Page 42 - Zero Net Energy Case Study Homes-Volume 1

CASE STUDY NO. 2 FORTUNATO HOUSE RENOVATION
(Opposite Page) Rear view of the newly renovated house, facing east.
Site Constraints
The site slopes from the rear of the lot toward the street level at the front, dropping a full story in height across the length of the property. The orientation of this sloping site is toward the south- west, which is the general direction of views to the ocean. This led to the planning decision to cluster the spaces of the new addition at that end of the house, which faces the street and coinci- dentally is exposed to the afternoon sun. The spaces naturally were given large window areas on that façade to maximize the view opportunity. The lower angle sun in the afternoon then became an important design consideration with regard to solar heat gain and glare in the afternoon.
The solar PV system had similar constraints because of site orientation. The southwest-facing panels were less productive in the winter months but had an advantage in summer months when the weather pattern in this marine location provides more available solar exposure in the afternoon.
Two palm trees located along the southeast property line abutting the neighboring lot were not a significant shading issue because the shadows were cast on the rear of the site and building by midday. The location of the new spaces with its high flat roof where the solar PV panels would be installed thus also minimized the potential effect of these trees.
Low Energy Design Strategies
General Design Considerations
Site constraints and the building program (how much additional space was needed) dictated some of the basic design considerations, as discussed above. Additional ideas resulted from the owners’ initial research and discussions with several professionals, as well as from various site visits to local green homes. They also adopted some design guidelines that would inform their choices as they proceeded: (1) select solutions that involved materials and “off-the-shelf” tech- nologies to demonstrate that ZNE homes were affordable and realistic for middle-class families; (2) choose no system that utilizes natural gas in order to guarantee a “net zero carbon” house without any “carbon offsets”.
Building Envelope – Insulation and Windows
The existing structure, which was to be reused, was built using standard 2X4 framing spaced at 16” on-center. For cost reasons, this framing system was used for the additional floor level also, with a modification of the wood stud spacing to a wider dimension of 24”. This was structurally equivalent (and allowed by the state building code exception for Advanced Framing). In addition, two layers of half-inch thick “insulated sheathing”3 board was installed over the exterior side of the wood studs. This wider spacing allowed for a higher overall insulation value and air-tightness for the wall assembly.
The “insulated sheathing” board also mitigated heat loss via thermal bridging through the wood studs by providing a continuous layer of material with an R-value equal to 2.6. (There are ad- ditional environmental advantages of the product, including absence of formaldehyde found in plywood glues and reduction of noise transmission through the exterior building walls.) The net result was an economical system of structural framing that also had good environmental perfor- mance characteristics.
3 There are several types of insulated sheathing products with different material properties. See Guide to Insulated Sheathing, Building Science Corporation (January, 2007), www.building- science.com.
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Zero Net Energy Case Study Homes: Volume 1