Page 105 - Zero Net Energy Case Study Homes-Volume 1

COLONIAL HOUSE MULTI-FAMILY HOUSING
CASE STUDY NO. 5
Site Constraints
The site was essentially cleared of all structures at the time of approval and the type of project was limited by zoning to the three levels proposed: ground level commercial and parking and two levels of apartment units. There were no other significant site constraints on the design of the project.
Financing Constraints
Because the project was financed largely through the USDA Farm Labor Housing Direct Loans and Grants Program as a result of the Pacific Company’s commitment to LEED Platinum certifi- cation and ZNE performance of the housing structure as a part of their funding application, this funding came with its associated constraints and requirements.
There were, for example, stipulations about rent and utility allowances. Rents at Colonial House are a fixed 30% of the tenant’s annual household income. A monthly “utility allowance” (in dol- lars) for each type of unit (1BR, 2BR, 3BR or 4BR) is determined by the combination of the output of the solar array and the average consumption of the building during that period, according to a formula given by the California Public Utilities Commission (CPUC). Tenants who consume less energy than the utility allowance receive a credit from the electric utility. Those that exceed the allowance are charged for their additional usage.
Low Energy Design Strategies
The LEED Platinum and ZNE energy performance goals required energy-efficient design strate- gies and construction features that are discussed in detail below. The low-energy/high-efficiency planning of the affordable housing part of the project was modeled during the design phases to assist in cost-effective decision-making. This energy modeling process is discussed in detail below in the section, Energy Design Analysis and Energy Performance.
General Design Considerations
Generally, the number of units was maximized to fit on the two levels, with a large solar PV array installed on the roof to serve the new community as a whole. To minimize the carbon footprint of the project, no natural gas was installed on the site and only electric equipment and appliances are specified. Thus, all heating/cooling, hot water and cooking are electrical only.
Building Envelope – Insulation and Windows
The building is standard wood frame construction with maximum fiberglass insulation used be- tween both the wall studs and the roof joists. No mitigation of thermal bridging through the wall studs is used, such as a layer of rigid insulation over the wood stud wall. The roof joists, however, utilize a layer of sloped rigid foam to slope the roof surface for drainage, which also serves to prevent thermal bridging across the large area of flat roof. The wall R-values range from R-15 to R-21 and the roofs average R-49. The concrete podium, which serves as the floor of the second- floor units, is insulated with panels that have an R-value of R=20.
The glazing properties average U=0.30 and solar heat gain factor (SHGF) equal to 0.37, which are values of good quality low-e glass2. So the windows have very good thermal property values.
2 Low-E, or low-emissivity, glass minimizes the amount of infrared and ultraviolet light that comes through glass, without minimizing the amount of light that enters the room. Low-E glass windows have a microscopically thin coating that is transparent and reflects heat.
Zero Net Energy Case Study Homes: Volume 1
93