Low-e coatings are applied to the surface of glazings or to films suspended in the airspaces between the panes of glass. They reduce radiant heat loss and gain and dramatically improve a window's insulating value. For example, double-glazed, low-e windows are about as energy efficient as triple-glazed windows using regular glass, but they cost and weigh less. Note that there have been reports that windows with less than 70% visible light transmittance might support plant growth.
When argon, sulphur hexafluoride, carbon dioxide, or other gas fills with higher insulating values than air are included between glazings, the energy efficiency of windows is further improved. Although the extra layers of glazing and low-e coatings lower total light transmittance somewhat, the reduction is more than offset by the increased amount of heat remaining in the room. Other new window technologies include spectrally selective coatings (next generation of low-e films) that reject heat while admitting light, electrochromic glazings that lighten and darken as small electric currents are applied and removed, and "super windows" that combine a number of features (e.g., low-e coatings, gas fills, and insulating frames and spacers) into one unit.
If you decide to use overhead glazing in the roof of your sunspace, invest in one of the glazing systems developed specifically for this purpose. Overhead glazing has a reputation for leaking, but excellent sealing systems are now on the market. Invest in a good system--this is not a place to cut corners. In some areas, building codes require that you use plastic glazing or tempered or laminated glass in overhead and sloped glazing sections for safety reasons.
Which glazing system is most appropriate for your project depends on your budget and the climatic conditions at your site. For more detailed information on current and future glazing options, contact the Energy Efficiency and Renewable Energy Clearinghouse.
Thermal Mass Considerations: Water is the most efficient thermal mass, because it holds the most heat per unit of volume. Anything that will not leak will work to hold the water, and designers and homeowners have used everything from plastic jugs to 55-gallon (208-liter) drums to specially designed (and often very attractive) containers.
Figure 88: Low-e glazings reduce radiant heat transfer.
Masonry materials (brick, concrete, or stone) are also good choices for thermal mass. Although they store only about half as much heat as water, they can also support the structure, form the floor, and serve as the wall between the house and the sunspace. Masonry is most effective in 4- to 6-inch (10- to 15-centimeter) thicknesses. If walls are built with concrete blocks, the holes in the blocks must be filled with concrete.
The surfaces of thermal mass materials should be dark colors of at least 70% absorptance. Black has about a 95% absorptance rate, deep blue has about 90%, and deep red approximately 86%. Nonstorage materials should be lighter colors so they will reflect light to the thermal mass not located in the sun. Thermal storage materials can be located in the floor and in the north, east, and west walls of the sunspace.
When masonry floors and walls are the only thermal storage materials in the space, 3 square feet (0.3 square meters) of 4-inch-thick (10-centimeter-thick) masonry surface per square foot of south glazing is probably adequate. When water in containers is the only heat storage medium used, the recommended ratio is 3 gallons (11.3 liters) per square foot (0.09 square meter) of glazing. These are only rules of thumb and should be confirmed by modeling your project on a computer or checking with a design or building professional in your area who is familiar with local design practices.
Insulation: To maximize comfort and efficiency, it is important that your sunspace be well insulated. The perimeter of the sunspace's foundation wall or slab should be insulated down to the frost line (i.e., the depth at which frost penetrates the soil) and underneath the slab if it is appropriate in your area. If you live in a very cold climate, insulate the east and west walls of the sunspace rather than glazing them. Always insulate the sections of exterior walls that are not glazed. Check with solar specialists in your area or guidance on your particular project.
Although overhead glazing can be beautiful, an insulated roof provides better thermal performance. When the highest part of the structure is well insulated, heat loss in winter is reduced, and the summer sun will not strike the interior wall and cause overheating. Instead, skylights can be used to provide some overhead light for plants. And, if they are the type that open, skylights offer a way to vent excess heat. Skylights are available with advanced glazings that reduce radiant heat loss to the night sky.
Window coverings, shades, and other forms of movable insulation help trap the warm air in the sunspace both after the sun has set and during cloudy weather. When closed during extremely hot days, window coverings can help keep the sunspace from overheating.
Thermally isolating the sunspace from the house at night is important. Large glass panels, French doors, or sliding glass doors between the house and the sunspace will maintain an open feeling without the heat loss associated with an open space.
Climate Controls: Overheating can kill plants and make the sunspace unlivable. To control overheating, some designers place operable vents at the top of the sunspace where temperatures are the highest and at the bottom where temperatures are the lowest. For times when you are not home to open vents manually, thermostatically controlled motors can be installed to automatically open them.
If passive (i.e. nonmechanical) circulation is not possible or practical, fans with thermostatic controls can be used to circulate air to the rest of the house. Other types of climate controls include shades or movable window insulation that can be operated with electric timers or sensors.
An Investment in Future Enjoyment: Few home improvements offer the aesthetic appeal and practical paybacks that a carefully designed and constructed sunspace can. Although you may be tempted to tackle the endeavor on your own, it is money well spent to consult with a solar engineer, architect, or contractor. They will provide feedback, as well as a computer analysis of your design. Remember: It is much less expensive to make changes on paper than to alter a sunspace once it is built. And after your sunspace is finished, you can enjoy it for years to come.