Life Cycle Assessment

Systems Thinking for Residential Buildings: prioritizing green building and waste prevention 

Research team: Earth Advantage Institute, Quantis, and the Oregon Home Builders Association

Executive Summary | Report only PDF  | Appendix 3-14 | Appendix 17

As we look toward the future, it is clear solving the climate change dilemma is of paramount importance, but additional issues of sustainability also deserve attention.  While building energy use is of utmost significant (representing 40% of US greenhouse gas (GHG) emissions), other key issues linked to the building industry are materials use and waste management.  The amount of waste generated on a per capita basis (to which construction and demolition is a major contributor) has increased significantly over time and it is important to identify practices that reduce material use to not only avoid the impacts of landfilling materials but more importantly to avoid the impacts of material production, which are significantly larger.  While efforts are underway to reduce the material use and waste generated from the building industry, sometimes these activities unintentionally result in overlooking opportunities for even greater reductions in lifecycle environmental impacts. 

This new research, funded by the Oregon Department of Environmental Quality (DEQ), seeks to help guide the integration of a systems approach into the evolving green economy, and the residential building industry specifically.    The best way to guarantee that green is built into the fabric of America’s tomorrow is to ensure that a holistic approach is utilized to prioritize efforts and achieve the best results possible – in terms of performance, cost and environmental gains.  The results of this life cycle assessment are informative for members of the design, construction, remodeling, deconstruction, and demolition industries; material suppliers, green building specialists, community organizations, building associations and nonprofit organizations; and government housing, code, planning, and energy agencies.

This report identifies green building practices that should be prioritized based on the greatest benefits provided over the life cycle of residential homes in Oregon.  The research examined the effects on life cycle impact of over 20 distinct green building practices ranging from design issues, construction techniques and material selections, such as advanced framing techniques, multi-story housing, utilization of durable materials, and placing ducts in conditioned space, among others.  Further, the research includes modeling the large scale (state-wide) impact of focusing on the practices and materials that are the best candidates for achieving environmental improvements, as identified by the aggregated GHG emission reductions over the life cycle of the homes.

While efforts are underway to reduce the material use and waste generated from the building industry, sometimes these activities unintentionally result in overlooking opportunities for even greater reductions in lifecycle impacts.  The full report, linked above, covers the study approach, results and conclusions of a life cycle assessment of residential homes and over 20 distinct green building practices to help guide practices in this sector, and a summary of findings is below. 

Results and Findings from the Study

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The chart indicates the relative impacts of different stages of residential home construction for a standard home built to Oregon’s 2008 residential building code. Impacts are measured for climate change, human health, ecosystem quality and resource depletion.  As you can see, the majority of the impacts occur during the occupancy (use phase) of the building.  In the case of climate change impact over 85% of the impact is accounted for by the energy use during the occupancy stage, with materials accounting for a little under 14% of life cycle greenhouse gas (GHG) impacts.  Transportation, construction and maintenance, and end of life treatment of the home account for a very small portion of life cycle GHG emissions.

Following is a summary of some additional key findings of the study:

• Of the 30 different material reduction and reuse practices evaluated, reducing home size and multi-family living achieved the largest greenhouse gas reductions along with significant reductions in other impact categories.
• Reducing home size by 50 percent (from approximately 2200 square feet to 1100 square feet) results in a projected 36 percent reduction in lifecycle greenhouse gas emissions.
• Reducing home size is a significant leverage point for environmental impact reduction and may be and may be equivalent to achieving minimum levels of “green” certification.
• Various sizes of multi-family housing show significant lifecycle reductions in greenhouse gases and most other pollutants.
• Families who choose or require more living space than a “small” home may lessen a large home’s impact by adding green building practices and increasing the home’s energy efficiency.
• New and existing homes of any size could incorporate internal accessory dwelling units (sometimes known as “mother-in-law apartments”) within the home as an option to increase density and reduce the square foot/ person ratio, provide flexible living spaces, and achieve the environmental benefits of both small and multi-family living.
• More than 80 percent of greenhouse gas emissions over a home’s 70-year life occur during occupancy and are attributed to electricity and fuel consumption. Approximately 14 percent of greenhouse gas impacts are tied to producing the original and replacement building materials.  Constructing and maintaining the home account for about 2 percent and transportation of building materials accounts for less than 1 percent.  Oregon’s existing material recycling and energy recovery system reduces greenhouse gas emissions by 4 percent over the typical 70-year life of a home.
• For other types of environmental impacts, materials contribute 10 to 40 percent of life cycle impacts.
• Only a small amount – about 6 percent – of building material-related waste generated occurs during home construction, with about 50 percent of waste generation occurring during 70 years of home repairs and maintenance. The remaining 44 percent of waste generation occurs at the time of the home’s demolition.
• Material reuse significantly reduces the amount of waste generated and material-related impacts of production but provides limited greenhouse gas reductions over the home’s life because reuse does not typically affect the operational energy consumption of a home, as do other practices evaluated.