Demolition vs. Redevelopment: Carbon Payback in Massachusetts

The question

This project began in Brookline, where debates about housing and demolition often focused on the immediate carbon released when an older home is replaced. That concern is real, but it is not the whole climate comparison. The longer-term questions are harder to see: how the old and new buildings use energy, where additional households would otherwise live, and how location affects driving.

The same questions arise in communities across Massachusetts. Towns and cities need to reduce greenhouse-gas emissions while adding housing in places with access to jobs, services, and transit. The calculator was developed to put the principal carbon effects on one timeline and make their assumptions open to examination.

These goals can appear to conflict when new housing requires demolition. Taking down an existing building and constructing a new one creates an immediate carbon cost. Most of that cost comes from producing the materials used in the new building. Demolition adds another, usually smaller, amount.

That initial cost matters, but it does not answer the whole question. Keeping an existing home also has consequences over time. The home continues to use energy. If a site could accommodate more households, those households need housing somewhere else. Where they live can affect both building emissions and how much they drive.

The MA Embodied Carbon Payback Calculator brings those parts of the comparison together. It asks how long it takes for annual differences in building and transportation emissions to offset the upfront carbon cost of redevelopment. It also shows what happens after that payback point.

A broader comparison

The calculator compares keeping one existing single-family home with demolishing it and building either one replacement home or a small multifamily redevelopment containing two to eight homes on the same site.

The first new home replaces the household associated with the existing home. Homes 2 through X provide capacity for additional households. This distinction matters because the calculator applies its substitute-housing and transportation comparisons only to those additional households.

Keep the existing home	Redevelop the site
The existing home continues to use energy.	Demolition and production of new building materials create upfront emissions.
Households beyond the first need housing elsewhere.	All new homes use energy at the project site.
That substitute housing creates operating emissions elsewhere.	Homes 2 through X avoid the need for the modeled substitute housing.
The location of those households affects expected driving.	The redevelopment location affects their expected driving.

The model does not assume that redevelopment creates new households. It asks where households are housed and compares the emissions associated with those locations.

All reported emissions and savings are totals for the complete comparison. They are not divided by the number of homes, buildings, or occupants.

This fuller comparison is especially important when evaluating housing in a walkable or transit-rich area. A comparison limited to demolition and new materials captures the carbon cost of construction but misses the operating, housing, and transportation effects that follow.

The reference scenario

The opening reference case reflects the setting that motivated this project: an older gas-heated home in transit-rich Brookline, replaced by four smaller homes under the Stretch/Specialized Code plus Fossil Fuel-Free setting. It is a useful starting point for that local policy question, not a claim about what is typical statewide or in every Brookline project. Users can change the inputs to match the context they want to examine. In the calculator, "older" names a stock-informed energy profile. No construction year is assigned.

The reference case compares:

one 1,800-square-foot older gas-heated home;
four new all-electric homes averaging 900 square feet each;
three additional households;
a transit-rich, walkable location;
the Stretch/Specialized Code plus Fossil Fuel-Free setting and a 26.35 kBtu/sf-year all-electric new-building energy estimate;
the reference estimates for construction carbon and demolition emissions;
the Mid future regional-grid pathway, with the optional Green CCA result turned off.

Using those inputs, the initial carbon cost is paid back in Year 6. The modeled cumulative net savings reach approximately:

Result	Reference-scenario value
Carbon payback	Year 6
Year 10 cumulative net savings	76.0 tCO₂e
Year 30 cumulative net savings	407.4 tCO₂e

Before Year 6, part of the upfront carbon debt remains. After Year 6, the annual differences continue to add to cumulative net savings. In this reference case, payback is a milestone rather than the end of the comparison.

Year 6 is the result of these inputs. It is not a general answer for every redevelopment. Changing the number or size of the homes, the condition of the existing home, building performance, upfront carbon, location, or future emissions pathways can move the payback point earlier or later. Some cases may not reach payback within the modeled period. The calculator is not designed to produce payback in every scenario: if the selected construction and operating emissions are not offset within 100 years, it reports no payback rather than forcing a favorable result.

Why the result looks this way

Several parts of the model work together to produce the result.

Building operations

Older gas- and oil-heated homes use both fossil fuel and ordinary household electricity. The calculator includes both so their whole-building site energy can be compared consistently with new homes. New homes must comply with current energy codes. That generally results in lower energy use than in the older-home examples. The Stretch Code and Specialized Code add stronger requirements, while the Municipal Fossil Fuel-Free Demonstration Program limits fossil-fuel use in covered new construction. The calculator uses a Base Code energy profile and a lower-energy profile for Stretch, Specialized, and Fossil Fuel-Free cases. It also estimates that all-electric construction becomes more common as the policy requirements grow stronger. In Advanced Options, users can instead test a known all-electric or mixed-fuel project and lower or higher energy use. Electricity use still has emissions, so the result reflects the regional power system in each model year. The emissions rate declines along the selected future pathway.

Four new homes use more total energy than one new home would. The comparison is therefore not based on new-building efficiency alone. It also accounts for the housing needs associated with the additional three households.

Housing somewhere else

If the existing home remains, the model assumes that households 2 through X need substitute housing elsewhere. For each additional household, it uses the same reference home: 1,000 square feet, 50 kBtu/sf-year of site EUI, and a Massachusetts residential energy mix. Site EUI means all energy used at the building per square foot per year.

The size is rounded from a Boston-area American Housing Survey sample. The energy-use value is based on EIA Massachusetts data and Massachusetts ResStock results for larger multifamily buildings. The energy mix comes from 2022–2024 Massachusetts EIA State Energy Data System consumption. The reference home represents existing regional housing rather than assuming a new code-built unit elsewhere.

This counterfactual—the comparison case if redevelopment does not occur—is central to the calculation. Excluding it would compare housing for one household with housing for several households without accounting for where the others live. The substitute building is a regional scenario, not a prediction of a specific apartment or destination. The same reference home is used for every location choice; location changes only the driving comparison.

Transportation and location

Housing location can affect expected driving. The reference case uses a 12.5-mile daily driving difference for each additional household in a transit-rich, walkable location. The value is informed by Massachusetts transportation and location evidence. It is an evidence-informed estimate rather than a prediction of how a particular household will travel.

Users can select a smaller transportation difference for a walkable town center with limited transit, or select zero to test a case with no transportation benefit.

Upfront construction and demolition

Redevelopment begins with a carbon debt from new-building materials and demolition activity. In the reference case, production of the new building materials is the larger upfront term because it applies to the full area of all four new homes. The demolition factor applies only to the area of the existing home.

The calculator provides lower and higher values for both terms in Advanced Options. These choices show how sensitive the result is to the included construction and demolition assumptions.

Future electricity and vehicles

Future electricity and vehicle emissions cannot be known exactly. The calculator therefore does not present one pathway as a forecast. Users can test Faster, Mid, and Slower regional-grid pathways. They can also test alternative paths for direct vehicle emissions.

When Green Community Choice Aggregation (Green CCA) is selected, the calculator shows a second estimate that gives the new homes credit for a qualifying municipal electricity product covering 100% of their modeled electricity use. This lowers the operating emissions assigned to the new homes in that view and can shorten carbon payback.

We show it separately because Green CCA programs generally use renewable energy certificates to match renewable purchases with electricity use. That is not the same as supplying or matching renewable electricity in every hour the homes use power. Green CCA is directionally positive, but it is not equivalent to 24/7 hourly matched renewables. The main result therefore continues to reflect emissions from the regional grid.

What can change the answer?

The result is most useful when users explore more than one reasonable case. Important choices include:

the number and average size of the new homes;
the size and energy profile of the existing home;
new-building energy use;
upfront construction carbon;
demolition emissions;
the transportation difference associated with location;
future regional-grid emissions; and
future direct vehicle emissions.

The main controls describe the site and proposed redevelopment. Advanced Options provides nearby sensitivity cases for assumptions that are uncertain or project-dependent. The aim is to see which factors drive the result and whether the overall conclusion remains stable when reasonable inputs change.

What the calculator includes

The upfront calculation includes the A1 through A3 product stages for the new building and the C1 demolition stage for the existing home. In plain language, that means raw-material supply, transport to manufacturing, product manufacturing, and the activity involved in taking down the existing building.

The annual calculation includes:

energy used by the existing or new homes;
operating energy for substitute housing associated with homes 2 through X;
transportation differences for those additional households;
changing regional-grid emissions; and
optional future direct vehicle-emissions scenarios.

The Technical Methods provides the equations, sources, and complete boundary definitions.

What the calculator leaves out

The model does not separately add construction-site activity, material transport to the site, major sitework, utility infrastructure, landscaping, parking, recurring embodied carbon, replacements, refrigerants, later renovations, most demolition transport and disposal, salvage or reuse credits, or the final end of life of the new buildings.

These effects can matter. They also vary sharply from one project to another. A small wood-frame project with little surface parking is different from a project with an underground garage, extensive excavation, or complex utility work. Applying one generic value to all projects could create a misleading sense of precision.

For the upfront carbon cost, the calculator uses the parts of construction and demolition that can be estimated most consistently across small residential buildings. Advanced Options lets users test lower and higher estimates for the included new-construction and demolition terms. Those sensitivity choices do not add every excluded lifecycle stage or turn the calculator into a complete whole-life assessment.

Projects with substantial parking, sitework, unusual structures, or detailed material quantities warrant a project-specific analysis.

How to use the calculator

The calculator is intended for comparative planning and sensitivity analysis. It can help municipal officials, board members, residents, advocates, designers, researchers, and journalists understand the carbon consequences of different housing scenarios.

It is not a parcel-specific engineering assessment, a traffic study, a code-compliance calculation, a certified lifecycle assessment, or a prediction of what a particular household will do. It does not decide that demolition or preservation is always preferable.

Its value comes from putting the main carbon tradeoffs in one transparent framework. Users can see the upfront cost, the annual effects, the payback point, and the assumptions that matter most. They can then test whether a conclusion holds under different reasonable choices.