- Passive Solar Design: Three Approaches
- Elements of Passive Solar Design
- Features to Enhance Passive Solar
Passive Solar Heating
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.
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.
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.
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.
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 “,” 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.
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.”
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.
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
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.”
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.”
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.
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.
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 “” (EBN June 2011) and , 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 “,” EBN Nov. 2011), this is a viable strategy for the right client. Automated interior shading devices are becoming more common as well.
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.
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.
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.”
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.