Multiplier Effect: High Performance Construction Assemblies and Urban Density in US Housing

Abstract

The suburban house—an emblem of the 20th century American Dream—has come to symbolize unsustainable excess in the new millennium. For the homeowner, the single family home is increasingly burdensome to finance and maintain; for planners and policy makers, suburban sprawl has undermined efforts to limit land consumption and mitigate anthropogenic greenhouse gas (GHG) emissions. While the link between sprawl and transportation emissions is well-established, the atmospheric impacts in the construction and operation of single-family houses are acknowledged but not as well understood. Using a readily available lifecycle assessment tool and building modeling software, this study compares the carbon emissions of low- and high-density housing morphologies and weighs the lifecycle embodied energy costs against the operational energy benefits of increasing thermal performance in the building envelopes of each housing type. The assessment shows that in spite of increasing energy demands embedded in the materially and technically intensive construction of high performance assemblies, the adoption of these techniques in both the house and multi-unit apartment dramatically reduces lifetime GHG emissions. However, the initial toll of building high performance houses—measured in emissions and extrapolated as construction costs—is burdensome to the environment and homeowner alike. As an alternative, high performance apartments can be built at a carbon and dollar cost only marginally higher than that of conventionally-constructed multi-unit dwellings, with a per-unit lifetime GHG footprint that is one quarter of that of a standard house. The economic and land-use efficiencies of enhanced construction assemblies deployed in dense urban residential development create a multiplier effect in potential GHG reduction; a critical factor for contemporary environmental planning and policy.

Appendix: Study Methodology in Detail

Embodied Emissions

Embodied GHG emissions were assessed with the Athena Impact Estimator (from here on: Athena). This life-cycle assessment (LCA) tool has a built-in life-cycle inventory (LCI) of different construction materials and assemblies that respond to regional differences in construction practices in North America. The LCI for New York City was used in this study because it is the closest geographic match. Athena GHG assessment does not account for carbon sequestered in any material. More detailed assumptions of Athena as well as LCI reports can be found at http://www.athenasmi.org/our-software-data/impact-estimator/.

The assessment was done for a 50-year lifespan and all dwelling units were assumed to be owner occupied (this decision has an impact on assumptions that Athena makes about building maintenance).

Building Assemblies

Table 1 shows building assemblies used in the LCA.

Table 1 Assemblies used in the LCA

Omitted Building Elements

The most notable omission in the embodied GHG calculation is the impact of gravel fill at foundations. While an important factor when looking at an individual building, the effect is small when comparing two or more structures. An apartment building is likely to need more fill but its impacts are divided between a large number of apartments. Other omissions include the mechanical and plumbing systems, and elevators. Since a full basement was included for both dwelling types, it can however be assumed that there is ample space for mechanical rooms. The GHG impact of building systems can be presumed to be negligible. In a LCA of a concrete apartment building Pasanen et al. (2011) estimate that elevators and mechanical equipment represent 0.2 % of the total weight of the building. Partitions (except the ones dividing apartments) were also excluded since they would have close to a matching GHG impact in both the apartment building and the house.

Operational Emissions

Operational emissions for the standard house and the apartment were estimated on the basis of U.S. data found in the Residential Energy Consumption Survey (RECS) 2005 (EIA).⁴ Average per square meter annual energy consumption was first derived for each dwelling type and this figure was then multiplied by the size of each dwelling. Basement or garage floor areas were not included for either dwelling but stair halls were included in the apartment building. Data included from RECS was narrowed to dwellings in cold climates (at least 6,000 heating degree days per year) and dwellings built since year 2000.⁵ Calculated in this manner, annual per-square-foot heating energy consumption for the house and the apartment is 595 and 228 MJ respectively. Electricity consumption was estimated at 222 MJ for the house and 296 MJ for the apartment. With no specific data for common area energy use available for Connecticut, these same figures were used for the stair halls of the apartment building.

Energy consumption for the high-performance dwellings were taken directly from the stipulations of the Passive House standard. For both dwelling types, annual per square meter heating energy consumption was estimated at 54 MJ. Electricity consumption was estimated at 112 MJ/m²/a on the basis of the maximum total primary energy consumption (divided between heating and electricity) of 433 MJ/m²/a and estimated 30 % efficiency in electricity production.

Operational GHG emissions were estimated on the basis of the fuel mix for heating in the U.S. North–East and Connecticut electricity emission factors. Carbon equal global warming potential factors of 25 for methane and 298 for nitrous oxide were used. It was assumed that the carbon intensity of both heating fuels and electricity production would be decreasing over time: by 2.4 % annually for electricity and 0.8 % for heating over the full lifetime.