HEM May/June 1991 - Home Energy Magazine Online May/June 1991

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Home Energy Magazine Online May/June 1991

SOLAR

Passive Solar Design: Housewarming With Many Efficient Returns

Cub Kahn

Cub Kahn is an environmental writer/photographer based in the Shenandoah Valley of Virginia.

New design strategies that rely on the sun can reward the architect, builder, and occupant with a home that is beautiful, comfortable, and energy-efficient.

The Passive Solar Industries Council (PSIC) and the Solar Energy Research Institute (SERI) have advanced the cause of passive solar design in residential construction by publishing passive solar guidelines for home builders. Passive Solar Design Strategies: Guidelines for Home Builders covers fundamental concepts and techniques of passive solar design and construction. The authors have included tables which illustrate how passive solar design can be applied to reduce energy needs by 20%, 40%, and even 60%. Guidelines also provides location-specific worksheets allowing the reader to compare the potential energy performance of various combinations of passive solar design elements in a planned house. The four worksheets allow builders and designers to compute conservation performance level, auxiliary heat performance level, thermal mass and comfort (that is, temperature swing on a sunny winter day), and summer cooling performance level. A computer program, BuilderGuide, has also been developed to assist the user in completing the worksheets.

Dr. Douglas Balcomb of SERI, whose research forms the technical basis of Guidelines, believes that passive solar design is really cost-effective in today's marketplace. He adds that builders in most regions will find that the public is receptive to the low initial cost, excellent building comfort, and lifetime energy savings offered by passive solar.

Versions of Guidelines have already been produced for 17 cities and regions from Burlington, Vt., to Atlanta, Denver, and Los Angeles. In addition, Guidelines can be generated that are specifically tailored to the climate, geography, and other location- specific factors for 2,400 other areas throughout the United States. Each one costs $350 for a newly generated version, $50 for one already created. PSIC also sponsors passive solar design workshops to facilitate the use of Guidelines by builders, architects, utilities, and energy agencies. (For information about Guidelines and BuilderGuide, see "Sources of Passive Solar Information.")

This article outlines some basic passive solar concepts covered in Guidelines and probes some of the issues related to the future of residential passive solar.

Basics of Passive Solar Design

Passive solar refers to the design and construction of residences that make use of the free energy inputs of the sun. "Passive" is distinguished from "active" solar technologies which use mechanical means to transfer energy. Guidelines defines passive solar techniques as going beyond conservation and "suntempering" to a complete system of collection, storage, and use of solar energy. These techniques entail use of more south-facing glass (or glazing), adding significant thermal mass, and taking steps to control and distribute heat energy throughout the house. (Suntempering refers to a modest increase in south-facing window area without other passive solar design elements such as increased thermal mass.)

The two fundamental elements of passive solar design are south-facing glass and thermal mass. The solar energy collected through the south-facing glass is stored in thermal mass-material that stores and slowly releases thermal energy. Thermal mass is typically contained in walls, floors, and ceilings constructed of material with high heat capacity such as tile, poured concrete, brick, or concrete masonry (See Table 1).

The most common residential passive solar system is known as direct gain, which simply allows sunlight coming through south-facing glass to pass directly into the space to be heated, and then be stored in the thermal mass of the interior walls or floor. A good example of direct gain is a parked car facing south on a sunny day; solar energy passing through the windshield is stored in the thermal mass of the seats and dashboard.

The greater the area of south-facing glass in a house, the greater the thermal mass that must be present for energy storage. Inadequate thermal mass can result in overheating- even on sunny winter days-and poor overall performance of passive solar systems. A proper balance of direct-gain glazing (window glass) and thermal mass depends on the quantity, type, thickness and location of the thermal mass. For example, brick has approximately 21/2 times the heat capacity of gypsum wallboard. Guidelines provides formulas for computing a suitable amount of south-facing glazing relative to the amount of thermal mass in floors, walls, and ceilings.

In addition to thermal mass, energy performance of direct gain systems depends on window orientation, type of glazing (for example, double, triple, or low-emissivity) and whether night insulation is used on the windows. The ideal orientation for solar glazing to obtain maximum solar gain is within five degrees of true south, but orientations up to 30 degrees from true south can produce a significant solar energy contribution. (In considering solar orientation, the difference between true north and magnetic north should be taken into account. For instance, in Denver, true north is 13 degrees west of magnetic north as measured by a compass.)

Thermal storage walls are south-facing glazed walls, most commonly made of heavy masonry. A small air space, generally no more than several inches in width, separates the glazing from the thermal mass. Masonry thermal storage walls are also known as Trombe walls. Alternatively, the thermal mass may be in the form of a water wall, consisting of lightweight but rigid containers of water.

A thermal storage wall is an indirect gain system in that sunlight first heats the thermal mass, which then indirectly heats the living space in the house. Summer heat gain provided by a thermal storage wall is substantially less than from a similar amount of direct gain glass. However, the slow energy release by thermal storage walls makes them ideal for providing heat to the house after sunset. To increase energy performance, special adhesive foils called selective surfaces may be applied to the exterior side of thermal storage walls to increase solar absorption and reduce back-radiation.

Sunspaces (sunrooms, solariums, greenhouses) are widely used passive solar design elements that can provide additional living space as well as improved energy performance for the whole house. A sunspace is termed an isolated gain system, as differentiated from a direct gain system, because solar energy collected during the day in the sunspace can be circulated through the house. At night the doors and windows in the sunspace can be closed to retain heat in the house. Important sunspace considerations-which depend on the local climate, site, and orientation-include the type and amount of thermal mass, type and amount of glazing, summer ventilation of the sunspace, and insulation of the common wall between the sunspace and the rest of the house.

Passive solar homes frequently take advantage of natural cooling provided by appropriate sizing and siting of windows, shading, and natural ventilation. Shading can be accomplished with roof overhangs, porches, landscaping, or with interior or exterior shades-awnings, screens, shutters, and blinds. The air movement provided by ceiling fans makes them another effective and inexpensive cooling strategy.

Passive solar elements and natural cooling features can be most effective when used in combination. Also critical to passive solar performance are site planning (for example, unshaded winter exposure on the south side) and interior space planning (for example, living areas on the south side). Other important design considerations for conserving energy include adequately insulating, limiting air infiltration, and carefully planning non- south glazing. Designing a passive solar system in tandem with mechanical heating and cooling systems is a must. Particular attention should be given to mechanical system sizing, automatic night setback, and duct design. Passive solar elements are only one part of a complete residential energy efficiency system.

The authors of Guidelines put it succinctly: "A sensible energy-efficient house uses a combination of techniques. Builders and designers are advised to take an integrated approach, keeping in mind the house as a total system."

Beyond improved energy efficiency, Guidelines touts passive solar homes for their attractive living environments, comfort, low maintenance, high owner satisfaction, resale value, and environmental sensitivity. Guidelines cites national surveys in the 1980s by the U.S. Department of Energy and SERI that revealed passive solar homes average approximately 30% less energy for heating than conventional houses. The surveys also found that more than 90% of passive solar home occupants are very satisfied with the performance of their homes.

SERI's Residential Class B Passive Solar Performance Monitoring Program carefully measured the performance of dozens of passive solar homes in the early to mid-1980s. The study found that these homes have low auxiliary heating needs, that very tight homes are more efficient than leaky ones, and that the performance of passive solar systems is dependent on the habits of building occupants.

The Future of Passive Solar

Barbara Harwood, a Dallas-based builder of low-income housing, believes passive solar is the heating of the future, but laments, "People don't even know what [passive solar] is." She feels that builders must create a market for residential passive solar by educating consumers about it.

Bruce Hunn, University of Texas research engineer, echoes Harwood, saying, "Education is the main issue." He adds, "Getting proper design tools into the hands of the decision- makers is the key. The decision-makers include builders, architects, and engineers, and homeowners and the design tools must match their needs and technical skills."

In that regard, Bion Howard, of the Alliance to Save Energy and a participant in the development of Guidelines, thinks these design tools are well matched to the technical acuity of builders. He sees Guidelines as a simple yet accurate means for use in the design phase of energy-efficient residences. Builders unfamiliar with passive solar design should read Guidelines, pick an analytical tool such as BuilderGuide, and become skilled at using the tool. After using it on several buildings, the builder can evaluate the actual performance of the houses and develop an intuitive feel for passive solar. Howard cautions that builders need to provide homeowners with information on proper use of their energy systems. Correct operation of elements such as shades and HVAC equipment can be critical to the energy performance of a passive solar house.

State and local building codes have to provide incentives for builders to use passive solar designs, declares Brady Bancroft of the Rocky Mountain Institute. On the other hand, there are those like Douglas Balcomb of SERI who think that building codes should be based on performance and that passive solar should get credit in the codes if it produces a required level of performance. He believes, however, that passive solar should not be mandated in codes. Likewise, John Spears of GEOMET Technologies and also an advisor on Guidelines, states, "For a building code to prescribe use of passive solar is not wise. For example, some building sites have excellent solar access but on other sites proper solar orientation of a house may be impossible."

Other influences may increase the popularity of passive solar strategies. Financial pluses such as value (as judged by lenders and appraisers), savings in energy costs, and utility rate incentives are the best selling points for passive solar homes, according to Fredericksburg, Va., builder Don Carr. He feels these carrots are preferable to the stick of prescriptive building codes.

Larry Sherwood, executive director of the American Solar Energy Society, is convinced that builders will be attracted to passive solar if they understand passive solar homes will provide them with a market niche. And their attractiveness to the buyer will partly depend upon how well they mesh with prevailing styles in an area. He observes that the added cost of passive solar features on a custom house is minimal. Energy efficiency is a quality issue, and today quality is a strong selling point.

Along with energy efficiency, enhanced quality of life is another advantage of passive solar homes, believes Subrato Chandra, director of research and development at the Florida Solar Energy Center. He observes that other types of designs-such as superinsulated homes-can produce energy performance similar to passive solar homes, and that ultimately the market will decide which designs are used.

Of course, superinsulation and passive solar systems are not necessarily exclusive. A passive solar home performs much better if it is also superinsulated, according to builder Bill Eich, president of the Energy Efficient Building Organization. He adds that it would be foolish to build a superinsulated house without including passive solar elements.

In any case, Guidelines has helped defuse the superinsulated versus passive solar debate of the past by offering builders the happy medium of a well-insulated, energy- efficient design that incorporates passive solar elements. Guidelines, in the words of John Spears, "helps you understand the relation between insulation, glazing, and thermal mass." He thinks Guidelines is most useful for someone who is just starting to design homes with passive solar features, because "it keeps you from getting into trouble with an unworkable design."

As for the cost of passive solar homes, Kansas City builder Craig Eymann has found the added cost of passive solar to be less than 1% of the total construction cost. And PSIC's Helen English asserts, "Passive solar design inherently does not cost any more to construct than conventional houses."

Checklist for Good Design

Suggestions from Passive Solar Design Strategies: Guidelines for Home Builders:

Building Orientation
A number of innovative techniques can be used for obtaining good solar access on less- than-ideal sites. No matter what the house design, and no matter what the site, some options for orientation will be more energy-efficient than others, and even a very simple review of the site will probably help you choose the best option available.

Upgraded Levels of Insulation
It is possible, of course, to achieve very high energy efficiency with a "superinsulated" design. But in many cases, one advantage of a passive solar design is that energy efficiency can be achieved with more modest increases in insulation. On the other hand, if very high energy performance is a priority-for example, in areas where the cost of the fuel is high-the most cost-effective way to achieve it is generally through a combination of high levels of insulation and passive solar features.

Reduced Air Infiltration
Air tightness is not only critical to energy performance, but it also makes the house more comfortable. Indoor air quality is an important issue, and too complex for a complete discussion here, but in general, the suntempered and passive solar houses built along the guidelines provide an alternative approach to achieving improved energy efficiency without requiring air quality controls such as air-to-air heat exchangers-which would be needed if the house were made extremely airtight.

Proper Window Sizing and Location
Even if the total amount of glazing is not changed, rearranging the location alone can often lead to significant energy savings at little or no added cost. Some energy- conserving designs minimize window area on all sides of the house. However, it is a fact of human nature that people like windows, and windows can be energy producers if located correctly.

Selection of Glazing
Low-emissivity (low-e) glazing types went from revolutionary to commonplace in a very short time, and they can be very high energy-efficient choices. But the range of glazing possibilities is broader, and the choice will have a significant impact on energy performance. Using different types of glazing for windows with different orientations is worth considering for maximum energy performance. Heat-rejecting glazing can be used on west windows, high R-value glazing for north and east windows, and clear double- glazing on solar glazing, for example.

Proper Shading of Windows
If windows are not properly shaded in summer-either with shading devices or by high- performance glazing with a low shading coefficient-an air conditioner will have to work overtime and energy savings during the winter may be cancelled out. Even more important, unwanted solar gain is uncomfortable.

Interior Design for Easy Air Distribution
If the rooms in the house are carefully planned, the flow of heat in the winter will make passive solar features more effective, and the air movements will also enhance ventilation and comfort during the summer. Often this means the kind of open floor plan which is highly marketable in most areas. Planning rooms with attention to use patterns and energy needs can save energy in other ways, too-for instance, placing less-lived-in areas like storage rooms as buffers on the north side.

Addition of Thermal Mass
Adding effective thermal mass-for example, tiled or paved concrete slab, masonry walls, brick fireplaces, or tile floors-can greatly improve the comfort in the house, holding heat better in winter and keeping rooms cooler in summer. In a passive solar system, of course, properly sized and located thermal mass is essential.

Selection and Proper Sizing of Mechanical Systems and Energy Efficient Appliances
High performance heating, cooling, and hot water systems are extremely energy-efficient, and almost always a good investment. Mechanical equipment should have at least a 0.80 Annual Fuel Utilization Efficiency (AFUE). Well-insulated passive solar homes will have much lower energy loads than conventional homes, and equipment should be sized accordingly. Oversized systems will cost more and reduce the house's performance.

Source: Passive Solar Industries Council

Booting Up With Solar Design-A Software Comparison

A variety of computer tools are available for designing energy-efficient buildings, taking passive solar strategies into account. The tools differ in the level of interpretation needed to depict a building in the computer models. Generally more detailed input data lead to more detailed analysis results. A few helpful computer tools are listed below. The first program requires little interpretation of data and is easier to use. The last is the most sophisticated, permitting more design fine-tuning. The other two have qualities of both. For all the programs, regional differences in latitude and climate necessitate varying regional applications.

BuilderGuide-The PSIC Software
Passive Solar Design Strategies: Guidelines for Home Builders is a residential passive solar analysis tool. A computerized version of the Guidelines worksheets is available from the Passive Solar Industries Council. It's called BuilderGuide. The price of the software for analyzing one location is $80 for members and $100 for non- members. Additional locations may be purchased for about $25 each. Members can buy the software bundled with all 224 locations for $250. The program will run on any IBM- type PC, although an AT model is recommended. The algorithms behind the program are based on the Solar Load Ratio (SLR) method with extensions to treat the effects of cooling. The program is designed for use by builders, architects, and engineers.

The software automates completion of the calculations for the Guidelines worksheets. It is very user-friendly, using pop-up menus and context-sensitive help facilities. Like its hard copy companion, it exudes Dr. Doug Balcomb's lifetime accumulation of knowledge in passive solar design and analysis.

The program determines annual heating and cooling loads. Suntempered designs, direct gain systems, masonry walls, water walls, and sunspaces can be modeled in the program. The guidelines also address related passive solar issues such as shading, site planning, natural ventilation system design, and fan sizing.

Contact: Helen English, Passive Solar Industries Council, 1090 Vermont Ave. NW, Suite 1200, Washington, DC 20005. Tel:(202)371-0357.

REM/DESIGN-Residential Energy Evaluation Program
REM/DESIGN is a residential energy analysis tool available for $395. Included in the purchase price are four weather files (including Denver). An additional set of three weather files can be selected from 217 stations for $50. REM runs on any IBM-type PC. The algorithms behind REM stem from general heat transfer equations and parametric correlations of hourly building energy simulation models. The program was developed with builders, HVAC contractors, architects, and engineers in mind.

The user interface of the program is simple. It uses a menu-driven, fill-in-the-blank type approach. Input values are typed in. In some instances, appropriate input values are listed in help screens or menus. The program determines heating and cooling annual and peak loads. Internal gains (including latent loads) and air infiltration rates are considered in the load calculations. Infiltration can be specified as air changes per hour or as equivalent leakage area. Suntempered designs, direct gain systems, sunspaces, and masonry walls can be modeled in the program. In addition, active solar systems for space heating and domestic hot water can be analyzed. A side-by-side comparison of two different designs is possible.

Contact: Michael Holtz, Architectural Energy Corp., 2540 Frontier Ave., Suite 201, Boulder, CO 80301. Tel: (303)444-4149.

ASEAM 2-A Simplified Energy Analysis Method
ASEAM 2 is an energy analysis tool for residential and commercial buildings. The price for the ASEAM 2 software is $200, and includes data for 46 locations. Purchase of additional weather files is possible. ASEAM 2 is a public domain program and users may make changes to the BASIC source code if they desire. The program will run on any IBM-type PC. The algorithms used in ASEAM follow the ASHRAE-modified bin method of load analysis. The program was developed for architects and engineers and is also an effective educational tool.

The program uses menu-driven, fill-in-the-blank screens. Some input values are selected from menus. Equations used in the energy model can be examined by the user. A visual display of HVAC equipment operating conditions is available. These diagrams show psychrometric parameter values (temperature, humidity, etc.) of the flow stream at different points in the HVAC system for one set of bin data.

The program has inputs for specifically describing residential buildings, but its system analyses are strongest for commercial buildings. Only suntempering can be analyzed. ASEAM 2 determines annual and peak heating and cooling loads. It considers the effects of conduction, infiltration, internal gains, lighting, and daylighting. It can model numerous HVAC central systems and zonal systems. It can also simulate the performance of many central plants and load management features.

Contact: Sherrie Snell, ACEC Research & Management Foundation, 1015 15th St., N.W. Suite 802, Washington, DC 20005. Tel:(202)347-7474.

CALPAS-3-California Passive
CALPAS-3 analyzes energy performance in residential buildings. It is an hourly simulation program using typical meteorological year (TMY) weather data. The software costs $300, which includes weather data for five locations. Additional weather files for location across the U.S. are available for $5 each. The program runs on an IBM-type computer. It is recommended that the computer be equipped with a math co-processor. The program uses ASHRAE algorithms, implicit differentiation for determining transient heat flows, and combined radiation-convection heat transfer coefficients.

The program does not prompt the user for input data. Instead, a building description file containing parameter names and values must be created. Editing an existing building description file facilitates this process to some extent.

The program can analyze annual and peak heating and cooling loads based on the aforementioned typical weather year. Suntempering, direct gain sunspaces, trombe walls, rock beds, and evaporative cooling systems can be modeled. Reports on loads and comfort conditions can be generated for the year, by the month, and by the hour.

Contact: Bruce Wilcox, BSG Software, 760A Gilman St., Berkeley, CA 94710. Tel:(415)525-6675.

Sources: Innovative Design, Inc. and Entropy Scoop Technologies

Guidelines In The Field

"It is a great tool for finding out what works," explained architect Michael Nicklas when asked about Passive Solar Design Strategies: Guidelines for Home Builders. He and his company, Innovative Design of Raleigh, N.C., have designed over 500 passive solar homes. The company regularly uses Guidelines as part of their design evaluation process. In their office, it has been particularly useful for the less experienced designers and for documenting the energy performance of different designs. Nicklas says it is a good means for demonstrating energy performance and comfort conditions to the client.

To Nicklas, the biggest strength of Guidelines is the consistency in analysis between the worksheet-based hand calculation and the computerized version. He sees this as an opportunity to use the computerized version, BuilderGuide, for sophisticated studies and the hand calculation approach for simple applications. For example, the computer program results can be used as a basis for an energy rating system. Users interested in checking the compliance of a design in relation to the rating system can use the worksheet-based hand calculation.

Nicklas' main complaint about Guidelines is that it does not determine heating and cooling equipment sizes. The load coefficient for the house is not explicitly calculated, making extracting information for equipment sizing even more difficult.

Ellen Franconi, a building energy engineer in Boulder, Colo., found the design advice and rules of thumb given in Guidelines helpful in identifying an appropriate starting point for more detailed studies: "I needed to evaluate an architect's building design using an hourly simulation computer program. The rules of thumb described in Guidelines made me quickly realize the building had more mass than needed. Use of the simulation program confirmed this finding. The guidelines helped me to quickly identify a starting point, and from there I started the fine tuning."

The model homes in the prize-winning subdivision, Black Walnut Estates in Fredricksburg, Va., was designed using Guidelines. Joe Butler and his firm, Butler Custom Homes, took Virginia's Top Energy Award in 1990 for the design of this passive solar subdivision. Butler laid out the lots to permit solar access to each home. He kept shade trees on the east and west sides of the sites and used topography to maximize summer breezes. Butler finds the Guidelines very effective for builders who want to build energy-efficient homes. "If a builder has any ability to read math tables and use a calculator, he can do it," Butler states, although he also warns that the tool isn't the only key to a successful design. At Butler Custom Homes, aesthetically pleasing designs and building quality control are also stressed.

Sources of Passive Solar Information

Organizations