Challenges to decarbonizing post-war buildings
Buildings of the mid-twentieth century face several specific hurdles to successful reuse. The integration of building structure and enclosure is one such challenge. The advancement of concrete as a building material through the early twentieth century led to the dissolution of the idea of separate internal frame and exterior wall systems and the emergence of the monolithic frame. Particularly in Brutalist design, concrete was not only used as a concealed structural material, but it was also celebrated and expressed as part of the building envelope. This application of concrete poses dual conservation and energy efficiency challenges. From a conservation standpoint, concrete exposed to the elements is highly vulnerable to water infiltration and temperature variation. Over time, this exposure will target any original material imperfections, ranging from reactive aggregate to under reinforcing to inadequate cover of ferrous element, causing progressive deterioration of the façade.
As relates to the building envelope, this integration of structure and façade creates significant limitations to the thermal performance of the building enclosure. Concrete structure directly exposed on the façade conducts heat from the conditioned interior to the exterior, creating a thermal bridge that can only be mediated by recladding or over-cladding, an approach that applies a new façade assembly over the plane of the original facade. Even when not exposed, concrete structural elements in plane with the exterior wall frequently interrupt existing or new interior insulation at columns and floor slabs; again, these thermal bridges can only be eliminated by recladding or overcladding outboard of the concrete structure. Cladding over or recladding original facades offers the opportunity to both protect exposed structural elements and provide a continuous thermal envelope; however, it challenges current heritage conservation practices and comes with its own upfront carbon costs that must be evaluated for value against potential improvements to performance.
There are also architectural challenges to upgrading HVAC systems in post-war buildings. While they were originally designed with active mechanical systems, these systems were typically not sized adequately for current requirements, and primary equipment is at the end of its life, if still in service. Many buildings of this era were designed with a structural economy that resulted in minimal floor-to-floor heights, making horizontal redistribution of systems particularly challenging. Additionally, distribution was often concealed within concrete structure, making it impossible to repair or reroute without destructive demolition. There are also risks to installing new materials and technologies without sufficient consideration for how they will perform within the existing building. For example, components such as chilled beams and radiant panels may see issues with condensation if environmental conditions are not controlled properly, and changing the flow of moisture across a building envelope may lead to condensation and deterioration of existing material to remain. Creative solutions and a thorough understanding of the existing building are required to understand which elements of the original systems may be kept in service through retrofit, where new systems are required, and how to design them appropriately.
Life cycle assessment approach for building reuse
Traditionally, environmental sustainability in building reuse has been evaluated based how well an existing building can perform relative to current standards. Energy performance, while important, only captures the impacts from one stage of a building's life cycle, ommitting the environmental footprint of the materials used and construction activities that occur over a building's life. Life cycle assessment (LCA) is a quantitative accounting methodology that predicts the environmental impacts of a material or assembly across the entire life cycle, from material extraction and manufacturing through use and end of life. When this analysis is applied to many or all of the materials that go into a building, this is referred to as whole-building life cycle assessment (WBLCA). As defined by ISO 14040, the international framework for life cycle assessment, the process of performing a LCA involves four phases:
Goal and scope definition: understand what the results of the assessment will be used for and thus which materials or assemblies are important to include and to what level of detail.
Inventory analysis: create an accounting of all the materials and activities that contribute to the scope defined in Step 1. This includes, but is not limited to, the bill of materials used in a building or portion of the building being evaluated.
Impact assessment: calculate environmental impacts by applying environmental impact factors to the inventory created in Step 2. This is most simply done using tools that include building-specific data but can also be performed manually using open-source data sets.
Interpretation: analyze and understand the results to draw conclusions and respond to the goals defined in Step 1.
Life cycle assessment can be applied to existing buildings to measure and reduce the impact of renovation and to quantify the avoided impacts of renovation compared to new construction. In both cases, it is critical to create a meaningful scope boundary. Existing building fabric that will remain in place constitutes a 'sunk cost' and can be excluded from the analysis; those embodied carbon emissions were released in the past and are not impacted by reusing the building. The environmental impacts that do occur from renovation are related to the new material we add to buildings as well as the demolition and end-of-life treatment of material that we remove – these added and subtracted materials are, therefore, the materials that are critical to include in the inventory to measure the impact of renovation. This paper illustrates two ways in which LCA can be applied to building reuse: the first, to quantify the avoided impacts –in this case the carbon emissions that are saved – by reusing and upgrading an existing building instead of replacing it; and the second, to make decisions about the lowest-carbon approach for reusing a specific building.
It is important to note that LCA is not an exact science. While there are standards and product category rules in place, inconsistencies and uncertainties exist with LCA data, with tools, and in the methodology with which these are applied to individual projects. For example, some materials have only industry average data rather than product specific data, and different manufacturers may include different life cycle stages in their numbers. Data includes assumptions about what happens to a given material over its life cycle, from how often it must be refurbished or replaced to what happens at the end of its life. There are also questions about how to count biogenic carbon, emissions associated with the natural carbon cycle of materials like wood that sequester carbon as they grow and then release it at end of life. LCA for existing buildings introduces its own set of challenges. Data can be more difficult to obtain for traditional building materials and techniques. Additionally, most LCA software tools are created with new construction in mind. While these limitations lead to a margin of error in LCA results, the analysis is still highly valuable as a comparative assessment technique and is the only accepted methodology for calculating life cycle environmental impacts. LCA is the basis of many embodied carbon policies and is incorporated as a requirement into many green building rating systems, including LEED, BREEAM, and LBC.
Case study: quantifying the carbon savings potential of whole building reuse
The first case study presented here is a residence hall at MIT designed by Josep Lluis Sert in 1974 (Figure 1). The 115,000 square foot, five-story building is located on the Charles River, a waterfront whose architecture was influenced by Sert's academic leadership at Harvard's Graduate School of Design and own design work of the mid-twentieth century. The residence hall comprises a minimal, cast-in-place structure clad in brick veneer with exposed concrete slab edges and cantilevers, and the fenestration displays Sert's signature multichromatic aluminum window frames, although in a stark black and white in this application. The building originally functioned as three distinct 'towers' housing nine independent social communities. Over its life, the building retained the same programmatic use; however, changing residential approaches, community sizes, and expectations of comfort and performance in conjunction with aging systems and envelope components had rendered the building in need of renovation and upgrade.
The student groups living in the building felt a strong affinity for the architecture of their home. However, there were tensions between the rigidity of the existing building and the evolving needs of the students living within it. The three towers of the building were connected only via a corridor at the ground floor, leaving communities physically isolated from one another. Additionally, as community sizes grew and shrank over time, the distribution of amenities became skewed with some communities physically disconnected from their assigned common spaces, which has led a lack of equity.
Physically, the building was facing systems at the end of their service life and deterioration and aging of exterior components. The original heating and cooling system consisted of a two-pipe hydronic system fed from a central plant, with a local chiller. The building was not mechanically ventilated, relying on operable aluminum, single-glazed windows for fresh air. The original envelope was brick veneer on metal studs, with a small amount of batt insulation in the stud cavity; roofs were entirely uninsulated. Additionally, all the building's MEP equipment was located on the first floor, which will become increasingly vulnerable to flooding through the coming decades.
After more than forty years with only minor renovations, the building needed a comprehensive renewal. The renovation project included new building systems to bring the building up to modern performance standards, upgrades to the building envelope to improve thermal comfort and reduce energy use, climate resilience improvements, and interior reconfigurations to support equity and flexibility in the future while maintaining the original design intent. Programmatically, the renovation physically connected the three towers to encourage strength of community and laid out residential groups via a series and 'anchors' and 'flex' spaces, which allow each community to maintain a central heart while accommodating a changing population over time. A new circulation route was created through the building, consolidated amenities and taking advantage of the building's daylight and views (Figures 2 through 4).
The exterior restoration of the building involved repair and localized rebuilding of the brick masonry veneer, repair and recoating of exposed concrete, and replacement of the failing aluminum windows with double-glazed, thermally broken frames. The existing building design and condition presented challenges to envelope improvement, including the thermal bridge created by the concrete cantilevers and slabs extending to the outside face of the wall (Figure 5) and the unknown condition of the original felt paper air barrier. Thermal modeling was performed during the concept phase to establish the existing thermal resistance (R-Value) of the opaque wall and thermal bridge conditions and to explore the potential improvements in R-Value that could be achieved by filling the stud cavity or furring the wall out to create continuous insulation inboard of the existing wall at each floor level to achieve current prescribed levels of performance. Hygrothermal modeling in WUFI, a software that calculates moisture and heat flow through building materials over time, was then completed to evaluate several insulation materials, including mineral wool batt and open and closed cell spray foam to ensure that the proposed solution did not introduce a risk of condensation. Ultimately, due to factors of human health, long-term performance, and to maintain the moisture migration paths of the original wall construction, the stud cavities were filled with mineral wool and an air-sealed variable permeability vapor membrane was applied to allow moisture to continue to move through the wall similarly to the original condition. The roofs were insulated to code. More aggressive envelope improvements, including additional roof insulation and triple glazed windows, were evaluated through energy modeling, which showed negligible annual energy savings from additional upgrades. While recladding the building to enable installation of continuous insulation at the exterior would have eliminated the existing thermal bridges created by the building's structural system, this approach was rejected due to impact to historic appearance and cost; ultimately the carbon payback of this scheme, while not evaluated during the project, may have been disadvantageous due to the high carbon footprint of additional enclosure materials.