The greatest opportunities for improving residential and commercial energy efficiency may be in improvements to the building shell itself, as opposed to improvements in lighting, HVAC systems, and other internal components. Although complete replacement of building shells may seem to be impractical compared to internal system improvements, the recent IEA report noted that the costs of demolition and reconstruction are of the same order of magnitude as new construction.
Even though demolition and reconstruction may offer greater opportunities for improving building shell efficiencies, CO2 will be generated as a result.
The IEA report notes that the CO2 generated during demolition and reconstruction (above that generated during renovation) is not likely to be quickly recovered by reduced energy consumption by the new building, unless the new construction is at a "high energy efficiency standard" . Palmer et al.  studied this issue in some detail, and found that there would be a net CO2 reduction after 7-15 years. This study evaluated life cycle emissions, including embodied energy and construction and demolition (C&D) waste disposal, but did not evaluate other environmental impacts that could result from the increase in C&D waste disposal.
New materials have also been proposed for use in building shells to improve operational energy efficiency, including phase-change materials (PCMs) in wallboard . While some of these materials use paraffin as the PCM, other compounds have also been proposed. These include methyl esters, methyl palmitate, methyl stearate, capric acid, lauric acid, coconut fatty acids , styrene maleic anhydride, hexadecane, octadecane and formaldehyde . The environmental implications of these materials in PCM wallboard have not been evaluated, and could result in undesirable emissions of organic compounds into the indoor environment, or could create increased organic emissions during production or generate unwanted emissions following disposal.
The building industry has also evaluated the use of insulated concrete forms (ICFs) as an alternative to wood frame structures to improve building operating efficiency. The potential reduction in GHG emissions when using alternative approaches requires a comparative life cycle analysis to evaluate the total changes in GHG emissions during material production, use, and disposal. The Portland Cement Association conducted a relatively thorough life cycle analysis for a model home located in five U.S. cities, and estimated that the net GHG emissions (and most other environmental impacts) would be reduced over a 100-year building life when using ICFs rather than wood frame construction . The analysis did not account for changes in emissions in the disposal stage, and also did not account for the potential sequestration of carbon in the wood used in the building, but did account for emissions during material production. The most significant area in which ICFs performed worse than wood was in emissions of polystyrene, which is used in the ICF insulation.
Building shell improvements combined with on-site power generation using renewable energy sources (primarily wind, solar, or biomass), when designed as a system, can result in buildings that require no energy from external sources ("net-zero buildings"), and can even lead to a net generation of electricity as on-site demand and production allows. The environmental impacts of the specific technologies have been described above. Net-zero buildings are sometimes considered to be sustainable, although additional evaluations of broader environmental impacts are needed to determine the sustainability of net-zero or energy-producing buildings .
A further approach to improving the environmental performance of building shells is the application of "green roofs." In this approach, grass or other vegetation is planted on available flat rooftop space, generally resulting in reduced heating load and acting as a sink for CO2. The use of green roofs could result in an increased level of nutrients in runoff during storm events if appropriate designs are not used or if the systems are improperly operated .
A separate environmental issue that must be considered in conjunction with increased building operating efficiency is indoor air quality. Initial efforts to increase building energy efficiency starting in the 1970s relied heavily on reducing ventilation rates and reducing leakage of air into and out of buildings. However, this strategy sometimes resulted in "sick building syndrome" in which building occupants became ill due to decreased indoor air quality [135-138]. More holistic approaches to reducing building energy consumption have been developed, and are now being applied in new construction [139, 140]. Reduced use of materials that emit organic compounds and improved ventilation systems that minimize energy loss while maintaining indoor air quality are two key approaches to minimizing indoor air quality impacts once associated with improved energy efficiency [141, 142].
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