Nathan Good Architects

We are often asked about the design of passive solar homes for Oregon’s Willamette Valley. The climate in this region is relatively mild with wet/cool winters and dry/warm summers. The average temperature during the winter months is about 45º F, and the overhead sky is dominated by cloud cover from November until April. For passive solar homes, when we need the sun the most is when we have it the least. Without a well-insulated building envelope and high-performance windows, it is easy to lose more heat during the winter months than that which is gained from exposure to the sun.

One of the key ingredients for a successful passive solar home is the proper window specification. We track two of the multiple window performance factors for passive solar design: the U-value and the Solar Heat Gain Coefficient (SHGC). The U-value rates the windows resistance to transmit heat from the inside out; the lower the value the better for energy savings and thermal comfort. The Solar Heat Gain Coefficient (SHGC) rates the ability of a window to block solar heat from entering a home; the lower the number the better to keep a home cool, the higher the number the better for passive solar design during the winter months.

Our preferred window for passive solar design in this climate is one with a low U-value of .25 and lower, and a high SHGC of .65 (65%) or higher. Until recently, this has been a difficult order to fill. We have recently specified a triple-pane window (suspended film with fiberglass frame) from Serious Windows (www.seriouswindows.com) that will approximate these performance criteria.