Topics of Interest
Passive Solar Design, Sustainable Design

For building sites with good solar exposure, passive solar heating can provide 25% of your heating needs here in Western North Carolina simply with south-facing windows.  The total south-facing glass area should be no more than 10% of your home's floor area unless you have thermal heat storage walls or other heat storing components beyond that in normal light wood frame construction.  Passive solar designs which "overglaze" with window areas that exceed this 10% rule, can become uncomfortably overheated on sunny winter days.

Sun exposure is most important between the hours of 10:00 AM and 2:00 PM during the winter months.  For sites where a portion of the sunshine is compromised by a land form, such that the sun does not appear until 11:00 AM in January, the amount of south-facing window area or aperture might be increased to as much as 13 or 14% of the floor area.  Some increase in aperture may also be made to offset shading and reduced solar exposure from deciduous tree trunks and limbs that will remain in a southerly direction from your building site.  It is very important that your heat collecting south wall be oriented to face due south, or no more than 15 degrees east or west of due south.

While 25% of your heat is a significant amount, this can tripled to 75% if the building is constructed with a no-holds barred approach to passive solar.  The south-facing glass or aperture can be increased to 20-30% of the floor area, as long as the heat from the additional area of solar aperture is directly absorbed into a heat storage mass.  The entire materials and fabric of the home must not only be a good solar heat collection device, it must be a good solar heat storage device, and like a thermos bottle, must be insulated to be a very effective heat trap.  For a high aperture, high mass, high performance passive solar home, you had better consult an expert in the design to be certain the heat collection and storage elements are balanced.  Attached sunspaces with large glass or aperture areas can further augment a passive solar design. The attached sunspace is a separate room or space that can be isolated from the main living area and therefore can have temperature swings which exceed the normal comfort range. This space can share heat storage walls with the main living space, and can have operable doors or vents to bring the heat collected into the main living area.  

Sustainable design refers to the broader realm of energy conservation measures  which reduce the use of fossil fuels which thereby reduces CO2 emissions into the atmosphere.  Sustainable design turns to renewable energy sources where possible and thus passive solar design is part of sustainable design.  Photo-voltaic panels for direct conversion of the sun's energy into electricity is another example of sustainable design, as are hybrid electric vehicles and higher efficiency furnaces.  Ultimately, all these can be combined on the right site to make an off-the-grid, autonomous dwelling, but renewable energy sources are finite and a fundamental aspect of all sustainable design is energy conservation.  

Recommended Reading:

1)        The Passive Solar Energy Book
           Edward Mazria,  1979
2)        Building with Passive Solar
           Southern Solar Energy Center, 1981
3)        Movable Insulation
           William Langdon, 1980