If a water wall is used between the sunspace and living space, about 0.20&nbsp;ft<sup>2</sup> of thermal mass wall surface per ft<sup>2</sup> of floor area being heated (0.2 m<sup>2</sup> per m<sup>2</sup> of floor area) is appropriate. In most climates, a ventilation system is required in summer months to prevent overheating. Generally, vast overhead (horizontal) and east- and west-facing glass areas should not be used in a sunspace without special precautions for summer overheating such as using heat-reflecting glass and providing summer-shading systems areas.

The internal surfaces of the thermal mass should be dark in color. Movable insulation (e.g., window coverings, shades, shutters) can be used help trap the warm air in the sunspace both after the sun has set and during cloudy weather. When closed during extremely hot days, window coverings can help keep the sunspace from overheating.

To maximize comfort and efficiency, the non-glass sunspace walls, ceiling and foundation should be well insulated. The perimeter of the foundation wall or slab should be insulated to the frost line or around the slab perimeter. In a temperate or cold climate, the east and west walls of the sunspace should be insulated (no glass).