# Frequently Asked Questions

**Applies to:** MA Embodied Carbon Payback Calculator 2.0.0

**Updated:** September 20, 2026

## 1. What question does the calculator answer?

It 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. It estimates when changes in building and
transportation emissions offset the upfront carbon from construction and
demolition.

It is a Massachusetts municipal scenario tool, not a prediction that a
particular project will be built or perform exactly as modeled.

## 2. What does “carbon payback” mean?

Redevelopment starts with a carbon debt from new construction and demolition.
Each modeled year then adds the emissions difference between redevelopment and
the keep-the-home **counterfactual**—the comparison case describing what the
model assumes would happen if redevelopment did not occur. Payback is the first
whole model year when the cumulative balance reaches zero or becomes a net
saving. A displayed result such as Year 6 is the payback year, not a special
six-year reporting period.

If payback does not occur during the 100-year analysis horizon, the calculator
says so rather than extrapolating.

## 3. Are the results per home or per person?

No. The results are totals for the selected redevelopment and its
counterfactual. Construction and operating emissions include all selected new
floor area. The first new home replaces the household associated with the
existing home; substitute-housing and transportation effects apply only to the
additional households represented by homes 2 through X.

The calculator does not estimate occupancy or divide emissions among individual
residents. It also does not model how many physical structures contain the new
homes. A normalized measure could answer a different question, but it would not
replace the total carbon consequence of the selected scenario.

## 4. Why does the first new home count differently from homes 2..X?

The first new home replaces the household associated with the existing home.
Homes 2..X are additional housing capacity. Only those additional homes receive
the substitute-housing and transportation counterfactuals. This prevents the
model from assigning an additional-household benefit to the replacement home.

## 5. Does the calculator assume redevelopment creates new households?

It assumes each additional home accommodates one household that otherwise
would need housing elsewhere. This is a comparison boundary, not a prediction
of household formation, occupancy, vacancy, tenure, or migration.

## 6. Why include substitute housing elsewhere?

Without redevelopment, the additional households represented by units 2..X
still need housing in the comparison. Counting only project-site operations
would omit that part of the counterfactual.

The model does not know exactly where an additional household would otherwise
live or whether it would occupy existing housing, induce new construction,
double up, leave the region, or make another choice. It therefore uses the same
Massachusetts-relevant reference home for every additional household:

- 1,000 square feet, rounded from approximately 1,007 square feet for renter
  units in buildings with two or more apartments in the Boston-Cambridge-Newton
  American Housing Survey sample;
- 50 kBtu/sf-year of **site EUI**, meaning all energy used at the building per
  square foot per year, based on the EIA's 47.3 Massachusetts all-homes value
  and the ResStock 50.8 result for Massachusetts buildings with five or more
  units; and
- an energy mix calculated from 2022–2024 Massachusetts residential energy
  consumption in the EIA State Energy Data System.

This represents housing in the existing regional market rather than assuming a
new code-built unit elsewhere. The reference home does not change with the
location choice; location changes the driving comparison. It is not a
prediction of a specific building or address. See
[Substitute housing](2026-08-03_Technical-Methods-v1.1.md#substitute-housing).

## 7. Why include transportation?

Location, access, walkability, and transit can affect expected vehicle travel.
That relationship is supported by established transportation and location-
efficiency research, and it belongs in a comparison of where additional
households are housed.

The reference value is grounded in Massachusetts Vehicle Census data
normalized with Census household information, with recorded Boston-region
location evidence providing additional context. It applies only to additional
households. The exact effect for an individual household cannot be predicted,
and 12.5 miles per day is not a universal transit-oriented-development result.
See the [MassDOT block-group data](https://www.arcgis.com/sharing/rest/content/items/68c6746085b8474789211df9427055fa/data)
and the [transportation method](2026-08-03_Technical-Methods-v1.1.md#transportation-and-location).

## 8. Is Year 6 the answer for every redevelopment?

No. Year 6 is the result produced by the calculator’s reference-case inputs.
Changing the building sizes, number of homes, existing-home energy use,
new-building energy use, upfront carbon, transportation assumptions, or future
grid and vehicle pathways can move the payback year earlier or later—and in
some cases may eliminate payback within the modeled period.

When sharing a result, identify the inputs used and explore reasonable
alternatives in Advanced Options.

## 9. Can the calculator produce no carbon payback?

Yes. A no-payback outcome can be the correct result, not an error. For example,
replacing a 1,800-square-foot home with several 5,000-square-foot homes creates
much more new floor area. The resulting construction and operating emissions
may exceed the modeled housing and transportation benefits throughout the
100-year period.

This means the selected redevelopment does not produce cumulative carbon
savings under the chosen assumptions. Users can change building sizes, energy
performance, upfront-carbon assumptions, location, and future pathways to see
which factors drive the result. The calculator does not force every scenario
to reach payback.

## 10. What carbon payback is good enough?

No single payback year is automatically “good enough.” The calculator provides
a consistent way to measure carbon payback and cumulative savings for a stated
set of assumptions. A municipality or other decision-maker must decide whether
to use a policy threshold, which planning horizon to consider, which
sensitivities should be tested, and how to weigh the carbon result alongside
housing, cost, design, preservation, and other community goals.

The calculator informs those policy choices; it does not make them.

## 11. Is this a forecast?

No. The calculator answers conditional “what if” questions. It estimates what
the carbon result would be if the selected building, transportation, and
future-energy assumptions occur.

Current-year values use observed data where available. Future grid and vehicle
values are transparent scenarios rather than predictions that one particular
future will occur. Advanced Options lets users test faster or slower future
changes. The current release fixes Model Year 1 at 2025, immediately after the
observed 2024 regional-grid baseline; it does not automatically advance the
model year when the website is deployed later.

## 12. Do the existing-home choices correspond to particular building ages?

No. The word "older" is not tied to a defined construction period. The gas and
oil profiles are simplified, stock-informed energy-use scenarios based on
Massachusetts results from the National Renewable Energy Laboratory’s ResStock
building-stock model, with additional regional context from the U.S. Energy
Information Administration’s Residential Energy Consumption Survey. They are
not measured averages for every Massachusetts home. See the
[Massachusetts ResStock results](https://oedi-data-lake.s3.amazonaws.com/nrel-pds-building-stock/end-use-load-profiles-for-us-building-stock/2022/resstock_amy2018_release_1/metadata_and_annual_results/by_state/state=MA/csv/MA_baseline_metadata_and_annual_results.csv)
and [EIA residential data](https://www.eia.gov/consumption/residential/data/2020/state/xls/ce1.1.st.xlsx).

Each active existing-home choice includes ordinary household electricity as
well as heating fuel, so it is compared consistently with the whole-building
energy use of new and substitute homes. The older gas total is split into
56.6686 kBtu/sf-year of fossil energy and 3.9072 kWh/sf-year of electricity.
The older oil total is split into 58.1331 kBtu/sf-year of fossil energy and
4.3572 kWh/sf-year of electricity. These splits come from the same
Massachusetts ResStock results and preserve the adopted 70 and 73 kBtu/sf-year
totals.

The weatherized-home case is informed by an
[evaluation of Massachusetts energy-efficiency programs](https://ma-eeac.org/wp-content/uploads/MA23R56-IESF-Impact-Report_FINAL_17SEP2024.pdf).
The partial-heat-pump case combines published evidence, including a
[Massachusetts and Connecticut metering study](https://ma-eeac.org/wp-content/uploads/Study-25-6-MA23R51-B-HPMS-Res-Heat-Pump-Metering-Study.pdf),
with explicit assumptions about heating load, heat-pump performance, and
backup-fuel use. Actual homes can differ substantially because of their
envelope, equipment, controls, occupancy, and weather.

## 13. How does the Energy Code choice affect the calculation?

The Energy Code choice affects both the modeled energy use of the new homes
and the expected share built all-electric. Base Code uses a 2021 IECC Climate
Zone 5A planning profile. Stretch Code, Specialized Code, and the Fossil
Fuel-Free Demonstration Program use a matched later-code profile with lower
modeled energy use. See the [DOE residential prototype models](https://www.energycodes.gov/prototype-building-models).

The expected all-electric shares are 20% for Base, 40% for Stretch, 75% for
Specialized, and 100% for a confirmed FFF-covered case. These are transparent
planning estimates informed by historical Massachusetts construction data,
current program activity, and policy requirements. They are not measured
current market shares. Specialized Code strongly encourages all-electric
construction but does not require it in every case. The FFF selection applies
the Specialized efficiency profile with 100% electrification for the modeled
end uses, subject to program coverage, exceptions, and waivers.

If a proposed project's fuel choice or energy performance is known, Advanced
Options can replace the expected mix with an all-electric or mixed-fuel case
and can test lower or higher energy use. The full method and sources are in
[New-building energy use](2026-08-03_Technical-Methods-v1.1.md#new-building-energy-use).

## 14. Does all-electric automatically mean zero emissions?

No. An all-electric building avoids burning fossil fuels on site, but the
electricity it uses can still be associated with emissions from the regional
power system.

The calculator begins with ISO New England’s reported annual average emissions
rate for electricity generated in and imported into New England. That rate
changes over time as the regional mix of power generation and imports changes.
See [ISO New England’s Air Emissions report page](https://www.iso-ne.com/about/key-stats/air-emissions).

Massachusetts policy calls for major reductions in electricity-sector
emissions, but the pace is uncertain. The calculator therefore uses a policy-
informed future pathway rather than claiming to predict one exact outcome. In
Advanced Options, users can test faster and slower grid-decarbonization
pathways and see how those assumptions affect carbon payback. The policy
context is described in the
[Massachusetts 2050 Clean Energy and Climate Plan](https://www.mass.gov/info-details/massachusetts-clean-energy-and-climate-plan-for-2050).

## 15. What is Green CCA, and why is it shown separately?

Community Choice Aggregation allows a Massachusetts municipality to purchase
electricity supply on behalf of participating residents and businesses.
Customers generally remain with their utility for delivery, billing, and grid
service. Participation is voluntary, although eligible customers are commonly
enrolled unless they opt out.

Programs differ. Some offer a standard product with more renewable content
than legally required, while others offer an optional 100% renewable product.
The calculator’s Green CCA option represents a qualifying high-renewable
municipal product; it does not assume that every Massachusetts aggregation has
the same design. Boston's
[Community Choice Electricity explanation](https://www.boston.gov/departments/environment/community-choice-electricity)
is one recorded municipal example.

## 16. Does Green CCA mean the electricity physically has zero emissions?

No. Renewable electricity products generally document their renewable content
by purchasing and retiring qualifying renewable-energy certificates over a
defined accounting period. That can support demand for renewable attributes,
but it does not mean that the electricity serving a home is physically supplied
by renewable generators during every hour. The U.S. Environmental Protection
Agency explains the role of
[renewable-energy certificates](https://www.epa.gov/green-power-markets/renewable-energy-certificates-recs).

When 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. The calculator does not validate a particular
municipal product or claim guaranteed additionality.

## 17. What embodied-carbon stages are included?

For new construction, A1 means raw-material supply, A2 means transporting
materials to manufacturing, and A3 means product manufacturing. The calculator
uses one benchmark covering those three product stages. For the existing home,
C1 means the activity of deconstruction or demolition.

This narrower boundary was chosen because these are the most supportable and
comparable estimates available for the initial carbon cost in this model.
Other stages can matter, but available evidence is less consistently comparable
across the small residential building types covered. Advanced Options lets
users test lower and higher values for the included assumptions; those choices
do not convert the tool into a complete whole-life assessment. See the
[lifecycle-module table](2026-08-03_Technical-Methods-v1.1.md#lifecycle-scope-and-definitions).

## 18. Does the model include parking garages, sitework, interiors, MEP, replacements, and refrigerants?

The new-building benchmark includes residential assemblies, the source study’s
garage materials, and a modeled mechanical, electrical, and plumbing (MEP)
estimate. Interior materials are included only to the extent represented in
that benchmark. The calculator does not separately model surface, structured,
or underground parking, sitework, utilities, later fit-out, recurring interior
work, replacements, maintenance, or refrigerants.

These elements vary too much by project to support one defensible generic
adjustment. A small wood-frame project with little surface parking is
materially different from a project with a podium, underground garage,
extensive excavation, or major utility work. Applying one parking or sitework
value to every scenario could create more false precision than excluding it and
disclosing the limitation. Projects with substantial structured parking,
excavation, or sitework require project-specific analysis.

## 19. Why is demolition a relatively small part of the result?

In the calculator’s reference case, demolition is small relative to new
construction because the model applies the C1 demolition-activity factor only
to the area of the existing home, while new-construction embodied carbon
applies to the entire area of all new homes.

The demolition value intentionally covers the machinery, energy, and activity
involved in taking the building down. It does not include transport,
processing, disposal, or potential salvage and reuse effects. Those terms are
excluded because available studies use inconsistent boundaries and do not
provide a strong Massachusetts small-building default.

This does not mean demolition has no broader environmental consequence. It
means that, within a transparent and consistently sourced carbon boundary,
construction of the replacement building is usually the larger modeled upfront
term. Users can test lower and higher C1 values in Advanced Options, while
projects with unusual demolition, disposal, or reuse conditions require more
detailed analysis.

## 20. What should I choose if I do not know the exact value?

Start with the option that most closely describes the general building and
location context. If you are uncertain, start with the reference case and then
test the nearby alternatives in Advanced Options. The purpose is to see whether
the overall conclusion changes under reasonable assumptions, not to manufacture
an exact property prediction.

For a real proposed project, use project-specific information only when its
scope and units match the calculator’s definitions. If the new building could
use more energy than the selected profile, test the Higher energy use option; higher
operating emissions can delay or eliminate payback within the modeled period.

## 21. Can this evaluate a specific parcel or proposed project?

It can support an initial scenario discussion, but it is not a parcel-specific
engineering, energy-code, traffic, or life-cycle assessment. A project decision
should use project geometry, energy modeling, material quantities, parking and
site scope, local transportation evidence, and applicable procurement details.

## 22. Does the calculator favor demolition or preservation?

No. It applies the same equations to the selected inputs and reports the
conditional cumulative difference. Some cases produce faster redevelopment
payback; others delay or eliminate payback within the horizon. The calculator
is intended to make those conditions visible, not to prescribe a land-use
outcome.

## 23. What if the grid decarbonizes faster or slower?

The future emissions of an all-electric building depend partly on how quickly
the regional grid becomes cleaner. Advanced Options provides Faster, Mid, and
Slower pathways. Faster change lowers future operating emissions sooner;
slower change keeps them higher for longer.

These are transparent scenarios, not confidence bounds or claims that one
pathway will definitely occur.

## 24. What assumptions matter most?

Important drivers include the number and size of new homes, existing-home
energy use and fuel, new-building energy use, upfront construction carbon,
substitute housing, the transportation difference, and future electricity and
vehicle emissions. Their importance changes with the selected scenario.
Advanced Options covers the exposed building, construction, grid, and vehicle
sensitivities. The substitute-housing bundle is fixed in this release. As
documented checks, making the 1,000 sf, 50 kBtu/sf-year substitute housing all-
electric leaves payback in Year 6 and lowers Year 30 net savings from 407.4 to
291.7 tCO₂e. Modeling a 900 sf all-electric substitute unit at 26.3528
kBtu/sf-year moves payback to Year 8 and Year 30 net savings to 228.7
tCO₂e.

## 25. How were sources selected?

The project favors primary and authoritative sources with clearly defined
values, units, geography, building type, and emissions boundaries.
Massachusetts-specific evidence is used where it matches the model question.
When no directly comparable Massachusetts value exists, the method explains
how a national, regional, or modeled source was adapted and what limitations
that creates.

The [Technical Methods](2026-08-03_Technical-Methods-v1.1.md) document cites
the evidence supporting each major assumption and includes a consolidated
reference list.

## 26. What are the calculator’s most important limitations?

The model uses simplified examples rather than parcel observations; transfers
new-building energy and embodied-carbon evidence across building types; uses a
narrow A1–A3 plus C1 upfront boundary; simplifies substitute housing, carrier
shares, occupancy, and transportation; treats future grid and vehicle paths as
scenarios; and keeps Green CCA as attribute accounting rather than a physical-
emissions claim.

It also excludes sitework, most parking effects, construction transport and
activity, recurring embodied carbon, replacements, refrigerants, upstream fuel
cycles, full demolition and disposal, market feedback, vacancy, and induced
behavior.

These limitations mean the calculator should be used for comparative planning
and sensitivity analysis, not as a certified prediction for a particular
project. It remains useful because it brings the principal carbon
tradeoffs—upfront construction, demolition, building operations, substitute
housing, transportation, and changing electricity and vehicle emissions—into
one transparent framework. Users can see which factors drive the result and
test whether the broad conclusion remains stable under different reasonable
assumptions.

Questions or feedback? [Send a note](/feedback/).