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Passive Solar Heating

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By Alex Wilson

I was fortunate to have been involved in the glory days of the passive solar energy movement. In the late 1970s I worked in Santa Fe for the New Mexico Solar Energy Association, which was at the epicenter of the burgeoning movement to create buildings that relied on south-facing glass, integrated thermal mass, and carefully designed building geometries to deliver heat in the winter and maintain natural cooling in the summer. We thought we had the answers to the world’s energy woes. It was a heady time.

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The 20,000 ft2 Congregation Beth David Synagogue in San Luis Obispo, California, was completed in 2006 at a cost of $161/ft2. Measured energy consumption shows 82% savings compared with a standard Title-24 building in California—and a 90% reduction in CO2 emissions. A combination of direct gain and water walls, with effective shading, provides passive solar heating, daylighting, natural cooling, and natural ventilation.

Above: David Bainbridge, Below: San Luis Sustainability Group

In some ways, passive solar was a low-tech response to the first wave of solar energy systems that emerged following the 1973 energy crisis. Those early active-solar systems were complex and prone to failure, and many of them were only marginally successful when they operated as designed. Passive solar systems were far simpler, with no moving parts. The buildings became the solar heating system, with solar collection, heat storage, and distribution handled passively and by virtue of the geometry, materials, and design.

While passive solar remains in the DNA of green building, we’re now more likely to hear about high-performing buildings with R-5 windows and R-40 walls, with rooftop solar-electric systems and high-tech energy recovery gadgetry. Is passive solar still relevant? I decided to find out.

Passive Solar Design: Three Approaches

In passive solar buildings, solar energy collection, storage, and distribution are integrated into the building design. There are three classic approaches to passive solar: direct gain, indirect gain, and isolated gain.

Direct gain: South-facing windows

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Designed by architect Bill Maclay, the River House is a net-zero-energy home in Vermont’s Mad River Valley with deep overhangs that shade the extensive area of south-facing, low-e glass. Removal of most trees to the south was necessary to provide good solar exposure on this wooded site.

Photo: Maclay Architects

South-facing windows (or north-facing for our readers in the Southern Hemisphere)—including clerestory windows that bring light deeper into a building—capture sunlight, while thermal mass that is distributed around the building interior absorbs and stores some of that solar energy, releasing it to the interior over an extended period. Direct-gain designs are the simplest and most practical passive solar systems, and they are by far the most common.

Indirect gain: Thermal storage walls

Rather than having sunlight shine directly into the occupied space, indirect-gain passive solar systems have thermal mass positioned between the glazing and the living space. With Trombe walls (named after the French inventor Felix Trombe), that thermal mass is provided by masonry or concrete. Water walls use containers of water in place of masonry. The glazing is positioned a few inches to a foot or more away from the south-facing wall. Sunlight is absorbed by the dark surface, and that absorbed heat slowly moves through the wall to the interior surface, where it radiates warmth to the living space. Indirect-gain systems are usually installed today as supplements to direct-gain designs.

Isolated gain: Sunspaces

In a third flavor of passive solar system, isolated-gain systems capture solar energy outside the insulated building envelope, usually in a “sunspace,” and allow it to flow into the building by convection through windows, doors, or vents through the wall. There is typically thermal storage in the space to moderate nighttime temperatures, but the useful heat is delivered primarily during the daytime. Some sunspaces provide both isolated gain and indirect gain—if the back wall of the sunspace (the south wall of the building to which it is attached) is uninsulated masonry and can serve to store and transmit heat.

Passive solar vs. suntempering

A distinction is often made between passive solar design and suntempering. With direct-gain passive solar designs, one often saw the rule of thumb that up to 12% of the floor area could be in south-facing glass, while suntempered spaces could have no more than about 7% of the floor area in south-facing glass. Suntempered buildings are essentially standard buildings with appropriate orientation and somewhat more of the glass placed on the south walls than on other façades.

True passive solar designs involve higher percentages of south-facing glass relative to floor area and depend on thermal storage to prevent spaces from being too hot during the day and too cold at night. Suntempered spaces rely only on the inherent thermal mass in standard light-frame construction systems and furnishings.

Why Passive Solar Is Important

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This 2,600 ft2 home in St. Peter, Minnesota, designed by Sarah Nettleton Architects, features passive solar design with concrete floors for thermal storage. Sliding shade screens can be seen on the south-facing fenestration. The house achieved LEED Gold, missing Platinum by just a few points.

Photo: Don Wong

There has been a strong trend within the green building movement in recent years to create buildings that go well beyond compliance with energy codes. For some, the goal is net-zero energy, for others carbon neutrality or fossil-fuel avoidance. With any of these approaches, creating a highly energy-efficient building envelope is the first priority, and it’s a requirement of Passive House certification (see “Passive House Arrives in North America,” EBN Apr. 2010).

But along with the energy performance of the building envelope, solar gain is also important with any of these performance targets—especially Passive House. Even a well-insulated box will eventually reach the ambient average outdoor temperature if there are no energy inputs to it. If the goal is to avoid fossil fuels or carbon emissions in operating that building, solar energy is the best option we have for achieving net-zero-energy performance.

One can begin to understand the importance of passive solar gain in a Passive House by examining Passive House Planning Package (PHPP) energy modeling program outputs. David White, a Passive House consultant and the owner of Right Environments in Brooklyn, New York, says that for a typical Passive House in the Northeast that relies on its heating system to provide 15 kWh/m2·yr (the maximum for Passive House certification), solar gain is supplying about 15 kWh/m2·yr and internal loads another 10 kWh/m2·yr. In other words, even though energy conservation still does the heavy lifting, the supplemental heating load would double if it weren’t for passive solar gain.

Not As Relevant As It Used to Be

When J. Douglas Balcomb, Ph.D., and his research associates at Los Alamos National Laboratory were refining definitions of solar savings and coming up with rules of thumb for south-facing glass and thermal mass, almost no windows exceeded R-2; walls and ceilings rarely surpassed R-15 and R-35, respectively. It's a different world today, with triple glazing, low-e coatings, and gas fills pushing center-of-glass window R-values above R-8 and insulation levels commonly reaching R-40 for walls and R-60 for ceilings—at least within the green building community.

In these ultra-low-energy buildings, heating simply isn’t as big an issue today as it was thirty years ago, argues Marc Rosenbaum, P.E., of South Mountain Company. “The Los Alamos folks never thought about buildings with as little heat loss as what we’re creating today,” he told EBN. When you’re at the Passive House level of performance, it’s not unusual to have heating account for as little as 20% of the total energy use of a home, according to Rosenbaum. Water heating is often a greater demand. In Passive House monitoring, Efficiency Vermont found 1,700–1,800 kWh per year for heat and about 3,000 kWh per year for water heating.

Rosenbaum is a leading proponent of renewable energy, but his interest in passive solar has waned over the years. “To be honest, I’m not doing it,” he admitted. “If I have to spend extra money to put extra mass in a house, it’s not going to come up very far in my list of priorities. I can spend that money better elsewhere.”

Other experts EBN contacted about passive solar design were more positive about its place today. Ken Haggard, founder of the San Luis Sustainability Group and coauthor with David Bainbridge of Passive Solar Architecture (Chelsea Green Publishing, 2011), has been incorporating passive solar design features into nearly all of his projects for thirty-plus years, and he remains a firm proponent. But he argues that it’s not just about passive solar heating. “It’s important not to define passive design strictly on the basis of heating,” he told EBN. He considers passive design to include cooling, ventilation, and daylighting as well.

Mike Nicklas, FAIA, of Innovative Design in Raleigh, North Carolina, has been a proponent of passive solar design and daylighting since the 1970s and has served several times as chairman of the American Solar Energy Society. While most of his work is with larger buildings where passive solar heating isn’t as relevant (due to internal loads), he always works passive solar into homes and into smaller, skin-dominated commercial buildings. He cringes at what he sees as a new trend with net-zero-energy buildings of forgetting about passive solar and relying on a larger PV system. “People just do what they want to do and throw on a bunch of PV at the end,” he complained.

Elements of Passive Solar Design

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The Science & Technology Facility at the National Renewable Energy Laboratory in Golden, Colorado, completed in 2006, demonstrates the potential of both passive and active systems, including advanced glazings, daylighting, natural cooling, and extensive use of interior thermal mass in a large (71,000 ft2) net-zero-energy laboratory building. This was the first federal laboratory building to achieve LEED-Platinum certification.

Photo: DOE/NREL, Eric Telesmanich

Incorporating passive solar into our buildings can mean a lot of different things. While some strategies have fallen out of favor, others aren’t likely to ever go out of style. Here are key elements of passive solar design, including heating as well as cooling-load avoidance and daylighting.

Applicability of passive solar

Passive solar heating is best suited to residential and smaller, skin-dominated commercial buildings. This means that the San Luis Sustainability Group pursues skin-dominated buildings even for its larger projects. “We feel that skin-dominated buildings are better ecologically and socially as well as being more balanced in regard to heating, cooling, and daylighting—as well as more aesthetically acceptable,” says Haggard. “Load-dominated buildings are easier to heat but more difficult to passively cool and ventilate due to their bulk.”

Orientation and geometry

Passive solar buildings should have the long axis oriented east to west, maximizing the south exposure (or north exposure, in the Southern Hemisphere). Except near the equator, south-facing glass transmits maximum sunlight in the winter, when the sun is low in the sky, and transmits proportionately less sunlight in the summer months because the sun is higher—with sunlight striking the glass at a more acute angle. With south-facing windows, very simple fixed overhangs or awnings can effectively shade the glass in summer.

Carefully balancing that shading is important for swing conditions. In the fall, outside temperatures are still fairly warm, so the fact that the sun stays lower in the sky (allowing more of it to reach south-facing windows protected by overhangs) can be problematic. In the spring, that sunlight is very beneficial because outside temperatures are still quite cool. Thus, a fixed overhang may provide more shading than desired in the spring but not enough shading in the fall. Adjustable awnings are one solution to that concern—although, with the help of simple computer modeling, fixed overhangs can be configured to provide a reasonable compromise. In larger buildings, a narrow footprint helps maximize solar access.

Bringing solar energy into a building

Glazing is the foundation of passive solar design. It has also proven to be one of the most challenging elements—particularly during the 1990s and early 2000s, when most U.S. window manufacturers offered only heat-rejecting windows with low-solar-heat-gain-coefficient (SHGC) glass. To function well for passive solar, glazing should transmit as much sunlight as possible. In the early days of passive solar design, clear, double-glazed insulating glass units (IGUs) were most commonly used. With the advent of low-e glazings, heat loss was greatly reduced—but so was solar gain.

Nicklas, who has battled against the nearly universal shift to low-e glazings, still specifies clear, double-glazed glass on the south—at least for higher glass used for daylighting (in commercial buildings, for comfort reasons, he may specify low-e for glass below seven feet). On the east and west façades, he usually specifies tinted low-e glass, and on the north untinted low-e.

On residential projects and in more northern climates, it is common to have only two window specifications: one for the south and another for all other façades. Marc Rosenbaum is now regularly specifying triple glazing with center-of-glass U-factors as low as 0.11 (R-9) and SHGC values as high as 0.62. With this glass, he’s getting about 80% of the solar gain that double-glazed clear IGUs used to provide—but with significantly less heat loss. To put that into a Btu framework, assuming a clear day and a 40°F temperature differential, these new top-performing windows provide about 300 Btu/ft2·day less solar gain than the older, clear-glass options, but the R-6 windows would lose just 160 Btu/ft2·day, compared with 480 Btu/ft2·day for those older windows (a difference of 320 Btu/ft2·day). Rather than using poorly performing windows and compensating with thermal mass, Rosenbaum argues, it makes better financial sense to use better windows and a smaller heating system.

Several designers EBN spoke with commonly specify different window dimensions for different orientations to avoid mix-ups on the jobsite. The problem of mix-up is so significant that building scientist Terry Brennan, when he was more involved in design and construction, gave up on tuning windows by orientation. “It’s hard to get the right windows installed in the right places,” he told EBN. “I go for the R-value over solar heat gain.”

In cold regions, snow cover can improve performance in the winter months—just when that additional solar gain is most beneficial. The typical increase in solar gain from snow-covered ground is about 5%, according to the manufacturer of the Solar Pathfinder, a site-assessment tool.

Absorption of solar energy

There’s a common (mistaken) assumption that surfaces need to be dark to absorb sunlight. It is true that darker surfaces absorb solar energy more effectively than light-colored surfaces, but dark surfaces in the building interior also prevent visible light from bouncing around, requiring additional electric lighting to be provided.

“One passive function must not conflict with another,” said Haggard. “From a lighting standpoint, dark interiors are inefficient and glare-producing.” Haggard feels that natural lighting is the prime function of passive solar building because it so strongly affects occupant health and also reduces the need for electricity. “Our overriding rule is that buildings should be as light as possible on the inside and as dark as possible on the outside; dark buildings tend to recede into their landscape setting in almost all natural environments.” With thermal masonry and water walls, it’s a different matter. These should have absorptivity as high as possible to maximize heat collection.

Storage of solar energy

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Inside the home of Jack and Mary Spear in St. Peter, Minnesota, thermal mass is provided by the poured-concrete floor.

Photo: Don Wong

Thermal storage is a critical component of passive solar heating—nearly as important as glazing. Without enough thermal mass in a building, air temperatures will rise too much during sunny days, and there will be inadequate carryover into the evening hours. Thermal mass also increases construction cost—so there is motivation to skimp. “The critical failure of direct gain [in the 1970s and ’80s] was not enough thermal mass,” says Rosenbaum. He feels it is still not adequately addressed in a lot of Passive House designs. “I’m concerned about overheating,” he told EBN. In PHPP modeling, “there’s only one input cell in the whole thing for thermal mass,” he said. German buildings typically have more mass, according to Rosenbaum, which he thinks may explain why thermal mass is not a significant focus of the Passive House program.

To work well as thermal mass, materials need adequate heat capacity; that’s why heavy materials work well. But thermal storage materials also have to be reasonably conductive so that heat can move into and out of them easily—though not too conductive. Copper has great heat capacity, but heat moves out of it too quickly for it to provide effective thermal mass in buildings. Wood has a reasonable heat capacity, but the conductivity is too low for it to work really well; it’s hard to push enough heat into the wood. Water has very high thermal capacity but also relatively high effective thermal conductivity (due to convection of the water). This property can make water walls or roof ponds great cooling systems in hot climates, but they have to be engineered carefully to ensure that they will also work well for heating.

Thermal mass can be distributed or concentrated. Ken Haggard likes a combination of the two. The distributed mass his firm provides is generally at least two inches of stucco on frame walls or exposed concrete-block interior walls along with an exposed slab floor. “We have finally trained the local contractors to do interior, two-inch-thick stucco walls without complaining about the curing requirements,” he noted. He generally aims for dedicating the entire floor, plus about half of the interior wall area, to thermal mass. “If at the end we find we need a little more mass, usually determined by the performance modeling, we add a double layer of 5⁄8" drywall to the ceiling.”

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Architect Ken Haggard used a combination of direct gain and water walls in the Congregation Beth David Synagogue; the water walls can be seen beneath the south-facing windows.

Photo: David Bainbridge

In addition to that distributed mass, Haggard usually provides indirect gain by placing glazed 9"-thick (240 mm) water tanks under the south-facing windows. Most of Haggard’s buildings end up with some water walls. “We have local welders who do the tanks inexpensively and have never in 30 years had a leak,” he reports.

Phase change materials (PCMs) offer an exciting thermal storage opportunity for passive solar buildings. “Advances in micro-encapsulated phase-change materials could be a game changer for increasing thermal capacitance in lightweight construction,” says Michael Holtz, FAIA, principal at LightLouver. Innovative Design has been incorporating PCMs into passive solar designs for decades—using numerous products that have come and gone.

Nicklas estimates that his firm has used PCMs on at least 50 projects over the years, but he mostly relies on more traditional materials: veneers of masonry and stone, plaster walls, tile floors, and brick. In spaces that are used a lot, he often aims for 4"–6" (100–150 mm) of thermal mass in the floor and a 4" (150 mm) veneer on the walls. “It’s hard to beat a brick,” he added, noting that bricks are both inexpensive and attractive.

Whatever the thermal mass, there is usually a cost associated with it. “We’ve found in our practice that thermal mass is the only passive component that adds expense to the construction of a passive building,” said Haggard. “This is why we’re so interested in the new nanotech phase-change materials that can be added to drywall or masonry.”

Distribution of passive solar heat

Passive solar buildings by definition do not rely on fans or pumps to distribute heat captured from the sun. With direct gain, the distributed thermal storage materials release heat based on temperature differences between that mass and the room. When the indoor air temperature is lower than the surface temperature of the mass, there is a net release of heat to the space. With adequate solar gain and properly sized mass, enough heat will be stored during the day to keep the space comfortable into the evening hours.

With indirect gain, the heat delivery is also primarily by radiant flow from the warm inner surface, but the flow is mostly in one direction—through the thermal storage material.

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Plastered straw-bale walls provide distributed thermal mass in the Congregation Beth David Synagogue, and the light wall colors effectively bounce daylight throughout the space.

Photo: San Luis Sustainability Group

With both direct- and indirect-gain systems, convection also plays an important role in heat distribution. Air in contact with the surface of the thermal mass warms up and rises through its natural buoyancy, establishing convective loops in the living space. Convection also helps distribute warm air throughout the occupied space—delivering solar heat to spaces that may not be directly heated by the sun. This is why open floor plans are beneficial with passive solar buildings.

Features to Enhance Passive Solar

Complementary strategies and components are what make passive solar designs really shine.

Exterior shading

While the winter sun is low in the sky and reaches windows beneath fixed overhangs that easily block the summer sun, the need for shading differs significantly in spring and fall. Fixed overhangs may not be as satisfactory as adjustable awnings, exterior roll-down shade screens, or exterior roller blinds at these times. These window attachments can provide a high level of controllability, and some systems can even be automated based on indoor and outdoor conditions. For more on window attachments, see “Making Windows Work Better” (EBN June 2011) and WindowAttachments.org, which BuildingGreen manages for the U.S. Department of Energy.

Moveable insulation on windows

For optimal passive solar performance, south-facing glass should be as clear as possible (with a high SHGC). Some passive solar experts, including Mike Nicklas, argue against the overuse of low-e coatings for this reason. To achieve the desired solar gain without causing excessive heat loss, moveable insulation can be used on clear-glass windows. “The best solutions are still double-clear glass with an insulating shade or panel behind it,” says Paul Torcellini, Ph.D., P.E., of the National Renewable Energy Laboratory.

While insulating cellular blinds and other types of moveable insulation have the drawback of requiring occupant actions to achieve effectiveness (and studies have shown that occupants may not regularly take those actions—see “Occupant Engagement—Where Design Meets Performance,” EBN Nov. 2011), this is a viable strategy for the right client. Automated interior shading devices are becoming more common as well.

Landscaping

While shading systems incorporated into the building provide the greatest flexibility, a lot can be accomplished with carefully planned landscape plantings. Deciduous trees provide excellent shade during the summer months and into the fall, then lose their leaves in late fall, admitting more solar gain. Be aware, though, that the branch structure of a typical deciduous tree will block about half of the direct sunlight. Placement of landscaping close enough to a building to provide effective shade can also cause interference with air circulation and rainwater drainage systems.

Tall annual plants and vines can also be very effective natural shading systems for passive solar buildings. They can even work well shading east and west façades that are difficult to shade with fixed overhangs and awnings.

Integrated ventilation of thermal mass

Passive solar expert David Bainbridge is a proponent of nighttime convective cooling. In places with reasonably high diurnal temperature cycling, directing ventilation air across thermal mass can cool the mass during nighttime hours, allowing it to absorb heat during the day and keep the house cooler. This strategy doesn’t work in climates that are too humid for night-flushing or that do not cool down adequately at night. The same approach can function on a building’s roof using a roof pond, though the complexity and the liability risk of having water sitting on a roof keep this approach far out of the mainstream—even though this was the most effective of the classic passive solar systems for both heating and cooling, according to Haggard.

Options for Back-Up Heat

As Norbert Lechner, FAIA, so clearly articulates in the textbook Heating, Cooling, Lighting: Sustainable Design Methods for Architects (John Wiley, 3rd edition, 2008), whenever possible we should rely on passive systems for heating, cooling, and lighting before we look to electrical and mechanical options. But in the vast majority of buildings, passive systems will not satisfy 100% of energy needs. With passive solar heating, what are the best options for back-up heat?

It may make more sense to focus on what systems aren’t appropriate. In general, when there is significant solar gain, one should avoid heating systems with a significant flywheel effect, such as masonry heaters or concrete slabs with radiant-floor heating. With these systems, heat may still be delivered to the living space many hours after the energy inputs into those systems have ceased, resulting in overheating.

Better complements to passive solar design are small, responsive heating systems, such as forced-air or hydronic central heating systems, or space heaters, such as through-the-wall-vented gas heaters, pellet stoves, or mini-split air-source heat pumps—the latter becoming a favorite of many leading-edge designers today. Reflecting on 30-plus years of low-energy building design, Andy Shapiro of Energy Balance in Montpelier, Vermont, noted that he started off with solar pioneer Norman Saunders “doing these incredibly complicated systems. It keeps getting simpler and simpler.” He now gets heating loads so low that small point-source heating systems are perfectly adequate; and they obviate the need for expensive distribution systems.

Rules of Thumb and the Passive Solar Design Process

Rules of thumb for direct-gain passive solar systems

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Source: Passive Solar Architecture by David Bainbridge and Ken Haggard, Chelsea Green Publishing, 2011

To provide guidance on how much solar glazing and thermal mass to provide, researchers in the ’70s and ’80s came up with detailed rules of thumb. Doug Balcolm and his team at Los Alamos National Laboratory published these rules in the much-cited Passive Solar Design Handbook, Volume 2.

How relevant are these rules of thumb today? Have they been replaced with newer values that account for better-performing glazings and building enclosures?

Surprisingly, most of the experts contacted for this article still rely on the old principles from around 1980. “The rules of thumb have not changed considerably over the past few decades,” suggests Michael Holtz, though he notes that products have changed—including insulation materials, glazings, and HVAC options—so the design guidelines have to respond to that.

A common design practice is to start off with simple rules of thumb and plug those into energy modeling software (see the table "Software to Aid in Passive Solar Design" for information on the most common energy modeling tools for that process) to test the outcome. NREL’s Torcellini calls the rules of thumb “a good starting point.”

Software to Aid in Passive Solar Design

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Bainbridge and Haggard published simple rules of thumb in their 2011 book Passive Solar Architecture (see table). In a cold climate with only heating demand, for example, the south glass can range from 10% to 25% of the building floor area, with 6–11 ft2 of distributed thermal mass needed per square foot of south glass.

Suntempering Is Increasingly the Right Solution

The goal of using solar energy to heat our buildings is every bit as relevant today as it was in the 1970s and ’80s. But as our insulation levels, window energy performance, and airtightness have improved, the risk of passive solar gain overheating these spaces has increased.

The safer approach today is to aim for suntempering, with smaller areas of south-facing glass (or lower-SHGC glass if larger glazing areas are desired for aesthetic reasons). Advanced low-e coatings mean that less heat will be lost back out through the glass at night, so more modest glazing areas will result in smaller temperature swings and fewer periods of overheating.

If true passive solar heating—with south glass areas exceeding about 8% of the floor area—are called for, very careful attention must be paid to thermal mass to keep temperature swings within reason. Newer phase-change materials may help provide enough thermal mass, but whatever the thermal storage provided, expect a cost to be associated with it.

Although it’s still prudent to reduce energy needs as much as possible as a first step, PV has an increasingly important role in low-energy buildings. In highly insulated buildings, some of the leading energy designers are coupling a suntempering strategy with supplemental electric heating using mini-split air-source heat pumps that are powered by rooftop PV modules. In such a system, the baseline heating can still be provided by direct-gain solar and the people in the space, while carefully controlled peak heating requirements can be met with electricity generated on the building site. Solar energy—indeed, passive solar if one defines passive as not requiring fans or pumps—will provide all of the needed heat, and comfort can be maintained within tight tolerances.

Continuing Education

Receive continuing education credit for reading this article. The American Institute of Architects (AIA) has approved this course for 1 HSW Learning Unit. The Green Building Certification Institute (GBCI) has approved this course for 1 GBCI CE hour towards the LEED Credential Maintenance Program. The International Living Future Institute (ILFI) has approved this course for 1 CEU.

Learning Objectives

Upon completing this course, participants will be able to:

  1. Explain the three classic approaches to passive solar.
  2. Understand the relevance of passive solar today.
  3. Describe some key elements of passive solar design.
  4. List some features to enhance passive solar heating.

To earn continuing education credit, make sure you are logged into your personal BuildingGreen account, then read this article and pass this quiz. In addition, to receive continuing education credit for ILFI, please add to the discussion forum on this page by providing a thoughtful comment on the article—for example, its effect on your practice and engagement with Living Building Challenge concepts and petals.

Discussion Questions

Use the following questions to inform class discussions or homework assignments.

  1. The article concludes that suntempering is most suited to the needs of low-energy, highly-insulated buildings, in which “PV has an increasingly important role.” Consider this in light of our need for more "resilient design."
  2. “Passive solar systems were far simpler, with no moving parts. The buildings themselves became the solar heating system, with solar collection, heat storage, and distribution handled passively and by virtue of the geometry, materials, and design.” Discuss the poetry or art, perhaps even spirituality, inherent in these concepts. Where else can this be found in the green building world? Where is it missing in the most glaring ways?
  3. Imagine you are designing two homes, side by side, in the Northeast, for twin brothers, Jim and Tim. Jim wants his to be passive solar. Tim wants his to be a true net-zero-energy building, but doesn’t want to feel “out-sunlighted” by his twin. How does your design process differ for the two? If possible, spend some time with one or more of the software programs (listed in the article) that aid in passive solar designs.
  4. If, for optimal passive solar performance, “the best solutions are still double-clear glass with an insulating shade or panel behind it,” but moveable insulation has “the drawback of requiring occupant actions,” explore some methods of reintegrating into society an appreciation for actively engaging with buildings.
  5. Along with heating, Ken Haggard includes cooling, ventilation, and daylighting in his definition of “passive design.” Explore how you might achieve such goals in a residential or smaller, skin-dominated commercial building in your area.

Comments (6)

1 Passive solar is more relevant and easier than ever posted by Mark DeKay on 07/18/2014 at 01:35 pm

This is a great general article on passive solar heating. Those interested in net-zero buildings please pay attention. PV is still more expensive than grid energy, and much more expensive than free passive heating. New window and envelope technologies that lower loads do not really change the old principles. They only modify them, meaning less glass is required, and since total heat flows in and out of mass are less on a daily basis, less mass is also needed to provide the same solar savings fraction. Maybe we need to tweak the old rules-of-thumb to account for extremely low loads. Some attention to different strategies appropriate for different climate zones would be useful in any passive solar discussion. For example, norther Europe depends more on isolated gain in hybrid air-collector systems to avoid glass losses at night. Large direct gain is less effective in cold climates, etc.

Mark DeKay, co-author, Sun, Wind & Light: architectural design strategies, 3rd edition

2 Heady Times posted by Dennis Latta on 05/31/2014 at 11:53 pm

I too was in my prime when the passive solar movement was beginning. I soaked up the news and worshiped the books but found that in northern California views often trump solar orientation. It is a rare find to have both a nice southern exposure and windows facing the direction you want to look. But my cat reminds me that all I need is a little direct sunlight to be happy. 

3 Passive Solar vs. Suntemperin posted by Amanda Sorell on 07/03/2012 at 09:44 am

Great overview addressing several questions I've been wondering about, Alex. --Cheryl Long, Mother Earth News

4 Passive solar article posted by Wayne L. Appleyard on 07/04/2012 at 03:50 am

Alex,

Great article on passive solar. The one thing that should have been emphasized more it the time lag you can get with Trombe walls. An 8" concrete wall with a selective surface on the outside, only starts raising its interior surface temperature at 3:30PM and maxes out at 11PM at about 95 degrees F. A 12" wall will further delay the heat so the wall doesn't taper off its heat output until sunrise. It is like having a very long solar day. This combination with direct gain allows for a larger solar fraction. You also missed mentioning the BGW2004 solar program which is easier to use than Energy Plus and Equest, will do combined systems easily but is limited in its glass input possibilities. I recently did a presentation for the local AIA2030 course on passive solar and really looked at modelling. The variability of annual weather is such that modelling is most useful for comparing designs and less so for predicting percent solar. Having designed passive homes for 35 years, the two comments that I get back are 1) " I didn't know how much difference it would make to live in a well daylit house" and 2) " I love my mass walls". One clients only complaint was that when they had their bed up against my 8" thick mass wall was that she would wake up at 4am feeling the reduced heat from the mass wall and wanting another blanket. I highly recommend Bainbridge and Haggards book. Good article, as always.

Wayne Appleyard Architect, LEED AP Sunstructures Architects Chairman of the Ann Arbor Energy Commission

5 A Different Strategy for Hot posted by Stephen Colley on 07/04/2012 at 02:03 pm

I was right out of architecture school when Mazria's Passive Solar book was published and I soaked up the information like a sponge. In actual architectural practice it became quite evident the passive solar in that excellent book and in subsequent studies did not work well in South Texas (or as I would imagine in most of climate zones 1 and 2). It's humid here, and summer low temperatures often don't fall below 80 degrees F. Our heating season is so short, fixed overhangs to allow direct solar gain in January don't help in spring and fall when sun is still coming in the windows and we are in cooling mode. I find that the building envelope needs to be very well insulated and roof overhangs should be extensive to block direct solar gain. I depend heavily on shade trees, trellises and awnings when wide overhangs may not be appropriate or practical, and this year, I'm beginning to design with compressed earth block which in addition to being a thermal mass, is shown to be a good phase change material as well. We are also going to investigate designs using a "rat trap" bond that allows for a significant thermal break between two wythes of earthen blocks.

6 Disadvantages of Passive Sola posted by Ibrahim El-Shair on 03/11/2014 at 09:37 am

1- Limited Power Capabilities. 2- Upfront Costs. 3- Climate Dependent. 4- Appearance. So, having all the windows facing South, may mean that a view is missed and windows overlook unpleasant aspects of the surrounding landscape.

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June 29, 2012