New York City High-Rise vs. Low-Rise Residential Energy Optimization

Abstract

Residential buildings create 37% of New York City’s greenhouse gas emissions, so determining the most effective way to reduce the emissions of this building type is critical to achieving the city’s 80% greenhouse gas emissions reduction goal by 2050. Since most of the NYC buildings that will exist in 2050 have already been built, the city needs to focus on programs to optimize existing buildings, instead of only establishing aggressive requirements for new buildings. This research determines the most effective way to renovate existing low-rise brownstone buildings to reduce operational and lifecycle greenhouse gas emissions, while ensuring that these renovations meet or exceed the operational emissions of new code-compliant high-rise residences. All construction materials considered are readily available, to permit less sophisticated owners and contractors to use these strategies. My research focused on the significant stock of existing low-rise brownstone buildings, two to five stories, in Brooklyn, New York. My analyses tested the hypothesis that the historic character of existing low-rise brownstones in Brooklyn can be maintained, while efficiently upgrading the interior envelope, roof, walls and windows, to achieve a more efficient envelope than the high-rise buildings currently being constructed. I first determined that existing brownstones can be renovated from the interior, without impact on the existing facades, to produce buildings that meet or exceed the performance of high-rise towers built to current energy codes. The functional unit used to compare the different building types was a floor area of 300-333 square feet per resident occupied for a period of 20 years. High-rise buildings have been benchmarked per functional unit to have an energy consumption of 48,000 kBtu, while low-rise buildings consume 49,000 kBtu. After renovations, low-rise buildings can be lowered to 35,000 kBtu per functional unit. My research considered 22 combinations of materials, including two different framing types, six types of continuous insulation and six types of non-continuous insulation. Wall and roof construction assemblies were modelled using Therm 7.4 software to identify the R-value of each assembly. The lifecycle analysis, considering eight environmental impact categories, was run using the Tally database Revit plugin to determine the comparative impacts of the iterations. The environmental impact categories were: acidification, eutrophication, global warming, ozone depletion, smog formation, primary energy, non-renewable energy and renewable energy. A brownstone retrofit using wood studs, 3” Gutex wood fiber board insulation and open cell polyurethane foam blown between the 2x4 wall studs for the wall construction provided the lowest environmental impacts without a significant cost premium to a typical renovation. A comparison of the material selections studied indicates that the iterations with the highest global warming potential and the highest primary energy demand were 54% and 37% worse, respectively, than the iterations with the lowest values in these categories. This supports the principle that consideration of lifecycle impacts of materials is critical to reduce lifecycle carbon impacts in building construction and renovation. When upgrading a brownstone with the recommended materials, the overall energy use is 27% more efficient than a typical code compliant high-rise building.