Effect of Thermal Bridging on Buildings' Energy Performance
Comparison of Area-Weighted vs. Additive Thermal Resistance in Facades
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Overview
Abstract
Heat transfer through building facades can occur by any combinations of conduction, convection, and/or radiation. Conductive heat transfer depends on materials’ thermal conductivity (λ) and thickness (d), which influence building envelope’s thermal resistance (R-value). The most common approach for calculating R-value of building facades is based on the additive method, where material components of the facade in sectional view, their relative thickness and thermal conductivity are considered. However, in order to account for thermal bridging caused by framing, area-weighted approach should be used to determine more accurate R-value. This approach also considers plan view of building facade, and the properties of framing components. The main objective of this research was to investigate the effects of facades’ thermal resistance (additive vs. area-weighted R-values) on buildings’ energy performance. Research methods included data collection, modeling, simulations and comparative analysis of results. An existing Campus Recreation Building on UMASS Amherst campus was used as a case study building. First, the original construction documentation was reviewed to create a 3D model in Revit. Facade material components and specifications were used to determine properties of the opaque facade system, consisting of brick cavity wall with steel stud framing. R-values for this facade system were calculated using additive and area-weighted methods. Then, a building energy analysis simulation program Green Building Studio was used to calculate annual and monthly energy consumption for the case study building, where one energy model was created to analyze the impacts of two different R-values on the overall energy consumption of this building. Other inputs, such as building geometry, occupancy schedules, glazing materials, etc. were identical in both simulation scenarios. Energy modeling results were compared to actual energy consumption data, collected over a period of one year. Simulation results showed that energy consumption, cost, energy usage intensity, carbon emission, and heating loads were higher with area-weighted method, and lower with additive approach.
Authors
Mahsa Farid Mohajer
Ph.D. Candidate & Teaching Assistant
University of Massachusetts Amherst
mahsafarid@umass.edu
Ajla Aksamija
Associate professor
University of Massachusetts Amherst
aaksamija@umass.edu
Introduction
Heat transfer through buildings’ facades significantly impacts buildings’ thermal performance, their energy consumption, energy costs, and carbon emissions. Therefore, it is essential to provide an appropriate level of insulation, thus
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Background
Heat transfer mechanisms include conduction, convection, and radiation. In conduction, heat travels through solid materials. In convection, heat circulates within the building through liquids and gases, and in radiation, heat
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Research Questions and Methods
The research objectives of this study were to compare area-weighted vs. additive R-values of a facade assembly, and to quantify their effects on various performance aspects of the building. The
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Analysis Results and Comparison
Analysis model was extracted as gbXML file from Revit, and uploaded into GBS in order to prepare for the simulation runs. The first simulation design alternative captured monthly and annual
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Conclusion and Future Work
Actual energy consumption data was a representation of energy usage based on weather data specific to only one-year cycle. In contrast, simulation results captured energy consumption based on typical metered
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Rights and Permissions
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