Thermal Bridging and Building Facades: The Missing Key to Low Energy Building Design

Lee, Ivan

Thermal Bridging and Building Facades

The Missing Key to Low Energy Building Design

Paper by Ivan Lee

Presented on October 13, 2022 at Facade Tectonics 2022 World Congress

Overview

Abstract

Thermal bridging through building façades have been overlooked by designers and building energy codes and standards in the past, which has led to higher space heating and cooling loads, occupant discomfort, and higher risk of condensation. There are numerous examples of energy efficient buildings consuming more energy than what has been predicted in energy models that may be attributed to a lack of understanding of thermal bridging.

Thermal performance of a building façade can drop significantly when thermal bridging is accounted for. A typical 40-story high-rise residential building with 40% window-to-wall ratio, and balconies equal to 20% of the floor perimeter can have 71% of the total heat loss from thermal bridging based on typical construction details, with the majority of the heat loss at the window to wall transitions (e.g. window heads, jambs, and sills).

While thermal bridging is mostly evaluated with 2D and 3D thermal simulations, there are many resources available. Thermal bridging catalogues such as the Building Envelope Thermal Bridging Guide (BETB)^{^[1]} and thermalenvelope.ca have over 600 details and an integrated calculator for calculating building envelope overall thermal performance that makes it easier to account for thermal bridging.

Recognizing and mitigating thermal bridging is an important part of the strategy to meet low energy targets for buildings. Many jurisdictions are starting to incorporate thermal bridging into their codes and standards. The NECB 2017 already requires thermal bridging to be considered, the NYC 2020 Energy Conservation Code now requires thermal bridging values be reported in designs, and both ASHRAE 90.1 and IECC will likely include thermal bridging requirements in their next update cycles. With net-zero energy targets in the horizon and more strict building energy requirements coming, it is time for designers to pay more attention to thermal bridging as this is a bridge we must cross for a better future.

^{^[1]} https://www.bchousing.org/rese...

Authors

Ivan Lee

Building Science Consultant

Morrison Hershfield

Keywords

3D thermal modeling, energy modeling, high-performance facade, building performance, finite element analysis (FEA), performance modeling, thermal break, thermal performance

Introduction

It is widely known that thermal bridging through façade components and details degrade building energy performance and cause issues such as condensation and thermal discomfort for occupants. Until recently, accounting for the impact of thermal bridging in façade design has been rare due to difficulty in measuring and evaluating its impact. As a result, the effect of thermal bridging in building façades were under reported leading to many buildings with higher energy consumption and energy costs than predicted by code or energy models due to higher space heating and space cooling loads. Mechanical designers also compensated for the added space conditioning loads by oversizing mechanical units that were more expensive and not always operating at peak efficiency.

Advances in computer simulations have made evaluating thermal bridging in façades easier for designers, first through two-dimensional heat transfer analysis made popular by programs such as THERM which is widely used for evaluating thermal performance of glazing systems. In recent years, three-dimensional thermal modeling has become popular as this analysis can better evaluate thermal bridging of assemblies with discrete thermal bridging components and account for complex heat flow through highly conductive components of curtain wall and window wall systems often within a few percent of measured heat flow through hot box measurements (Norris et al. 2015). Thermal bridging resources such as the Building Envelope Thermal Bridging Guide (Morrison Hershfield 2021) has made accounting for thermal bridging in façade design much more accessible to designers with a comprehensive catalogue of details and assemblies that are typically found in North American building construction.

Building energy codes and standards have also made thermal bridging more prominent in façade design by placing a greater emphasis in including the impact of thermal bridging in façade U-value calculations and whole building energy simulations. Progressive building energy codes and standards that are targeting Net-Zero Energy or Net-Zero Energy Ready performance levels, such as Passive House, the BC Energy Step Code, Vancouver Zero Emissions Building Plan, and Toronto Green Building Standard, all stipulate the inclusion of thermal bridging in façade thermal performance calculations. Other codes that has less stringent energy requirements, such as the National Energy Code for Buildings Canada (NECB) and the NYC Energy Conservation Code 2020, are also starting to require thermal bridging be accounted for in assessing façade thermal performance.

Despite the changing requirements for designers to account for thermal bridging, many are not fully aware of the impact of thermal bridging on building energy performance, how to integrate it in their analysis, and how to mitigate the impact of thermal bridging. This paper discusses the impact of thermal bridging and mitigation strategies at common façade details through examples and tools that are readily available to façade designers.

Thermal Bridging Types and Mitigation Strategies

Thermal bridging in building façade assemblies may be categorized into three types which are typically found on most buildings. Façade details such as balconies, intermediate floors, parapets, corners, soffits, penetration