In passive solar building design, windows, walls, and floors are designed to gather, hold, bounce off, and spread solar energy as heat during winter and block excess heat during summer. This method is called passive solar design because it does not use machines or electrical systems, unlike active solar heating systems.

The main goal of designing a passive solar building is to use the local climate effectively by carefully studying the area. Important factors to consider include the size and position of windows, the type of glass used, materials that keep heat in or out, materials that absorb and release heat slowly, and ways to block sunlight. Passive solar design works best for new buildings, but older buildings can also be changed or updated to use these techniques.

Passive energy gain

Passive solar technologies use sunlight without using machines (unlike active solar, which uses thermal collectors). These systems change sunlight into heat that can be used in water, air, and materials that store heat. They also move air for ventilation or future use, using very little other energy. A common example is a solarium on the side of a building near the equator. Passive cooling uses similar design ideas to lower the need for cooling in summer.

Some passive systems use a small amount of regular energy to control parts like dampers, shutters, and night insulation. These parts help collect, store, and use solar energy while reducing unwanted heat transfer.

Passive solar technologies include direct and indirect solar gain for heating rooms, solar water heating using thermosiphon systems, materials that store heat and change temperature slowly, solar cookers, solar chimneys to improve natural airflow, and earth sheltering. More broadly, solar technologies include solar furnaces, but these usually need some outside energy to align mirrors or receivers. Historically, solar furnaces have not been practical or cost-effective for large use. Lower energy needs, like heating spaces and water, have been better suited for using solar energy passively.

As a science

Passive solar building design is based on several scientific fields, including the study of weather patterns, how heat moves (through conduction, convection, and electromagnetic radiation), how air and water naturally move without machines, and how humans and animals feel comfortable in different temperatures. These principles apply to homes, greenhouses, and other buildings where people or plants live.

Designers focus on factors like the building’s location, how it faces the sun, the local weather patterns, the amount of sunlight the area receives, the quality of materials used, the size and placement of windows and walls, and the use of materials that store heat.

While these ideas can be used in any building, creating an efficient and cost-effective design requires careful planning and combining these scientific principles into a complete system. Modern tools, such as computer programs like the U.S. Department of Energy’s "Energy Plus" software, and lessons learned since the 1970s energy crisis, help reduce energy use and environmental harm without sacrificing comfort or appearance. Features like greenhouses or sunrooms can improve a home’s livability, lighting, and value at a low cost.

Since the 1970s, many attempts to build zero-energy buildings (buildings that use no heating or cooling energy) have failed because they relied on incorrect ideas instead of science.

Passive solar building construction can be simple and affordable, using common materials and technology. However, designing it properly requires careful study of past lessons and time to test and improve designs using computer models.

After a building is completed, tools like thermal imaging cameras can help identify areas where heat is lost or gained, such as windows or skylights that cause problems in extreme weather.

Sophisticated computer programs, like "Energy Plus," have been developed to help engineers apply scientific knowledge to building design.

Designing passive solar buildings with scientific methods is complex and not easy for someone without experience. Poor designs and unscientific ideas have led to wasted money and disappointing results.

Using scientific design could save the United States over $250,000,000 each year in energy costs and pollution if applied widely since the 1980s.

Since 1979, passive solar design has been important in experiments by schools and governments, including the U.S. Department of Energy. While the cost-effective methods were proven long ago, changing how buildings are designed and built has been slow.

New subjects like architectural science and technology are being added to some schools to teach these scientific and engineering principles in the future.

The solar path in passive design

The ability to reach these goals at the same time depends on how the sun's path changes with the seasons. This happens because Earth's axis is tilted compared to its orbit around the sun. The sun's path is different for each latitude.

In the Northern Hemisphere, areas more than 23.5 degrees away from the equator experience:
• The sun reaching its highest point toward the south (toward the equator)
• As winter solstice nears, the sun rises and sets further south, and daylight hours shorten
• In summer, the sun rises and sets further north, and daylight hours increase

The opposite happens in the Southern Hemisphere. However, the sun always rises east and sets west, no matter which hemisphere you are in.

In equatorial regions (less than 23.5 degrees from the equator), the sun’s position at solar noon moves back and forth between north and south throughout the year.

In areas closer than 23.5 degrees to the North or South Pole:
• During summer, the sun moves in a full circle in the sky without setting
• During winter, the sun never rises above the horizon

The 47-degree difference in the sun’s height at solar noon between winter and summer is important for passive solar design. This information, combined with local climate data (degree day) for heating and cooling needs, helps determine when solar heat is helpful for comfort and when it should be blocked using shading. By placing features like windows and shading devices strategically, the amount of solar heat entering a building can be controlled year-round.

One challenge in passive solar design is that even though the sun is in the same position six weeks before and after the solstice, the temperature and solar heat needs differ because of the time it takes for Earth to warm or cool (thermal lag). Movable shutters, shades, or window covers can adjust to daily and hourly changes in solar heat and insulation needs.

Careful room placement completes passive solar design. A common suggestion for homes is to place living areas where the sun is highest at noon and sleeping areas on the opposite side. A heliodon is a traditional tool used by architects to model sun paths. Today, 3D computer graphics can simulate sun path effects and predict performance.

Passive solar heat transfer principles

Personal thermal comfort depends on health factors (medical, psychological, social, and situational), air temperature, how warm or cool surfaces around you feel, air movement (such as wind or turbulence), and humidity (which affects how well your body cools itself through sweating). Heat moves in buildings through convection (heat moving with air), conduction (heat moving through solid materials), and radiation (heat moving as waves from one surface to another) through the roof, walls, floor, and windows.

Convective heat transfer can help or harm. Poorly sealed buildings can lose up to 40% of heat in winter through air leaks, but opening windows or vents strategically can improve airflow and cooling in summer if outside air is comfortable. Systems that filter air can remove dust, pollen, and germs from ventilation.

Warm air rises and cool air falls, creating uneven heat layers in a room. This can cause temperature differences between the top and bottom of a space, help vent hot air, or be used intentionally to distribute heat naturally. Fans can help cool the body by moving air, but they might disrupt warm air layers near the ceiling or increase heat transfer through windows. High humidity makes it harder for sweat to evaporate and cool the body.

Radiant energy is the main way heat moves, with the sun as the primary source. Solar heat enters buildings mainly through roofs and windows. Warm surfaces emit heat toward cooler ones. Roofs receive most solar heat. Cool or green roofs with reflective materials can keep attics cooler than outside temperatures.

Windows allow heat to enter during the day and escape at night. Heat moves through windows using light particles called photons. Even on cold, clear days, sunlight can add heat. Insulated glass, shading, and window placement reduce heat gain. Windows are harder to insulate than roofs or walls. Air movement around window coverings can also reduce their insulation. External shading (like overhangs) is better at blocking heat than internal coverings.

East and west-facing windows can provide light and warmth but may overheat in summer without shading. South-facing windows let in light and warmth in winter but can be shaded with overhangs or trees that lose leaves in fall. The amount of heat from the sun depends on location, altitude, cloud cover, and the angle of sunlight.

Some building materials can store heat and release it slowly to balance temperature changes between day and night. Understanding how heat moves can be tricky for new designers. Computer models help plan buildings effectively without costly mistakes.

Site specific considerations during design

• Latitude, the path of the sun, and the amount of sunlight received
• Changes in how much the sun warms or cools a place during different seasons, such as measures of temperature changes, sunlight levels, and moisture in the air
• Changes in temperature throughout the day
• Local climate factors such as wind patterns, moisture levels, plants, and the shape of the land
• Features that block sunlight or affect wind patterns, such as buildings or trees

Design elements for residential buildings in temperate climates

• The location of rooms, internal doors, walls, and equipment within the house.
• Positioning the building to face the equator (or slightly east to capture morning sunlight).
• Designing the building to be longer from east to west.
• Making windows the right size to allow sunlight during winter and block it during summer.
• Reducing the number of windows on other sides, especially on the west side.
• Installing roof overhangs or shading features (like plants, trees, fences, or shutters) that are the correct size for the building's latitude.
• Using the right amount and type of insulation, including radiant barriers and bulk insulation, to reduce heat loss in winter and heat gain in summer.
• Using materials that absorb and store extra heat during the day in winter, which is then released at night.

The exact amount of glass facing the equator and the use of thermal mass should be decided based on the building's latitude, altitude, local climate, and the area's heating and cooling needs.

Factors that can reduce energy efficiency:
• Not aligning the building correctly with the equator or having an improper east-west length-to-height ratio.
• Too much glass, which can cause overheating, glare, and damage to furniture when it gets cold outside.
• Installing glass in areas where sunlight and heat cannot be easily controlled, such as on the west side, at angles, or in skylights.
• Heat loss through windows that are not insulated or protected.
• Not having enough shading during times of high sunlight, especially on the west wall.
• Using thermal mass in a way that does not help control daily temperature changes.
• Open staircases that cause warm air to rise and uneven heating between upper and lower floors.
• A building shape with too much surface area compared to its volume, such as too many corners.
• Poor weatherization that allows too much air to leak in or out.
• Missing or improperly installed radiant barriers during hot weather. (Also see cool roof and green roof.)
• Using insulation materials that do not match the main way heat moves (such as through convection, conduction, or radiation).

Efficiency and economics of passive solar heating

Passive solar heating (PSH) is very efficient. Direct-gain systems can use 65–70% of the energy from sunlight that reaches the collector or aperture.

The passive solar fraction (PSF) is the percentage of heat needed that is provided by PSH. This shows how much heating costs could be reduced. RETScreen International has reported PSF values of 20–50%. In sustainability, saving even 15% of energy is considered important.

Other sources report these PSF values:
• 5–25% for modest systems
• 40% for "highly optimized" systems
• Up to 75% for "very intense" systems

In favorable climates, such as the southwest United States, highly optimized systems can achieve PSF values greater than 75%.

For more information, see Solar Air Heat.

Key passive solar building configurations

There are three main types of passive solar energy systems, plus one type that combines features of two systems:

• direct solar systems
• indirect solar systems
• hybrid direct/indirect solar systems
• isolated solar systems

In a direct-gain passive solar system, the inside of the building collects, absorbs, and spreads heat from the sun. Windows facing south (or north in the southern hemisphere) let sunlight into the building. The sunlight directly heats surfaces like concrete or brick floors and walls, or indirectly heats them through air movement. These surfaces, called thermal mass, store heat during the day and release it into the room at night.

In cold climates, a sun-tempered building is a simple form of direct-gain system. It uses slightly more south-facing windows than usual but does not add extra thermal mass. The building is well-insulated, long from east to west, and has most windows on the south side. South-facing windows should cover about 5 to 7% of the total floor area, less in sunny areas, to avoid overheating. More windows can be added only if more thermal mass is included. This system saves only a small amount of energy and is low-cost.

In true direct-gain systems, enough thermal mass is needed to keep indoor temperatures steady. Too little or poorly designed thermal mass can cause the building to overheat. About half to two-thirds of floors, walls, and ceilings should be made of materials that store heat, like concrete, brick, or water. These materials should be exposed to sunlight, so avoid thick carpets, large furniture, or heavy wall hangings.

For every 1 foot of south-facing glass, about 5 to 10 feet of thermal mass is needed. This means thermal mass should cover about 5 to 10 times the area of the glass. Thermal mass should be no thicker than 4 inches (100 mm) and placed where sunlight hits it for at least 2 hours daily. Dark, heat-absorbing colors work best for surfaces in direct sunlight. Lightweight materials, like drywall, can be any color. Covering windows with insulation panels at night improves performance. Water in containers heats faster and more evenly than solid materials because of natural air movement.

In a direct-gain system, south-facing windows should cover about 10 to 20% of the floor area. This is based on the actual glass area, not the full window size. Too much glass can cause overheating, glare, or fabric fading.

In an indirect-gain system, thermal mass (like concrete or water) is placed behind south-facing windows, away from the inside space. This prevents direct sunlight from entering the room and blocks the view through the glass. There are two types: thermal storage wall systems and roof pond systems.

A thermal storage wall, also called a Trombe wall, is a thick wall behind south-facing glass. It absorbs sunlight during the day and releases heat at night. The wall is made of materials like concrete or brick, coated with a dark, heat-absorbing surface, and covered with glass. The glass is placed 3/4 to 2 inches away from the wall to create an air gap. This system avoids glare and large windows but limits views and daylight. If furniture or cabinets block the wall’s interior, its performance decreases.

A classic Trombe wall has vents near the top and bottom of the wall. During the day, sunlight heats the air between the glass and wall, causing it to rise and flow into the room through the top vent. At night, vents must be closed to prevent heat from escaping outside.

Additional measures

Steps should be taken to keep heat inside at night, such as using window coverings or movable insulation.

The sun doesn't shine all the time. Heat storage, like materials that hold heat, keeps the building warm when the sun isn't shining.

In solar homes that rely on daily solar heating, the storage system is built to last for one or a few days. The usual method is a specifically built heat storage system. This includes a Trombe wall, a ventilated concrete floor, a cistern, water wall, or roof pond. It is also possible to use the natural heat storage of the earth, either as it is or by building into the structure with materials like rammed earth.

In very cold areas, or places with long periods without sunlight (such as weeks of freezing fog), building special heat storage systems is expensive. Don Stephens developed an experimental method to use the ground as a large heat storage system for the whole year. His designs use a special underground pipe system 3 meters below the house and cover the ground with a 6-meter waterproof barrier.

Thermal insulation or extra thick insulation (type, placement, and amount) reduces heat escaping. Some passive buildings are made mostly of insulation.

The effectiveness of solar heating systems is improved by using insulating materials (such as double glazing), special window coatings (low-e), or movable window insulation (like window quilts or interior shutters).

Windows that face the equator should not use coatings that block sunlight.

Many homes in Germany use highly insulated windows. The choice of special window coatings depends on how much heating or cooling is needed in the area.

Windows facing the equator are different from other sides of a building. Reflective coatings and multiple layers of glass can reduce useful sunlight. However, direct solar heating systems rely more on double or triple glazing, or even quadruple glazing, in colder areas to reduce heat loss. Indirect or isolated solar systems may still work well with single-pane glass. The best solution depends on the location and system design.

Skylights let in bright sunlight from above, which can cause glare on flat or sloped roofs. Horizontal skylights sometimes use reflectors to increase sunlight, but this can make glare worse. In winter, when the sun is low, most sunlight reflects off sloped glass. In summer, when the sun is high, sunlight hits sloped glass directly, causing too much heat. Skylights should be covered and well-insulated to reduce heat loss in winter and heat gain in summer.

In the Northern Hemisphere, the equator-facing side is south. Skylights on non-equator-facing roofs mainly provide indirect light, except during summer when the sun may rise on the non-equator side. Skylights on east-facing roofs get the most sunlight and heat in the morning during summer. Skylights on west-facing roofs get sunlight and heat in the afternoon, which is the hottest part of the day.

Some skylights use expensive glass that reduces summer heat but still allows light. However, if visible light passes through, some heat also enters.

To reduce summer heat from sloped glass, install skylights under trees that lose leaves or use movable insulation on the inside or outside. This would block summer light.

Landscaping and gardens

Energy-efficient landscaping uses hard materials and plants to help control how the sun's energy is used. Designing landscapes with trees, hedges, and structures like trellises or pergolas covered in vines can provide shade during hot summer months. For winter, deciduous plants that lose their leaves in the fall allow sunlight to enter homes, helping to keep them warm. Evergreen shrubs and trees, which keep their leaves year-round, can be planted at different heights and distances to block cold winter winds. Using xeriscaping methods, such as planting native species that need little water, drip irrigation systems, mulch, and organic gardening practices, reduces the need for water, energy, and equipment that use gas. Solar-powered lights, fountains, and covered pools with solar water heaters also help lower the energy use of outdoor spaces.

• Sustainable gardening
• Sustainable landscaping
• Sustainable landscape architecture

Other passive solar principles

Passive solar lighting methods help make better use of natural light inside buildings, which reduces the need for artificial lighting. This can be done by carefully designing buildings, positioning them correctly, and placing windows to collect sunlight. Other methods include using reflective surfaces to bring daylight into buildings. Windows should be the right size, and to prevent too much light, they can be protected with features like Brise soleil, awnings, trees, special glass coatings, and other passive or active tools.

A major challenge for many windows is that they can allow too much heat to enter or escape from a building. While high windows or skylights can bring light into areas that are not well positioned, controlling unwanted heat can be difficult. The energy saved by reducing artificial lighting is often canceled out by the energy needed to operate heating, ventilation, and air conditioning systems to keep the building comfortable.

Solutions include using window coverings, insulated glass, and new materials like aerogel that let light through, optical fibers in walls or roofs, or hybrid solar lighting developed at Oak Ridge National Laboratory. Reflective surfaces, such as light shelves, light-colored walls and floors, mirrored sections, glass panels in walls, and clear or translucent doors, help direct captured light deeper into buildings. This light can come from windows, skylights, solar light tubes, or active daylighting systems. In traditional Japanese buildings, Shōji sliding doors with translucent Washi screens are an early example. Architectural styles like International, Modernist, and Mid-century modern designs also used these methods in homes, offices, and other buildings.

There are many ways to use solar energy to heat water for homes. Different active and passive solar water heating systems have different costs and benefits depending on the location. Passive solar water heating systems do not use pumps or electricity and are very cost-effective in areas with mild weather that is not too cold or cloudy. Other active systems may be better suited for certain locations.

Some active solar water heating systems can work without being connected to the power grid and are considered sustainable. This is achieved by using photovoltaic cells, which convert sunlight into electricity to power the pumps.

Comparison to the Passive House standard in Europe

In Europe, more people are supporting the methods promoted by the Passive House (Passivhaus in German) Institute in Germany. This approach uses all possible natural heat sources, reduces energy use, and focuses on strong insulation and careful construction to prevent heat loss and cold air entering buildings. Most buildings built to the Passive House standard also include a ventilation system that recovers heat, sometimes with a small heating unit (usually 1 kW).

The energy design of Passive House buildings is planned using a tool called the Passive House Planning Package (PHPP), which is updated regularly. The latest version is PHPP 9.6 (2018). A building can be certified as a "Passive House" if it meets specific requirements, including that the total heat needed each year should be no more than 15 kWh per square meter.

Comparison to the Zero heating building

With improvements in advanced window technology, a Passive House-based building that uses almost no heating energy is being suggested to replace nearly-zero energy buildings in the European Union. These zero-heating buildings use less reliance on designs that capture sunlight for warmth, allowing more flexibility in traditional building styles. The yearly heating energy needed for these buildings should not go over 3 kWh per square meter. Zero-heating buildings are easier to plan and manage. For example, they do not require adjustable window coverings to control sunlight.

Design tools

Traditionally, a heliodon was used to show the position of the sun in the sky shining on a model building at any time during the year. Today, computer programs can create models of this effect and use information about the local weather and environment, such as shadows and physical barriers, to predict how much sunlight a building design can collect throughout the year. Smartphone apps that use GPS technology can now do this cheaply using a handheld device. These design tools help passive solar designers check local conditions, building features, and direction before construction begins. Improving a building's energy performance usually needs a process that involves trying different designs and making improvements step by step. There is no single passive solar building design that works well in all locations.

Levels of application

Many homes in suburbs can lower heating costs without changing their look, comfort, or how they work. This is done by placing the house well, positioning windows correctly, using some thermal mass, good insulation, weatherization, and sometimes adding a heater like a radiator connected to a solar water heater. Sunlight can warm a wall during the day, and the wall will then release that heat into the house at night. Features like shading or a reflective barrier with space between can help reduce too much heat from the sun in summer.

This method is part of a larger idea called "passive solar," which captures heat in warm months and stores it for use in colder months. These designs use large amounts of thermal mass or connect to the ground to store more heat. Some people say these methods work, but no official studies have proven they are better than other methods. These systems can also help cool buildings during hot months. Examples include:

• Passive Annual Heat Storage (PAHS) – by John Hait
• Annualized Geothermal Solar (AGS) heating – by Don Stephen
• Earthed-roof

A "purely passive" solar-heated house would not use a furnace, relying only on heat from sunlight and small amounts of heat from lights, computers, cooking, and people. Natural air movement, instead of fans, helps circulate air. Some systems use small fans or solar-heated chimneys to improve airflow. A way to measure how well these systems work is by using their coefficient of performance (COP). For example, a heat pump might use 1 unit of energy to deliver 4 units of heat, giving a COP of 4. A system that uses a 30 W fan to spread 10 kW of solar heat would have a COP of 300.

Passive solar design is often a key part of a cost-effective zero-energy building (ZEB). While ZEBs use passive solar ideas, they also use active systems like wind turbines, solar panels, and geothermal energy. Passive solar is also important for passive survivability, which means a building can stay livable even during emergencies.

Recently, there has been interest in using the large surface area of skyscrapers to improve energy efficiency. Skyscrapers use a lot of energy, so using passive solar methods could save a lot. A study on a proposed building in London found that rotating the building for better airflow and daylight, using thick walls, and installing special windows could lower energy use by 35%. Other methods included using thick walls, covering windows with special glass, and using a Trombe wall to store heat. Overhangs block summer sunlight and allow winter sunlight, and reflective blinds help control heat in summer.

Another study looked at using green walls on skyscrapers in Hong Kong. These walls, covered with plants, can reduce air conditioning use by up to 80%.

In temperate climates, methods like using windows, adjusting how much of a wall is glass, adding shading, and designing roofs can save 30% to 60% in energy use.