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The Solar Envelope: How to Heat and Cool Cities without Fossil Fuels

Modern research, which combines ancient knowledge with fast computing techniques, shows that passive solar cities are a realistic option, allowing for surprisingly high population densities.


Illustration by Diego Marmolejo for Low-tech Magazine.

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Architects all over the world have demonstrated the usefulness of buildings which are heated and cooled by design rather than by fossil fuel energy. What has received much less attention, however, is the possibility of applying this approach to entire urban neighbourhoods and cities.

Designing a single, often free-standing, passive solar house is quite different from planning a densely populated city where each building is heated and cooled using only natural energy sources. And yet, if we want passive solar design to be more than just a curiosity, this is exactly what we need. Modern research, which combines ancient knowledge with fast computing techniques, shows that passive solar cities are a realistic option, allowing for surprisingly high population densities.

Passive solar design has been around for thousands of years, and even predates the use of glass windows.

Passive solar design requires the knowledge to design and orientate buildings so that they can be heated by the sun. Coupled with other low-tech solutions such as thermal underwear, heated clothing and the creation of microclimates, passive solar design could all but eliminate the use of fossil fuels and biomass for heating buildings throughout large parts of the world.

Indirectly, a passive solar house can also cancel the energy requirements for cooling and ventilation (passive cooling), and for lighting during the day. Of course, passive solar buildings can be outfitted with solar water heaters and PV solar panels, further reducing the use of unsustainable energy resources. Passive solar design does not involve any new technology. In fact, it has been around for thousands of years, and even predates the use of glass windows.

For most of human history, buildings were adapted to the local climate through a consideration of their location, orientation and shape, as well as the appropriate building materials. This resulted in many vernacular building styles in different parts of the world. In contrast, most modern buildings look the same wherever they stand. They are made from the same materials, they follow forms that are driven by fashion rather than by climate, and are most often randomly located and oriented, indifferent to the path of the sun and the prevailing wind conditions.


Illustration by Diego Marmolejo for Low-tech Magazine.

Modern buildings rely on a massive supply of cheap fossil fuels for heating, cooling, and lighting. Take the supply of cheap fossil fuels away, and they become completely uninhabitable for most of the year: they are too cold, too hot or too dark. This radical change in architectural design was caused by both the arrival of cheap and abundant energy sources and the resultant urbanisation (See part 2).

The Industrial Revolution relocated millions of people from the countryside to the cities. When most of us lived and worked on farms or in hamlets, it was fairly easy to orientate one’s house towards the sun. In an urban environment, however, building orientation is generally determined by street layout, and one building can easily overshadow another. High-rise buildings further complicate solar access.

From solar oriented buildings to solar oriented cities

This does not mean that passive solar design could not be applied to entire cities. It just takes more sophisticated planning. Solar access to an individual building is determined by only four factors: latitude (the distance north or south from the equator), slope, building shape and orientation.

Solar access to a city (or any other built-up environment) is determined by seven factors: the four just mentioned, plus the height of the buildings, the width of the streets, and the orientation of the streets (See part 2). Providing ventilation in an urban environment is determined by the same factors, with the exception that latitude is replaced by prevailing wind conditions.


Image: Ralph Knowles developed and refined a method that strikes an optimal balance between population density and solar access: the “Solar Envelope”.

While most research in passive solar design during the 1970s was directed at individual buildings, one man began forty years of research into solar oriented cities: Ralph Knowles, professor emeritus at the USC’s School of Architecture and author of three fascinating books on the topic (1974, 1981, 2006).

Knowles developed and refined a method that strikes an optimal balance between population density and solar access: the “Solar Envelope”. It is a set of imaginary boundaries, enclosing a building site, that regulate development in relation to the sun’s motion—which is predictable throughout the seasons for any place on Earth.

Buildings within the solar envelope do not overshadow neighbouring buildings during critical energy-receiving periods of the day and the year

Buildings within this imaginary container do not overshadow neighbouring buildings during critical energy-receiving periods of the day and the season, and assure solar access for both passive and active solar systems. On the one hand, the solar envelope allows architects to design with sunlight without fear that their ideas will be cancelled out by future buildings. On the other hand, the solar envelope recognizes the need for development and high population densities, by defining the largest container of space that would not cast shadows off-site at specified times of the day.


Image: Floor-to-area ratio and solar access. Ralph Knowles.

Knowles and his students have reached densities that are far above the average in European and American cities (See part 3), with the exception of high-rise centers such as Manhattan.

Modification of traditional zoning practices

The solar envelope is actually a relatively simple modification of existing zoning practices, which also set imaginary boundaries that enclose a building site—determining the maximum height, width and depth of future buildings. The most rigid approach in conventional zoning prescribes maximum building heights, set in feet or metres, number of floors, or both. A second, more flexible approach, sets limits based on a ratio between developeable land and floor area within the building on that site.

For example, a floor-to-area ratio (FAR) of 6 means that architects can develop 6 times the developeable square footage of land within the setbacks. They could cover the entire site with 6 stories, or cover only half of the site with 12 stories, for example.

Although both zoning methods offer a certain degree of solar access in a city, they are far from optimal. The main problem is that they do not design building orientation with its solar impact in mind, which can be as critical as building height. For example, a skyscraper with its broad flat sides facing east and west will cast a relatively small midday winter shadow, while one oriented with its broad flat sides facing north and south will shade a much larger area during the sunniest periods of the day (illustration above). Taking orientation into account would greatly improve solar access for surrounding buildings, without sacrificing housing density.

The geometry of the solar envelope

Compared to conventional zoning practices, the solar envelope produces a different geometry—the limits of the envelope derive their vertical dimensions from the sun’s daily and seasonal movements. Thus, while conventional zoning envelopes are shaped like a box, the solar envelope has both vertical and sloping spaces.

As a result, the buildings and city blocks that fill these imaginary solar envelopes are more likely to have unique shapes. One side of a building would not look like the other, nor would each side of the street. In the northern hemisphere, development would tend to be lower on the south side of a street than on the north where a major southern exposure would be preserved. Streets take on a directional character where solar orientation is clearly recognised.

Adjacent buildings can meet each other gently, rather than abruptly, across property sidelines. Tall buildings would group together at the site’s southwestern end, and those of moderate height at the northeastern end, with the shortest buildings taking up the site’s midsection. Buildings on corner lots will be taller because their shadows can extend accross the street in two directions instead of one.


Image: Solar envelopes on the Spanish street grid system in Los Angeles. Ralph Knowles.


Buildings within the solar envelopes shown above. Ralph Knowles.


Building designs under the solar envelope are characterised by roof terraces, courtyards and clerestories. Ralph Knowles.

Within the solar envelope, certain architectural characteristics have great consistency. For instance, roof terraces appear where the sloping sides of the envelope intersect the rectilineair geometry of buildings. Courtyards are another crucial element, as they introduce sunlight and heat to deep interiors. Clerestories allow for the penetration of winter sun down stairways to enliven otherwise darker, lower floors. Sunscreens and porches are everywhere, keeping the sun out in summer.

Defining solar access

The solar envelope is not only defined by the path of the sun, but also by fixed parameters set by the designer. Choosing these will determine the balance between solar access and development potential.

The most important choice is the definition of the hours during which we want to avoid casting shadows on adjacent land—the ‘cut-off times’. The longer the period of daily solar access, the smaller the developeable volume under the envelope. Obviously, setting the cut-off times as equal to the period between sunrise and sunset would not work, because in that case few or any buildings could be constructed. For passive solar design, a minimum of 4 to 6 hours per day in winter is considered practical, depending on the climate.


Image: Generating a solar envelope.

The duration of solar access could also be set by a minimum percentage of available energy instead of determining a minimum hours of sunshine. In that case, cut-off times would change over the course of the year. Another parameter to be set is the ‘shadow fence’. It determines the minimum height to which solar access has to be assured; for instance zero, 3 or 6 metres above street level. For example, one can choose to allow shadowing of garages and shops in order to improve the density under the solar envelope.

What about existing buildings?

Solar envelopes can be designed for individual buildings or as a single envelope for a group of houses, a neighbourhood, a district or even an entire city. This is a rather straightforward process when a site is being designed from scratch, but often current buildings will complicate the generation of a solar envelope. When the solar envelope is applied in line with existing buildings, new construction would always be shaped and proportioned with reference to the old. Each new phase of development changes the surroundings and thus the context within which the next envelope is generated.


Image: A solar envelope casting its maximum shadow in winter. The smaller, previously built houses retain their solar access. Ralph Knowles.


Image: One of the building designs within the solar envelope shown above. Ralph Knowles.

It is important to note that the solar envelope only protects neighbouring properties. It is the architect who must ensure solar access to the buildings within the envelope, tackling problems of overshadowing within the envelope itself. For larger sites, the volume of a solar envelope is therefore larger than the volume of the buildings that actually fill it, at least when solar access is assured to all dwellings on site.

Solar oriented cities in Antiquity

Knowles’ research draws on ancient knowledge, most notably the solar planned cities in Ancient Greece and the solar communities of the Ancient Pueblo People in what is today the Southwestern United States. The Ancient Greeks built entire cities which were optimal for solar exposure.

In the fifth century BC, for example, a neighbourhood for about 2500 people was built in the city of Olynthus. The streets were built perpendicular to each other, running long in the east-west direction (the horizontal streets shown in the plan below), so that all houses (five on each side of the street) could be built with southern exposure.


Image: Passive solar house in Ancient Greece.


Image: Street plan of the Ancient Greek City Olynthus.

A gridirion street plan oriented at the cardinal points was not new at the time, and neither is it proof of a design aimed at maximum solar exposure. But the Greeks did more. In “*A Golden Thread: 2500 Years of Solar Architecture and Technology“, Ken Butti and John Perlin note that all houses were consistently built around a south-facing courtyard:

The houses that faced south on the street and south to the sun were entered through the court, straight from the street. The houses that faced north to the street and south to the sun were entered through a passageway that led from the street through the main body of the house and into the court, from which access was gained to all other spaces.”

In keeping with the democratic ethos of the period, the height of buildings was strictly limited so that each courtyard received an equal amount of sunshine:

In winter, rays from the sun traveling low across the southern sky streamed across the south-facing courts, throgh the portico, and into the house - heating the main rooms. The north walls were made of adobe bricks one and a half feet thick, which kept out the cold north winds of winter.”


Image: The Ancient Greek city of Priene.

Another obvious example of Ancient Greek solar planning was Priene (illustration above), rebuilt in 350 BC and located in present-day Turkey. The city had about 4000 inhabitants living in 400 houses. Its buildings and street plan were similar to those in Olynthus, but because the city was built on the slope of a steep mountain, many of the fifteen secondary streets (running north-south) were actually stairways. The seven main avenues were terraced on an east-west axis.

Native Americans

The Ancient Pueblo People or “Anasazi” built a number of sophisticated solar oriented communities during the 11th and 12th centuries AD in what is now the Southwestern United States: Long House at Mesa Verde, Pueblo Bonito in Northern Mexico and the “sky city” of Acoma.

These communities followed a different building style than that of the Greeks. The Ancient Pueblo People constructed terraced buildings of up to three floors high. These were buildings that would fit perfectly in a solar envelope with slanting lines.


Image: Illustration of Acoma Pueblo, by Gary S. Shigemura (from “Energy and Form”, Ralph Knowles).

Acoma pueblo (illustration above) is one example of these orderly, solar planned communities. It consists of three rows of houses built along streets running east and west, so that each building faces south. The streets that separate the houses have a width that allows winter shadows to cover the whole of the adjoining street, stopping just before the following row of buildings.


Knowles’ research combines the best elements of these historical designs and incorporates modern technology that greatly facilitates the generation of a solar envelope. The heliodon, invented in the 1930s, is a contraption that creates a geometrical relationship between an architectural scale model and (a representation of) the sun. More recently, software versions of the heliodon have made the technology much more affordable, while allowing for the fast generation of even very complex solar envelopes.


Illustration by Diego Marmolejo for Low-tech Magazine.

On larger sites in particular, and when already existing buildings complicate the generation of a solar envelope, the available computer software saves time and can result in more building volume.

Continue reading: 1 / 2 / 3.

Kris De Decker (edited by Deva Lee)