The U.S. Department of Energy‘s Solar Decathlon is a competition in which twenty university teams compete to design and build the best completely solar powered house. The biennial competition culminates with the teams reconstructing their houses on the National Mall in Washington, DC for a week of tours and contests. In order for houses to be successful they must take advantage of passive solar design techniques while maximizing the solar energy collected through photovoltaic and thermal collectors.

Internal temperature, internal humidity, overall energy balance, site insolation, site temperature and site humidity data were measured for all twenty houses in 15 minute intervals for the eight days of the competition period. Photovoltaic conversion and thermal energy collection were predicted through simple equations which utilized site insolation and temperature data along with collector parameters and orientation. The construction documents for all twenty houses were analyzed for passive solar design techniques. Passive solar data was quantified through use of several rules of thumb. The passive solar design techniques analyzed were direct–gain, indirect–gain, thermal mass, daylighting, insulation, window placement, and shading.

Measured internal temperature and energy usage data were compared with the quantified passive solar rule of thumb values and plotted. Trendlines were fit to the resulting plots using simple linear regression. These trendlines were used to discuss patterns that emerged correlating each passive solar design technique to house performance. These correlations were also used to discuss the validity of the rules of thumb as a design tool.

The results for adherence to the passive solar rules of thumb and solar energy collection were combined to provide rankings of the best designed solar houses. The top three designs based on these criteria were Team Ontario/BC, Team Alberta, and University of Minnesota. These rankings were compared to combined placement in the comfort zone competition and the energy balance competition. Only one of the three predicted best houses was also in the top three for actual competition performance: Team Ontario/BC.

Strong relationships emerged between insulating values and internal temperature control. Strong relationships were also found between thermal storage volume and predicted total thermal collection. The plotted comparisons contradicted many of the limits associated with the passive solar rules of thumb. The rules of thumb are best used as quick calculations and estimations and a more refined computer model should be used for more accurate results.