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Sunspace Basics

Summary: This section will explain how a sunspace can help heat your home, as well as the basic design elements and guidelines to consider when building one. Anyone who lives in a home with a sunspace will tell you that the sunspace is the most enjoyable room in the house. Many times the homeowner's only regret is that the sunspace is not larger. Although aesthetics often drive the decision to add a sunspace or include one in a new home design, sunspaces can also provide supplemental space heating and a healthy environment for plants and people. In fact, a well-designed sunspace can provide up to 60% of a home's winter heating requirements.

Basic Elements: In a basic design, sunlight passes through glass or other glazing and warms the sunspace. The glazing is either vertical (as typical windows are installed) or sloped at an angle. To moderate temperature swings, massive materials (e.g., masonry or water) can be used to store the sun's thermal energy and absorb the heat. At night or during extended periods of cloudy weather, this "thermal mass" releases the heat it holds to warm the interior of the sunspace. Ceiling, wall, foundation, and window insulation in the sunspace minimize heat loss at night and during cold weather. Climate control features include operable windows, vents, and fans to keep the sunspace from overheating and to circulate the warm air to other parts of the house.

Energy Efficiency Comes First: Design Considerations for Different Functions - Sunspaces serve three main functions: they are a source of auxiliary heat, they provide space to grow plants, and they are enjoyable living areas. The design considerations for these functions are very different, and although it is possible to build a sunspace that will serve all three functions, some compromises will be necessary.

If the primary function of the room is only to provide heat, you can maximize heat gain by using sloped glazing, few plants, little thermal mass, and insulated, unglazed end walls. If the winters are sunny in your area, carefully sized thermal mass will prevent extreme overheating during the day. In practice, sunspaces are rarely built to serve only as heaters, because there are less expensive ways to provide solar heat.

If the space will mainly be used as a greenhouse, remember that plants need fresh air, water, lots of light, and protection from extreme temperatures. Greenhouses consume energy through the growth processes of plants and the evaporation of water: one pound of evaporating water uses about 1000 Btu that would otherwise be available as heat. Plants require overhead glazing, which complicates construction and maintenance, and glazed end walls, which are net heat losers. The bottom line is that a sunspace designed as an ideal horticultural environment is unlikely to have much energy left over for supplementary space heating.

Most people want to use their sunspaces as year-round living areas, so sunspaces should have minimum glare and only moderate humidity. Carefully sized thermal mass will greatly improve comfort levels by stabilizing temperature extremes. Thermal mass materials should be placed in direct sunlight and should not be covered with rugs, furniture, or plants. Movable window insulation or advanced glazings minimize nighttime heat losses and greatly improve comfort.

  • Sunspace Design Guidelines:

  • Passive solar structures are conceptually simple, but sunspace designers and builders must pay close attention to detail to ensure maximum performance and reliability of the structures.

  • Computer software is now available to help establish design and performance criteria for specific passive solar projects like sunspaces. This software makes it relatively easy to avoid making uninformed, potentially costly, and disappointing decisions about a sunspace addition.

Siting: A sunspace must face south. Due solar south is ideal, but 30 degrees east or west of due south is acceptable. If your project is a retrofit, consider how the new addition will look on the south side of your house. If the south side faces the street, the design must be well integrated into the home to avoid a "tacked-on" look. And, you will need to protect your family's privacy. If the south side of your house faces the backyard, privacy may be less of an issue.

Because the sun is low in the sky in the winter, any obstacle over 10 feet (3 meters) tall within 15 feet (4.6 meters) of the south glazing is likely to block solar gain. If the sunspace will be shaded only in the early morning or late afternoon, there is no major cause for concern. It is important, however, that the space receive direct sunlight between 10:00 a.m. and 3:00 p.m. Do not plant trees near the south glazing, and seriously consider removing existing trees from the area. Contrary to prior opinion, even deciduous trees that lose their leaves in the winter are capable of blocking the sun. In fact, a mature, well-formed deciduous tree will screen more than 40% of the winter sunlight passing through its branch structure.

If you have a choice, locate the sunspace so that the walls of the house serve as one or both end walls of the sunspace (to reduce heat loss) and the addition is adjacent to kitchens, dining areas, children's playrooms, and family living areas occupied during the day and early evening.

Heat Distribution: Warm air can be blown through ductwork to other living areas. It can also move passively from the sunspace into the house through doors, vents, or open windows between the sunspace and the interior living space. Strategically placed openings in the common wall can distribute the warmed air from the sunspace to the house by the "thermosiphoning" circulation of the air. In a thermosiphon, warm air rises in the sunspace and passes into the adjoining space through an opening, and cool air from the adjoining space is drawn into the sunspace to be heated.

The minimum opening should be about 8 square feet (0.7 square meters) per 100 square feet (9.3 square meters) of glazing area. If the design calls for two openings--one high in the sunspace and one low--the minimum area for each opening is approximately 2.5 square feet (0.2 square meters) per 100 square feet (9.3 square meters) of glazing, with 8 vertical feet (2.4 meters) of separation. Again, these are rules of thumb that should be refined through computer modeling or confirmed with local experts. An uninsulated masonry wall between the house and the sunspace will also transfer some heat into the living space by conduction.

Glazing: Sloped or Vertical? Although sloped glazing collects more heat in the winter, many designers prefer vertical glazing or a combination of vertical and sloped glazing. Sloped glazing loses more heat at night and can cause overheating in warmer weather. Vertical glazing allows maximum gain in winter, when the angle of the sun is low, and less heat gain as the sun rises toward its summer zenith. A well- designed overhang may be all that is necessary to shade the glazing in the summer. Compared with sloped glazing, vertical glazing is less expensive, easier to install and insulate, and not as prone to leaking, fogging, breaking, and other glazing failures. Vertical glazing is often more aesthetically compatible with the design of existing homes.

Sunspace Components: Glazing  - Glazing is the clear or translucent material that allows sunlight to enter and warm the space. Glass is the most common glazing material, and many sunspace builders choose glass for its durability, clarity, and appearance. However, plastic glazings can be cheaper, stronger, lighter, and easier to work with--making them popular choices with the 20% of homeowners who build their own sunspaces. Some plastics even transmit solar energy more effectively than glass. On the down side, plastics scratch more easily, expand and contract more in response to temperature extremes (making them harder to seal), and generally are less durable than glass.

Deciding on which glazing to use is only the first step in the decision-making process, however. Advances in glazing technology make it possible for designers to fine-tune performance by choosing glazings that meet the specific needs of their projects.

Historically, manufacturers have used multiple layers of glass to improve the insulating value of a window. In addition to making the unit more energy efficient, extra layers of glass also increased the weight and bulk--as well as the price--of the unit. However, today's low-emissivity (low-e) coatings--thin, invisible metal or metallic oxide films--have revolutionized the glazing industry.

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