One of the goals for the Solar Decathlon is to design and build houses that are innovative, yet affordable to some individuals who want to build a small custom home. The target construction costs were set at $250,000. A metric of cost-per-square-foot was not used, as it’s the final dollar amount that determines if a homeowner would purchase the house. In addition, larger houses tend to have a lower cost per square foot, given the wall-to-floor ratio. The final cost estimates include all building materials, including Americans with Disabilities Act-compliant items and fire-suppression systems—things that may not be routinely found in the consumer marketplace. The cost also includes estimated costs of hiring a contractor to build the house as documented; transportation of the house; and any unique foundation system. The teams were required to declare the target construction cost and those costs were verified by a professional cost estimator. If the information to complete a thorough estimate was missing, the estimators were to err on the high side of construction costs to accommodate for uncertainty.
Only three teams earned the available 100 points by not exceeding the $250,000 goal: Norwich University ($168,385), Stanford University ($234,092), and Kentucky/Indiana ($248,423). Norwich University made affordability a primary design component by targeting clients who earn 20% less than a Vermonter’s median income. Norwich University took the approach of reducing costs from the very beginning stages through the home’s final construction, minimizing skin-surface-to-floor-area ratio; using wood as the sole material for the superstructure; and choosing readily available cellulose and mineral wool batts for insulation. The team also prioritized conservation and efficiency so that the resulting PV and solar thermal systems could be as small (and inexpensive) as possible.
On the opposite end of affordability was the University of North Carolina, Charlotte home, which came in at $350,686. Some of the increased costs for their home may be attributed to the unique building materials and complex PV racking and control system.
The comfort zone contest measures the temperature and humidity values during specified time periods during both the day and nighttime hours. Houses received full points by maintaining indoor temperatures between 71°F and 76°F, and relative humidity below 60%.
Comfort zone winner Santa Clara University used a hydronic system embedded in the plasterboard of the ceiling for heating and cooling. To obtain adequate air changes and dehumidification, a low-flow ducting system was built into the wood subfloor. One interior wall, coated with a natural earthen clay plaster, helps regulate humidity by absorbing excess moisture in the house. It can also be misted with water to help cool the interior. A whole-house control system provides real-time energy performance data while simultaneously controlling the heating and cooling system, lights, sliding doors, windows, and blinds, to maintain indoor comfort.
To receive full points in this contest, the house needed to provide 15 gallons of water heated to 110°F (average temperature) in no more than 10 minutes. Several times over the course of the event, event judges drew hot water from each of the houses. These draws were designed to simulate typical clothes washing, dishwashing, and bathing at various times of the day, with no more than three draws in a 24-hour period.
Six teams tied for first place in this category, earning the maximum number of points. Teams took multiple approaches for their hot water needs, although most integrated solar water heating systems. Some teams chose to use heat pumps and heat recovery units to heat their domestic water. For their water heating, Team Austria tapped into the two high-efficiency air-to-water heat pumps that also provide space heating and cooling. They also included a heat-recovery system for the shower to reduce water-heating energy use for that task. For teams that incorporated solar thermal systems, typical strategies were to use two or three 4-by-8-foot flat-plate collectors.