The following excerpts include a majority of the text from the completed Affordable Passive Solar Planbook for North Carolina. The complete APS Planbook includes all illustrations, details, floor plans and elevations and is available in pdf format here.
Please scroll through the text as you wish or choose a section to review here:
Key Features of Passive Solar Design
Increased south-facing glass area - allows sunlight to help warm the home in winter months. South-facing windows receive close to three times as much sunlight as east and west windows in the winter and a third less sunlight in the summer. Sun-tempered homes have no more than 7% of the floor area as southern glazing. In passive solar homes the area of the south facing glazing is 7-12% of the floor area. This amount of glazing requires the use of thermal mass to temper the heat gain. A home with increased southern glazing up to 7% is considered sun-tempered and can be effective with out the use of thermal mass.
Lower east and west glass areas - reduce summer cooling needs because it prevents unwanted sun from entering the home in the morning and afternoon. Eliminating the windows also lowers construction costs.
Orientation and site selection - are critical in passive solar design. The passive solar windows must face within 15 degrees of due south to maximize solar gain in winter and minimize overheating in summer. Be aware that magnetic south is different than true south. To find how many degrees they vary at your site visit NOAA's Geophysical Data Center. The house should be designed on an east-west axis so the long side faces south. Trees on the site reduce summer cooling bills, but should not shade south-facing windows in winter. Effective passive solar design is not possible on all sites due to the fact that the site must receive direct sunlight on December 21st between 9 am and 3 pm. Privacy is also a factor, so if the south side is exposed to the street or neighboring houses it may not be conducive to passive solar design.
Energy efficient design - the first step in a successful passive solar home includes proper installation of recommended levels of insulation, air-tight design, and efficient heating and cooling systems.
Thermal storage mass - materials such as concrete floors, interior brick walls, brick pavers, and tile store heat and regulate interior temperatures both in winter and summer.
Effective window shading - reduces summer cooling needs and glare. Window shades lowered at night can also be used to help trap the heat absorbed by the thermal mass.
Moisture control systems - increases the home’s durability, improve indoor air quality, and provide comfort in both summer and winter.
Plan the room layout - to take advantage of the sun’s path. Rooms should match solar gain to the time of day the room is used.
Homes in North Carolina need to be equipped to handle both cold winters and hot humid summers. Depending on your climate you may need to reduce the amount of glass on the southern side to prevent high cooling bills due to overheating.
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Types of Passive Solar Designs
The three main categories of passive solar design are direct, indirect and isolated gain. Within these broad categories there are four primary types of passive solar design. The first type is a basic direct gain system in which the sun’s rays directly enter the living area. Thermal storage walls are indirect systems that store and distribute the thermal energy. Sunspaces and solar air collectors are isolated systems which can be closed off or opened to the main living area.
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Design Principles of Passive Solar Homes
This section introduces the basic design principles. Current trends in housing, such as expansive glass areas, daylighting, sunrooms, great rooms, tile floors, fireplaces, and open floor plans fit well into passive solar designs. Effective designs reduce heating and cooling bills and provide greater comfort.
Heating Season
In the winter months, three primary elements interact to provide a significant portion of a home’s heating needs:
energy efficiency features including effective insulation, airtight construction, and efficient HVAC systems, minimize the demand for heating;
increased south-facing windows bring additional sunlight into the home which can be captured as heat energy;
thermal mass can supply a means to store heat inside the home. Concrete or tile floors; walls made of masonry materials such as brick, stone, concrete; a masonry fireplace; or water-filled containers all provide thermal mass for heat storage and can be incorporated to meet the aesthetic requirements of the space.
Cooling Season
In summer months, passive solar homes in North Carolina must compensate for the hot, humid climate and the large amount of heat that can come into the home through windows. The true challenge of passive solar design is to ensure low summer cooling bills compared to those of a similar, standard home.
Many passive solar homes have significantly lower cooling bills because they:
have energy efficient features – high insulation levels, airtight construction, and effective air conditioning system design and installation;
have few, if any, windows on the east and west minimizing solar gain in the mornings and afternoons;
provide shading for south-facing windows;
incorporate thermal mass to balance temperature extremes;
can be ventilated during milder outdoor weather with open windows and fans, which help maintain indoor comfort.
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Passive Solar Design Guidelines
In passive solar design it is necessary to be sensible about your expectations of the sun. Do not assume that the sun and the house design will provide all of your heating and cooling needs. The climate in North Carolina can vary from cold, relatively cloudy winters to hot, humid, sunny summers. Well-designed passive solar homes provide their owners with low energy bills and year-round comfort, as well as natural daylight. However, improperly designed passive solar homes may actually have uncomfortable temperature swings both in summer and in winter, thereby reducing potential energy savings. When designing the home remember rooms with large expanses of glass should include thermal storage. It is also important to consider the layout of the rooms in passive solar design. Whether adapting passive solar features to a standard home plan or designing an entirely new plan, consider the following design ideas.
Frequently-used rooms (morning to bedtime)- Family rooms, kitchens, and dens work well on the south side. Be aware of potential problems with glare from sunlight through large expanses of windows.
Day-use rooms- Breakfast rooms, sunrooms, playrooms, and offices work well on the south side of the house. They should adjoin rooms that are used frequently to take full advantage of solar heating.
Sunspaces- These rooms can be isolated from the house if unconditioned. In winter, the doors can be opened to let solar heat move into the home. At night, the doors can be closed, and the sunspace buffers the home against the cold night air. In summer, sunspaces protect the home from outside heat gain. For best performance, they should not be air conditioned.
Privacy rooms- Bathrooms and dressing rooms can be connected to solar-heated areas, but are not usually located on the south side.
Night-use rooms- Bedrooms are usually best on the north side, unless used often during the day. It is often difficult to fit thermal storage mass into bedrooms, and privacy needs may limit opportunities for installing large glass areas. However, some homeowners may prefer bedrooms filled with natural light and can use passive solar features effectively.
Seldom-used rooms- Formal living rooms, dining rooms, and extra bedrooms are best on the north side, out of the traffic pattern and air flow.
Buffer rooms- Unheated spaces such as closets, laundries, workshops, pantries, and garages work best against the north, east, or west exterior walls. They protect the conditioned space from outside temperature extremes.
Exterior covered areas- Porches and carports on the east and west provide summer shading. However, west-facing porches may be uncomfortable in the afternoon. Avoid covered porches on the south side, as they shade winter sunlight.
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Windows
The windows of a home produce benefits such as light, fresh air, ventilation, and good aesthetics. Properly sited windows can provide a significant amount of heat for a home in the winter. In passive solar design, it is important to choose the right windows and place them in the optimum location. Improperly sited windows can lead to unwanted glare from the sun causing deterioration of finishes and fabrics and summer time overheating which could double cooling costs.
When selecting a window, you should make good quality a high priority. Double-glazing, solid construction, and effective weatherstripping should be minimum considerations in the choice of windows. To improve performance, consider low-emissivity (low-e), gas filled, and tinted windows, or units with reflective coatings. In passive solar design you want to have a higher solar heat gain coefficient on south facing glazing. A higher solar heat gain coefficient allows more solar energy to enter the home, but the windows are usually non coated, double paned units that are seldom low-e. Shades and shutters should be used to prevent the solar energy gained throughout the day from escaping in the winter and help prevent unwanted gain in summer months. Low-e windows can be used on the south side, but you will lose a small amount of solar gain. On winter evenings low-e windows will more effectively trap gained heat while blocking the sun’s heat in summer. Non coated, double paned units or low-e windows can be used on the south side effectively, but it is up to the homeowner if they are willing to actively participate in the conditioning of their home by lowering shades when appropriate. For more information on different types of windows and glazing refer to the North Carolina Builders Guide. The following chart has basic window terminology which will be helpful in shopping for windows.
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Thermal Mass Storage
Thermal mass materials, including concrete, tile, masonry, stone and other heavy building materials, absorb water and store heat. These materials are key elements in passive solar homes. Homes with substantial south-facing glass areas and no thermal storage mass do not perform well.
Providing adequate thermal mass is usually the greatest challenge to the passive solar designer. The amount of mass needed is determined by the area of south-facing glazing and the location of the mass. Sun-tempered homes, having less than 7 percent of the floor area in south facing glass, rely on incidental mass in the construction of materials and furniture. The guidelines that follow will help ensure an effective design.
Guideline 1
Locate the thermal mass in direct sunlight.
Thermal mass installed where the sun can reach it directly is more effective than indirect mass placed where the sun’s rays do not penetrate. Houses that rely on indirect storage need three to four times more thermal mass than those using direct storage.
Guideline 2
Distribute the thermal mass evenly.
Passive solar homes work better if the thermal mass is thin and spread throughout the living area. The surface area of the thermal mass should be at least 3 times, and preferably 6 times, greater than the area of the south windows. Slab floors and masonry walls that are 3 to 4 inches thick are more cost effective and perform better than those that are 6 to 12 inches thick.
Guideline 3
Do not cover the thermal mass.
Carpeting with a carpet pad substantially reduces the energy savings from the passive solar elements. It is generally acceptable to cover no more than 5 percent of the area with carpet or furniture. Masonry walls can have drywall or plaster finishes, but should not be covered by large wall hangings or lightweight paneling. The drywall should be attached directly to the mass wall, not to purlins fastened to the wall that create an undesirable insulating airspace between the drywall and the mass.
Guideline 4
Select an appropriate mass color.
For best performance, thermal mass elements should be a dark color. A medium color, which can store 70 percent as much solar heat as a dark color, may be appropriate in some designs. A matte finish for the floor reduces reflected sunlight, thus increasing the amount of heat captured by the mass and having the additional advantage of reducing glare.
Guideline 5
Insulate the thermal mass surfaces.
Insulation levels required by North Carolina code should be viewed as a bare minimum. The slab should be insulated around the perimeter as well as underneath.
Guideline 6
Make thermal mass multipurpose.
For maximum cost effectiveness, thermal mass elements should serve other purposes as well. Tile-covered slab floors store heat and provide a finished floor surface while masonry interior walls provide structural support, divide rooms, and store heat. Thermal storage walls are one type of a passive solar design that is often cost prohibitive because the masonry walls only function as thermal mass.
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Thermal Mass in the Heating Season
10:00 am to 5:00 pm
Sunlight enters south-facing windows and strikes the thermal mass inside the home. The sunlight is converted to heat energy, which heats both the air and thermal mass materials. On most sunny days, solar heat maintains comfort during the mid-morning and late afternoon periods.
5:00 pm to 11:00 pm
As the sun sets, it stops supplying heat to the home. However, a substantial amount of heat has been stored in the thermal mass. These materials release the heat slowly into the passive solar rooms, keeping them comfortable on many winter evenings.
11:00 pm to 6:30 am
The homeowner sets back the thermostat at night, so only minimal back-up heating is needed. Energy efficient features minimize heat losses to the outside.
6:30 am to 10:00 am
The cool early morning hours are the toughest for passive solar heating systems to provide comfort. The thermal mass has usually given up most of its heat, and the sun hasn’t risen enough to begin heating the home. The homeowner may have to rely on a supplemental heating system.
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Thermal Mass Storage in the Cooling Season
8:00 am to 10:00 am
The sun’s rays strike the outside of the east walls, which have minimal glass area. Thus, the home suffers less heat gain than a comparable standard home.
10:00 am to 4:00 pm
Direct sunlight on the south windows of the home is shaded by roof overhangs. Diffuse sunlight on hazy days is blocked by interior or exterior shades. The energy efficient features minimize heat gain through walls and attics, and stop air leaks which add both heat and humidity to the home.
On warm Spring and Autumn days, natural ventilation or mechanical ventilation, such as that provided by a ceiling fan or whole house fan, helps maintain comfort. On hot summer days, most homeowners prefer the comfort provided by an air conditioning system. The high capacity of the thermal mass to store heat regulates indoor temperatures so that the house is less likely to overheat during the middle of the day.
3:00 pm to 8:00 pm
Sunlight coming from the west is once again deflected because the home has little or no west-facing glass.
8:00 pm to 9:00 am
On mild nights, the windows can be opened to provide nighttime ventilation. On cool evenings, nighttime ventilation can help flush heat from the thermal mass to the outside. The cooler mass will absorb more heat the following day. On hot, humid evenings, air conditioning may be preferred.
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Incorporating Thermal Mass
Thermal mass can be incorporated into a passive solar room in many ways, from tile-covered floors to masonry walls. When selecting thermal mass materials, consider the aesthetics, costs, and energy performance.
• Slab-on-grade floor- used in most passive solar homes. Slab floors can be stained or stamped into a variety of patterns or finished with tile, stone or brick. Concrete floors can be expensive to install on upper stories. Floors made of brick, brick pavers, or tile on a thick bed of mortar also may be used.
• Interior mass walls- solid mass walls between interior rooms. Since they have living area on both sides, they can be up to 12 inches thick, although thinner 4- to 8-inch walls deliver heat more quickly. Masonry fireplaces that are several feet thick store heat but are not as effective as thinner mass walls with greater surface area. Since masonry is not a good insulator, keep fireplaces on interior walls.
• Thermal storage walls- a solid masonry wall fronted by exterior double-glazed windows. Sometimes known as Trombé walls, these designs are one of the least cost-effective passive solar options for North Carolina. They are expensive to build, and many researchers question whether the mass wall has sufficient time to warm between the periodic spells of cloudy weather experienced by most of the Southeast in the winter.
• Water-filled containers- water stores heat twice as effectively as masonry by volume and five times as effectively by weight. However, water containers look unusual in most living areas. Since they store more heat per pound, less weight is required to store the same amount of solar heat; therefore, they are easier to use in upstairs rooms. Commonly used water containers include fiberglass cylinders and 30- or 55-gallon metal drums.
• Hot tubs, saunas, and indoor pools- some homeowners have tried to use hot tubs, saunas, and indoor pools as thermal storage mass. In most cases, these forms of water storage do not work well. The desired water temperature for comfortable use of these amenities is hotter than the passive solar contribution can possibly achieve.
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Natural Cooling
Design features known as natural cooling measures can further reduce the air conditioning needs of the house. Natural cooling guidelines are especially important for passive solar homes because their large expanses of south-facing glass can cause overheating if unprotected from the summer sun.
Window Shading Options
The effectiveness of window shading options depends on the position of the incoming sunlight. On a clear day, most sunlight is direct, traveling as a beam from the sun to a home’s windows without obstruction. In winter, most of the direct sunlight striking a window is transmitted. However, in summer, the sun strikes south windows at a steep angle, and much of the direct sunlight is reflected. In developing a strategy for effectively shading windows, both direct and indirect sources of sunlight must be considered.
Landscaping and Trees
According to the U.S. Department of Energy report, “Landscaping for Energy Efficiency”, careful landscaping can save up to 25% of a household’s energy consumption for heating and cooling. Trees and vines are effective means of shading in the summer and combined with a lawn or other ground cover, can reduce air temperatures as much as 9 degrees F in the surrounding area. When located in the front of open windows on the windward side of the house, bushes and other vegetation can cool the air coming in. Trees must be located to provide shade in summer and not block the winter sun. Even deciduous trees that lose their leaves during cold weather block some winter sunlight; a few bare trees can block over 50% of the available solar energy.
Overhangs
Overhangs shade direct sunlight on windows facing within 30 degrees of south. Overhangs above south-facing windows should provide complete shade for the glazing in midsummer, yet still allow access to winter sunlight. Overhangs above tall, south-facing windows should generally extend 2 to 2½ feet horizontally from the wall. It is not necessary for south windows to extend vertically all of the way to the overhang because the top one to two feet will be shaded year round.
Shades and Shutters
Exterior window shading treatments are effective cooling measures because they block both direct and indirect sunlight outside of the home. Solar shade screens are an excellent exterior shading product with a thick weave that blocks up to 70% of all incoming sunlight. They should be removed in winter to allow full sunlight through the windows.
Shutters and shades located inside the house include curtains, roll-down shades, and Venetian blinds. More sophisticated devices such as shades that slide over the windows on a track, interior movable insulation, and insulated honey comb shades are also available. Interior shutters and shades are generally the least effective shading measures because they try to block sunlight that has already entered the room. However, if passive solar windows do not have exterior shading, interior measures are needed. The most effective interior treatments are solid shades with a reflective surface facing outside.
Reflective Films and Tints
Reflective film, which adheres to glass and is found often in commercial buildings, can block up to 85% of incoming sunlight. The film blocks sunlight all year, so it should not be used on south windows in passive solar homes. These films are also not recommended for windows that experience partial shading because they absorb sunlight and heat the glass unevenly. The uneven heating of windows may break the glass or ruin the seal between double-glazed units.
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Ventilation
In spring and fall, ventilation measures can help cool a house and bring in fresh air. Air movement keeps people cooler by evaporating moisture from the skin. Research has shown that people feel as comfortable in rooms at 85 degrees F with air movement as in rooms at 75 degrees F with still air. Both natural ventilation and mechanical ventilation measures are important for low cost cooling.
All houses need ventilation to remove stale interior air and excessive moisture and to provide oxygen for the inhabitants. There has been considerable concern recently about how much ventilation is required to maintain the quality of air in homes. While it is difficult to gauge the severity of indoor air quality problems, most experts agree that the solution is not to build an inefficient, “leaky” home. Research studies show that standard houses are as likely to have indoor air quality problems as energy efficient ones. Most building researchers believe that no house is so leaky that the occupants can be relieved of concern about indoor air quality. They recommend mechanical ventilation systems for all houses.
Natural Ventilation
Breezes can generate air movement inside the house. All rooms used frequently should be designed for ventilation; however, natural breezes are unpredictable throughout most of North Carolina. They usually do not blow from any one direction reliably in summer and are not very strong. It is important to place windows on opposite sides of a space to allow for cross ventilation because they can capture cooling, flow-through breezes. However, do not rely on the wind as the only source of air movement.
Another form of natural ventilation, called the stack effect, occurs when hot air can exit the house through a high opening. A low opening lets in outside air to replace the exiting air. The stack effect is not a reliable form of ventilation, particularly on hot days when the outside air drawn into the house is uncomfortable.
Mechanical Ventilation
Mechanical ventilation provides an inexpensive means of creating a cooling air flow. In addition, ventilation systems can expel stale air from the home to improve indoor air quality.
Portable fans or ceiling fans can provide comfort inexpensively, even when the air conditioner operates. For each degree that the thermostat is raised, air conditioning costs drop 3 to 8 percent. By setting the thermostat between 80 and 85 degrees and using fans that blow directly on room occupants, homeowners can save 20 to 50 percent on cooling bills.
Whole house fans, also called attic fans, blow hot room air into the attic and pull supply air into the home from outside. They generate substantial air flow within the home. They cost 4 to 6 times less to operate than a central air conditioning system. The primary disadvantage of whole house fans is that they bring in outside air containing dust, moisture, pollen and other allergens. Whole house fans are primarily recommended for houses without air conditioning or for homes whose occupants are committed to saving energy and are willing to control the operation of their home carefully. For most homeowners, they provide an excellent means of cooling a home during warm days in the spring and fall.
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Estimating Passive Solar Savings
The following rules of thumb approximate the annual heating energy savings of passive solar homes:
• Each square foot of double-glazed south-facing window that is unshaded in the winter will save 40,000 to 60,000 Btu per year on a home’s heating requirement, if sufficient thermal mass exists.
• Low-emissivity glass will increase the savings 15 to 30 percent.
Thus, an energy efficient home with 200 square feet of passive solar windows and sufficient thermal storage mass could save 8 to 12 million Btu of energy on home heating needs each year. Movable insulation or low-e glass would save an additional 2 to 4 million Btu.
The cost of space heating with a standard heat pump or gas furnace in North Carolina is about $10 per million Btu. Thus, the passive solar home described above could save as much as $160 per year on heating bills with movable insulation or low-e windows.
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Key Energy Efficiency Steps:
Energy features save money, improve indoor air quality, enhance comfort, prevent moisture problems, and increase the long term durability of the building. Keep in mind that investing in energy efficiency not only saves energy and increases comfort over the life of the home, it also decreases the required size of the heating and cooling equipment. Therefore, the added investment in efficiency also reduces the cost of a properly sized heating and cooling system.
Key Feature Checklist: - Moisture barrier system
- Air barrier system
- Continuous insulation system
- Design heating and cooling system
- Ductwork design and installation
- Minimize hot water costs
- Choose energy efficient appliances and lighting
- Provide intentional ventilation
(the pdf download of the entire Planbook has a complete checklist)
[top] North Carolina’s Renewable Energy Tax Credit
North Carolina provides a tax credit for the construction or installation of a renewable energy system to heat, cool, or provide hot water or electricity to a building located in state. The credit is 35 percent of the installation and equipment costs of a system, including passive and active space heating ($3,500 maximum per system), active solar water heating ($1,400 max) and residential electricity generating systems such as photovoltaics, wind, and micro-hydro. The credit is distributed over five years. The NC Solar Center can provide details and guidelines to determine the income tax credit, or visit The Database of State Incentives for Renewable Energy.
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