Passive solar systems structures are conceptually simple. However sunspace designers and builders must pay close attention to details to ensure maximum performance and reliability of the structure.
Computer software is now available to help design and establish criteria for specific projects such as passive solar sunspaces. This software makes it relatively easy to avoid making uninformed, potentially expensive and disappointing decisions on a sunspace addition.
A sunspace must toward the Equator (to the south in the Northern Hemisphere). Due 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 at the southern side of the house. If the south side of your home faces the street, the design must be well integrated in the existing building to avoid a “tacked-on” look. And you have to protect your family’s privacy. If the south side of the house faces the backyard, privacy may be less of an issue.
When the sun is low in the sky in winter, any obstacle of 10 feet (3 meters) high within 15 feet (4.6 meters) of south-facing windows is likely to block solar heat. If sunspace will be shaded only in the early morning or late afternoon, there is no greater cause for concern. However, it is important that the room receives direct sunlight between 10:00 and 3:00. Don’t plant trees near the south windows, and consider removing existing trees from the area. Contrary to previous opinion, even deciduous trees that lose their leaves in winter are able to block the sun. Indeed, a mature deciduous tree will filter more than 40% of winter sunlight.
To reduce heat loss, if possible, locate the sunspace so that one or more walls are adjacent to kitchens, dining rooms, children’s play rooms and family living areas occupied during the day and early evening.
Warm air created by the sunspace can be blown through ductwork to other living areas. It can also move passively into the house through doors, vents or open windows between sunspace and interior accommodations. Strategically placed openings in the common wall may distribute heated air from sunspace to the house with “thermosiphoning” circulation of air. In a thermosiphon, warm air rises in sunspace and passes into the adjacent compartment through an opening and cooler air from the adjacent compartment is drawn into the sunspace to be heated.
The minimum opening should be approximately 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 sunspace and a low minimum area of each opening is approximately 2.5 square feet (0.2 square meters) per 100 square feet (9.3 square meters) of glass panes, with 8 vertical feet (2.4 meters ) of separation. Again, these are rules of thumb to be refined by computer modeling and confirmed by experts. An uninsulated masonry wall between the house and sunspace will also transfer some heat to the living space via conduction.
Although sloped glazing collects more heat during the winter, many designers prefer vertical glazing or a combination of vertical and sloped glazing. Sloped glazing loses more heat at night and can lead to overheating in warmer weather. Vertical glazing is better for maximum heating in the winter when the angle of the sun is low. A well-designed overhang may be all that is needed to shade the windows in summer.
Compared with sloped glazing, vertical glazing is cheaper, easier to install and insulate. It is less prone to leaking, fogging, breaking, and other window failures. Vertical glazing is often more aesthetically compatible with the design of existing dwellings.