by
Adam Bond, Architectural Preservationist
Responsible Stewardship
The case for stewardship over ownership — for the custodial relationship over the transactional one — does not rest primarily on sentiment or civic rhetoric, it rests on the physics of what buildings are and on the economics of what their replacement costs us in the long run.
The global building sector’s contribution to carbon emissions is among the most consequential and least publicly understood facts in the climate conversation. The United Nations Environment Programme’s 2022 Global Status Report for Buildings and Construction established that the construction and operation of buildings accounts for approximately 37 percent of global energy-related CO₂ emissions — more than any other single sector, including transportation or heavy industry.
The significance of embodied carbon is that it is emitted upfront at the moment of construction. Unlike operational carbon, which can be reduced over decades by improving building systems and energy sources, embodied carbon is committed at the time the building is built. Once a building is demolished and its materials are landfilled, that carbon commitment cannot be recaptured by building a more efficient replacement — the replacement starts its own carbon clock from zero, accumulating new embodied carbon before it has offset what was destroyed.
The specific figures for Allentown’s building stock have not been independently calculated, but the National Trust for Historic Preservation’s widely cited Preservation Green Lab research provides useful reference data. Their 2011 analysis found that a 50,000-square-foot commercial building embodies approximately 80 million BTUs of energy — equivalent, in raw energetic terms, to approximately 640,000 gallons of gasoline. Demolishing such a building generates approximately 4,000 tons of construction waste. If only 40 percent of the original building’s materials are retained in the replacement, the new building — even a highly energy-efficient one — requires between 40 and 65 years of improved operational performance to offset the embodied carbon lost in the demolition.
The 40- to 65-year payback estimate is, if anything, conservative for the conditions prevailing in 2026.
As electricity grids decarbonize, the operational advantage of a newer, more efficient building shrinks: the energy savings from a more efficient HVAC system matter less when the grid powering that system is increasingly renewable. Embodied carbon, by contrast, is emitted regardless of the grid’s energy mix — it is locked in the moment the steel is smelted, the concrete is poured, and the lumber is milled. A 2023 report by the Climate Heritage Network, produced in collaboration with Architecture 2030 and the Carbon Leadership Forum, made this point with particular sharpness: under scenarios of substantial grid decarbonization, the payback period for demolition and replacement can extend beyond 100 years, at which point the embodied carbon of the demolition cycle is simply a permanent addition to the atmospheric burden with no legitimate offset.
The embodied carbon of a building is not simply the energy required to build it. It is also — and for old buildings, primarily — the carbon that has already been sequestered and stabilized in its materials over time. Lime mortars and plasters, for instance, hardens through carbonation: the calcium hydroxide in the mortar absorbs atmospheric CO₂ and converts it to calcium carbonate, the mineral of which limestone is composed. A lime-mortared masonry or plastered building that has been standing for 150 years has been actively sequestering carbon in its walls for that entire period. The trees whose timber frames the building absorbed carbon during their growth. The brick, fired in kilns that have long since gone cold, embodies the energy of that firing — energy that, whatever its original source, has now been spent and cannot be spent again.
Historic England’s 2023 research on the embodied carbon of traditional building materials makes this point with reference to the specific materials common in pre-industrial and early-industrial construction: natural materials like lime, timber, and unfired brick have significantly lower embodied carbon than their modern substitutes (portland cement, steel, synthetic insulation), in part because they were produced with less energy-intensive processes and in part because some of them sequester carbon as they cure or as they age. The replacement of these materials with modern construction products amplifies the embodied carbon cost of demolition not once but twice — destroying a low-carbon material and substituting a higher-carbon one.
For Allentown, whose pre-war building stock is predominantly structural brick and lime mortar construction, with lime plastered interiors, the implications are direct. These buildings were built from materials whose production was comparatively low-carbon (hand-fired brick kilns at much lower volumes than modern industrial production; local lime from regional quarries; locally or regionally harvested timber), and they have been sequestering and stabilizing carbon in their fabric for 80 to 150 years. Their demolition is not the end of a lifecycle. It is the interruption of one.
There is a logical structure to stewardship arguments that does not always receive the emphasis it deserves: the decisions involved are not symmetrical in their reversibility. A decision to preserve a building and subsequently find it unsuitable can be revisited — the building can be rehabilitated differently, adapted to a different use, modified to meet changed requirements. A decision to demolish cannot be revisited. The building is gone, and what is gone includes the embodied energy, the cultural material, the neighborhood context, and the specific material qualities — the old-growth timber, the hand-fired brick, the lime mortar — that cannot be replicated at any price.
This asymmetry should produce a systematic bias toward preservation in cases where the evidence is uncertain or the costs are closely balanced. It is the same asymmetry that produces precautionary principles in environmental law and irreversibility premiums in resource economics. When the cost of being wrong about demolition is permanent and the cost of being wrong about preservation is recoverable, the rational default is to preserve until the case for demolition is overwhelming — not to demolish until the case for preservation is made.
In practice, the burden of proof runs the other way: preservation must justify itself against a default of demolition in most American planning and development contexts. This is irrational by the logic of irreversibility, and it is irrational by the logic of embodied carbon. Baker, Moncaster, and colleagues argued in a 2021 paper in the Journal of Architectural Conservation that heritage conservation practice should be reframed explicitly around the “retention not demolition” principle, treating demolition as the exceptional case requiring positive justification rather than the routine default requiring no justification at all. This is not a novel position in conservation theory. It is, however, still novel in the practice of most American municipalities.
The concept of stewardship — as distinct from ownership — has a long history in environmental philosophy, public resource management, and cultural heritage theory, and its application to the built environment follows a consistent logic. Buildings inserted into urban contexts are not private goods in the simple sense. They are, as urbanists from Jane Jacobs to Charles Montgomery have argued, nodes in a network whose value is collective and whose quality is produced and sustained communally. The building owner benefits from the street that the neighboring buildings create; the street’s character depends on the decisions of all its building owners; those decisions are therefore matters of collective concern whether or not they are matters of collective legal authority.
The transgenerational dimension of this collective responsibility is, if anything, more important than the spatial one. Allentown’s historic fabric was built over roughly a century, from approximately 1840 to 1940, and it has been inhabited — and shaped, and maintained, and damaged — by many generations since. The current generation of building owners inherited it from the previous one, and will bequeath it, in whatever condition they leave it, to the next. The decisions being made now about what to maintain, what to repair, and what to demolish will be the environment that Allentown’s children inhabit. This is not a platitude. It is a statement about the actual temporal structure of urban change — slow enough that individual decisions accumulate over generations, fast enough that a single generation of poor stewardship can irreversibly degrade what took a century to build.
United Nations Environment Programme. Global Status Report for Buildings and Construction 2022. Nairobi: UNEP, 2022.
National Trust for Historic Preservation / Preservation Green Lab. The Greenest Building: Quantifying the Environmental Value of Building Reuse. Washington: NTHP, 2011.
Historic England. Investing in Heritage to Avoid Embodied Carbon Emissions. Heritage Counts series. London: Historic England, 2023.
Historic England. The Embodied Carbon Emissions of Construction and Retrofit Materials for Traditional Buildings. London: Historic England, 2023.
Baker, Harriet, Alice Moncaster, Hilde Remøy, and Sara Wilkinson. ‘Retention not demolition: how heritage thinking can inform carbon reduction.’ Journal of Architectural Conservation 27, no. 3 (2021): 176–194.
Röck, M., A. Saade, M. Balouktsi, F.N. Rasmussen, H. Birgisdottir, R. Frischknecht, G. Habert, T. Lützkendorf, and A. Passer. ‘Embodied GHG emissions of buildings — the hidden challenge for effective climate change mitigation.’ Applied Energy 258 (2020): 114107.
Climate Heritage Network, Architecture 2030, and Carbon Leadership Forum. CARE Tool: Carbon Avoided Retrofit Estimator. Caretool.org, 2023.
Targeting Zero. ‘458 Oxford Street: Why a Comprehensive Retrofit Is More Carbon Efficient than the Proposed New Build.’ London: Targeting Zero, 2022.