Page 122 - Zero Net Energy Case Study Homes-Volume 1

ZERO NET ENERGY CASE STUDY HOMES OBSERVATIONS
Observations
This first zero-net-energy (ZNE) case study monograph about residential buildings, Zero Net En- ergy Residential Buildings, Volume 1, examines the design and performance of five very different types of ZNE housing design, ranging from simple renovation of a single-family home to a low- rise multi-family project. The initial goal of each project is ZNE performance, so all the case study projects have the installation of a solar PV system that offsets the annual energy demand as a base component of the designs. The projects differ, however, in the choice of energy-efficiency design strategies utilized to minimize that annual energy demand, which are based on financial criteria peculiar to the type of project. These criteria depend on whether the project is to be owner-occupied or, as is the case for three of the projects, if the house or unit is to marketed for sale or rent. Not unlike other design features of the particular housing project, the perceived mar- ketability and expected profit margins are a determining factor in the choice of system or feature.
Yet, even though the decisions made may reflect different financial or marketing criteria, there is also a commonality among the case study projects for technical choices because of the basic similarity of construction type (generally wood-frame, “Type 5” construction) and the common “program” of spaces. In this regard, it is possible to observe and compare some differences in the design choices made and their relative success, which are informative for other ZNE residential projects.
Observations from the Case Studies in this Volume
Single-Family Residential
This type of housing, representing four of the case study projects, has the same basic type of construction and each utilizes similar energy-efficient systems and features that are unusual when compared to standard housing construction. The most significant such feature, which is not found at all in standard construction, is described in the case studies as air-tightness of the overall construction. Furthermore, for each ZNE case study project, this characteristic was ac- tually measured by physical testing of the completed house and rated on a scale according to its rate of air leakage. What is remarkable about this is the importance placed on this aspect of design for all of the case study projects and the recognition that air leakage is the major cause of heating and cooling demand in a highly-insulated house.
The second observation is that the design of each of these case study houses recognizes the importance of indoor air quality when air-tight design is a characteristic of the construction. How- ever, none of the projects include real-time monitoring of indoor air quality or installation of more elaborate ventilating systems, which may be advisable for extremely air-tight construction.
All of the single-family case study houses utilize electric heat pumps, either air-source or air- water type, for energy-efficient heating and cooling. This is preferable to traditional gas-fired fur- naces for heating combined with a conventional air-conditioning system for cooling. Heat pump technology has made perhaps the largest advances in energy efficiency, practically doubling in COP in the last ten years. Cost effectiveness has also improved and will continue to do so, mak- ing heat pumps the logical choice for future ZNE residential projects.
Furthermore, in all but one single-family case study house, electric heat pump water heaters were used instead of the standard gas-fired water heaters. This is more energy-efficient, is eas- ily supported by the solar PV system and brings the house closer to an all-electric (zero-carbon) design. With the one exception, the cooking appliances in these case study houses are electric powered—either electric range or induction cooktop with electric oven—achieving that zero- carbon design.
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Zero Net Energy Case Study Homes: Volume 1