# Technical Methods **Applies to:** MA Embodied Carbon Payback Calculator 2.0.0 **Prepared by:** [David Mendels](/about/) **Updated:** September 20, 2026 ## 1. Purpose and policy context Massachusetts communities are confronting two closely connected challenges: reducing greenhouse-gas emissions and expanding the supply of housing, particularly in places with access to jobs, services, and transit. Growing attention to embodied carbon has raised legitimate questions about the climate cost of demolishing existing buildings and constructing new ones. At the same time, redevelopment decisions intersect with housing availability and affordability, density and walkability, transportation emissions, and the energy standards that govern new construction.[1][2] This model was developed to clarify those tradeoffs. It compares the one-time carbon cost of demolition and new construction with changes in building operations, transportation, and the housing that additional households would otherwise occupy. Its purpose is not to determine that every demolition is justified or unjustified, but to help users understand the conditions under which keeping or redeveloping an existing property may produce lower greenhouse-gas emissions over time. The calculator is a general Massachusetts municipal scenario model. Its intended audience includes municipal staff and boards, policy analysts, researchers, designers, advocates, journalists, and interested residents. ## 2. Appropriate use The calculator is suitable for: - comparing a defined retention scenario with a defined redevelopment scenario; - understanding which modeled terms drive carbon payback; - testing whether a conclusion changes under reasonable alternative inputs; and - supporting transparent municipal policy discussion. It is not a parcel-specific engineering assessment, a regulatory or code-compliance calculation, a forecast, a traffic study, a certified life-cycle assessment, or a substitute for project energy models and material quantities. Results should always be reported with the selected inputs and important exclusions. ## 3. Compared scenarios and household boundary The calculator compares: 1. keeping one existing single-family home; and 2. demolishing it and constructing either one replacement home or a small multifamily redevelopment containing two to eight homes. The first new home replaces the household associated with the existing home. Homes 2 through X provide capacity for additional households: ```text additional households = max(number of new homes - 1, 0) ``` Substitute-housing and transportation terms apply only to those additional households. A **counterfactual** is the comparison case: what the model assumes would happen if redevelopment did not occur. A one-home redevelopment therefore has no additional-household counterfactual. The comparison contains four conceptually different parts: - **Project-site upfront emissions:** new-building materials and manufacturing, plus demolition activity for the existing home. - **Project-site operations:** energy used by the retained home or all new homes. - **Emissions avoided elsewhere:** modeled operation of housing that units 2..X allow additional households not to occupy elsewhere. - **Transportation difference:** expected direct-use vehicle emissions associated with where those additional households are housed. The ordinary result uses annual regional-grid electricity emissions. Green Community Choice Aggregation (Green CCA) is a separate renewable-attribute accounting result and never replaces the regional-grid calculation. All reported results are whole-scenario totals for the selected redevelopment and counterfactual. They are not normalized per home, physical structure, or person. The model specifies homes, households, and floor area; it does not estimate the number of buildings or the number of occupants in each home. ## 4. Lifecycle scope and definitions The model uses the lifecycle-module terminology summarized below. The definitions follow accepted whole-life-carbon guidance.[3] The calculator applies one A1–A3 benchmark; it does not calculate A1, A2, and A3 separately. | Module | Plain-language meaning | Treatment in this model | | --- | --- | --- | | A1 | Raw-material supply | Included within new-building benchmark | | A2 | Transport of materials to manufacturing | Included within new-building benchmark | | A3 | Product manufacturing | Included within new-building benchmark | | A4 | Transport to the construction site | Excluded | | A5 | Construction and installation activity | Excluded | | C1 | Deconstruction or demolition activity | Included for the existing building | | C2 | Transport of demolition materials | Excluded | | C3 | Waste processing | Excluded | | C4 | Final disposal | Excluded | Other terms used in this document are: - **EUI:** energy use intensity. **Site EUI** is all energy used at the building per square foot per year, regardless of whether it arrives as electricity, gas, oil, or propane. - **MEP:** mechanical, electrical, and plumbing systems. - **VMT:** vehicle miles traveled. - **CCA:** Community Choice Aggregation, a municipal electricity-supply procurement program. - **REC:** renewable-energy certificate, a contractual instrument representing renewable-electricity attributes. - **tCO₂e:** metric tonnes of carbon-dioxide equivalent. Included terms are new-building A1–A3 emissions, existing-building C1 demolition, direct stationary combustion, regional-grid electricity, direct-use vehicle emissions, substitute-housing operations for units 2..X, and the optional Green CCA attribute-accounting treatment. The model does not separately add A4–A5, sitework, utility infrastructure, landscaping, a parking module, C2–C4, salvage or reuse credits, recurring embodied carbon, maintenance, replacements, later fit-out, refrigerants, water, upstream fuel cycles, vehicle or battery manufacturing, or final building end of life. The A1–A3 source includes garage materials while using conditioned floor area as its denominator; the calculator neither subtracts those materials nor adds a generic parking value.[4] These boundaries focus the comparison on the upfront terms for which the project found the most comparable evidence across small residential buildings: A1–A3 product-stage emissions for new construction and C1 demolition activity for the existing home. Excluded effects can matter, particularly for projects with substantial sitework, structured parking, complex construction logistics, or unusual disposal and reuse conditions. Their exclusion does not mean those effects are zero or unimportant; it avoids applying one generic adjustment to highly project-specific conditions and creating false precision. Advanced Options lets users test lower and higher values for the included A1–A3 and C1 terms. Those sensitivities show how the result responds, but they do not add every excluded stage or turn the calculator into a whole-life assessment. ## 5. Calculation method ### 5.1 Units, notation, and sign The calculation uses: ```text 1 square foot = 0.092903 square metres 1 kilowatt-hour = 3.412 kBtu 1 MMBtu = 1,000 kBtu 1 metric tonne = 1,000 kilograms ``` Let: - `N` = number of new homes; - `H_add` = additional households; - `A_exist` = existing-home area, square feet; - `A_new` = average new-home area, square feet per home; - `A_sub` = substitute-housing area, square feet per additional household; - `EUI_AE,e` = all-electric new-building electricity EUI, kBtu/sf-year; - `EUI_MF,e` and `EUI_MF,g` = mixed-fuel new-building electricity and gas EUI, kBtu/sf-year; - `p_e` = expected share of new homes using the all-electric profile; - `m` = selected relative new-building energy-use multiplier; - `EUI_sub` = annual substitute-housing site EUI, kBtu/sf-year; - `g_y` = regional-grid factor in model year `y`, kg CO₂/kWh; - `v_y` = direct-use vehicle factor in model year `y`, kg CO₂e/mile; - `I_new` = new-construction A1–A3 intensity, kg CO₂e/m²; - `I_demo` = demolition C1 intensity, kg CO₂e/m²; - `U_net` = net time-zero upfront emissions, tCO₂e; - `B_y` = redevelopment-minus-counterfactual emissions in year `y`, tCO₂e/year; and - `C_y` = cumulative balance through year `y`, tCO₂e. Positive cumulative balance means redevelopment has higher cumulative emissions than keeping the existing home under the modeled counterfactual. Negative balance means cumulative net savings. The interface presents the absolute magnitude with the applicable plain-language direction: **cumulative net savings** or **cumulative net emissions increase**. An exact zero is described as no net emissions difference. All calculations use full precision; displayed values are rounded afterward. ### 5.2 Upfront emissions ```text new area (m²) = N * A_new * 0.092903 existing area (m²) = A_exist * 0.092903 new-construction emissions = new area * I_new / 1,000 demolition emissions = existing area * I_demo / 1,000 U_net = new-construction emissions + demolition emissions - modeled retained-home retrofit emissions ``` The four current existing-home choices describe pre-existing conditions, so no new retained-home retrofit intervention is added. Time-zero emissions occur before the first operating year. ### 5.3 Existing-home annual emissions For a profile with fossil EUI, electric use per square foot, and fossil factor: ```text fossil MMBtu = A_exist * fossil EUI / 1,000 electricity kWh = A_exist * electric kWh/sf-year existing emissions in year y = (fossil MMBtu * fossil factor + electricity kWh * g_y) / 1,000 ``` Every active existing-home profile now includes both ordinary household electricity and the named heating fuel. The gas and oil carrier splits come from weighted Massachusetts ResStock results and are scaled to preserve the adopted total site EUI. The weatherized profile retains ordinary household electricity and assigns the modeled energy reduction to gas. The partial-heat- pump profile adds heat-pump electricity to ordinary household electricity and keeps the remaining gas load separate. Existing, new, and substitute homes are therefore compared on a consistent whole-building site-energy basis.[6] ### 5.4 New-building annual emissions The Energy Code selection supplies an all-electric profile, a mixed-fuel profile, and an expected all-electric share. A project-specific Advanced Options selection can replace that share with 100% all-electric or 100% mixed-fuel. The energy-use sensitivity multiplier is applied to both complete profiles, preserving the selected code context. First calculate emissions per square foot for each profile: ```text all-electric profile kg CO2e/sf in year y = m * EUI_AE,e / 3.412 * g_y mixed-fuel profile kg CO2e/sf in year y = m * EUI_MF,e / 3.412 * g_y + m * EUI_MF,g / 1,000 * natural-gas factor new-building emissions in year y = N * A_new * [p_e * all-electric profile + (1 - p_e) * mixed-fuel profile] / 1,000 ``` The first division by 1,000 converts gas kBtu to MMBtu; the last converts kilograms to metric tonnes. Electricity and gas remain separate because they use different emissions factors and change differently over time. This term covers all new homes, including the first replacement home and units 2..X. ### 5.5 Substitute-housing annual emissions The substitute-housing bundle applies only to `H_add`: ```text substitute-housing kBtu = H_add * A_sub * EUI_sub ``` For each fossil carrier `c`, with energy share `s_c` and factor `f_c`: ```text fossil emissions c = substitute-housing kBtu * s_c * f_c / 1,000 / 1,000 ``` For retail electricity share `s_e`: ```text electric emissions in year y = substitute-housing kBtu * s_e / 3.412 * g_y / 1,000 ``` The two divisions in the fossil equation convert kBtu to MMBtu and kilograms to tonnes. Substitute-housing electricity always uses the regional-grid factor, including when Green CCA is shown. ### 5.6 Transportation emissions difference ```text transportation savings in year y = H_add * daily VMT difference * 365 * v_y / 1,000 ``` This is an avoided-emissions term and applies only to units 2..X. Green CCA does not affect transportation. ### 5.7 Annual balance, cumulative balance, and payback ```text counterfactual building emissions in year y = existing-home emissions + substitute-housing emissions B_y = new-building emissions - counterfactual building emissions - transportation savings C_0 = U_net C_y = C_(y-1) + B_y ``` Negative `B_y` means redevelopment emits less during that year. Carbon payback is the first whole model year for which `C_y <= 0`. The model reports Year 0 if `U_net <= 0`; otherwise, if no crossing occurs during the analysis horizon, it reports no payback within the modeled horizon. It does not interpolate a fractional payback year or extrapolate beyond the horizon. Payback timing and a reported-year result answer different questions. A scenario can still show a cumulative emissions increase at Year 30 and reach payback later. Conversely, if `C_y` remains positive in every modeled year, **no payback within 100 years** is the correct result rather than an error. ### 5.8 Green CCA result When Green CCA is selected, the model recalculates covered new-building electricity with the selected attribute-accounting factor. With coverage fraction `c`: ```text effective attribute factor in year y = c * attribute factor y + (1 - c) * regional-grid factor y ``` The regional-grid result is calculated and retained independently. Existing home electricity, substitute-housing electricity, and transportation keep their ordinary treatment. The deployed Green CCA case uses `c = 1.0`, meaning that the attribute factor is applied to 100% of modeled new-building electricity in the secondary result. ## 6. Time treatment and future pathways Model Year 1 is the first full operating year and maps to calendar 2025: ```text calendar year = 2025 + model year - 1 ``` This is a fixed baseline for the current model release. Year 1 follows the observed 2024 ISO New England emissions year used to establish the starting grid factor. It is not a claim that each calculator run begins in its web- deployment year. Moving the baseline requires a coordinated update to the model inputs, pathways, and documentation. Regional-grid and optional future-vehicle factors use piecewise-linear interpolation between recorded anchors. For adjacent anchors: ```text factor(year) = factor_0 + (factor_1 - factor_0) * (year - year_0) / (year_1 - year_0) ``` The 2050 factor is held constant after 2050. The analysis horizon is 100 years, with displayed cumulative results at Years 10, 30, 60, and 100. These are reporting choices, not claims that actual buildings, occupants, vehicles, or policies remain unchanged for a century. The starting regional-grid factor is 0.254 kg CO₂/kWh, converted from ISO New England's 2024 generation-plus-imports annual rate of 560 lb/MWh.[5] Faster, Mid, and Slower future pathways retain the implemented anchors below. They are policy-informed scenarios, not forecasts, probabilities, or confidence bounds. | Calendar year | Faster | Mid, reference | Slower | | ---: | ---: | ---: | ---: | | 2025 | 0.254 | 0.254 | 0.254 | | 2030 | 0.100 | 0.150 | 0.190 | | 2040 | 0.030 | 0.065 | 0.115 | | 2050 and later | 0.005 | 0.020 | 0.060 | The direction of the pathways is informed by Massachusetts climate policy,[1][2] but ISO New England did not produce these future values. The reference vehicle factor remains 0.35 kg CO₂e per mile. Optional Slower, Moderate, and Faster decline pathways use the anchors below. They are transparent sensitivity cases, not fleet forecasts, and exclude charging electricity, upstream fuel, and vehicle and battery lifecycle emissions. | Calendar year | No modeled decline, reference | Slower | Moderate | Faster | | ---: | ---: | ---: | ---: | ---: | | 2025 | 0.350 | 0.350 | 0.350 | 0.350 | | 2030 | 0.350 | 0.320 | 0.270 | 0.220 | | 2040 | 0.350 | 0.200 | 0.120 | 0.060 | | 2050 and later | 0.350 | 0.080 | 0.040 | 0.015 | ## 7. Assumptions and evidence The descriptions below separate the external evidence, the project adaptation, and the resulting model value. A value can be suitable for comparative planning without being a measured Massachusetts average. ### 7.1 Geometry and household counting The reference geometry is a 1,800 sf existing home and four 900 sf new homes. Users can enter one replacement home or two to eight homes in a small multifamily redevelopment and change both areas. Geometry is a planning input, not a statement about a typical Massachusetts project. The first new home replaces the existing household; only the remaining three homes in the reference case generate additional-household terms. ### 7.2 Existing-home energy The older gas case uses 70 kBtu/sf-year of total site EUI: 56.6686 kBtu/sf-year of fossil energy plus 3.9072 kWh/sf-year of electricity. The older oil case uses 73 kBtu/sf-year total: 58.1331 kBtu/sf-year of fossil energy plus 4.3572 kWh/sf-year of electricity. NREL ResStock modeled Massachusetts stock provides fuel-specific totals near 69.0 and 72.5 and the electricity share for each profile, while EIA Residential Energy Consumption Survey (RECS) data provide Northeast context.[6][7] The ResStock components are scaled proportionally to preserve the adopted 70 and 73 totals. These are simplified, selectable older-home examples, not measured averages for all older Massachusetts homes. Here, "older" does not identify a specific construction period. ResStock describes fuel-specific building stock, and the calculator adapts those results into planning profiles. Homes from the same period can perform very differently after renovations or equipment changes. Envelope condition, occupancy, and weather also affect energy use. The weatherized gas case uses 60 kBtu/sf-year total: 46.6686 kBtu/sf-year of gas plus 3.9072 kWh/sf-year of ordinary household electricity. It preserves the older gas-home electricity component and assigns the accepted 10 kBtu/sf-year reduction to fossil energy. The reduction is informed by Massachusetts program-evaluation evidence, but it is not a measured post- retrofit EUI or a guaranteed Mass Save outcome.[8] The partial-heat-pump case uses 29.7186 kBtu/sf-year of remaining gas plus 6.4348 kWh/sf-year of electricity. The electric term includes both the 3.9072 kWh/sf-year ordinary household load and approximately 2.53 kWh/sf-year for the heat pump. Project calculations apply a 55% heating share to the adopted 70 kBtu/sf-year total, then apply 70% heating-load displacement, a seasonal coefficient of performance of 2.5, and 80% gas-furnace efficiency. The choices are informed by RECS end-use data, Massachusetts and Connecticut heat-pump metering, and Massachusetts retrofit evaluation evidence.[8][9][10] Actual performance varies with sizing, controls, backup fuel, envelope, occupants, and weather. All four profiles describe existing conditions and include whole-building site energy. The model adds no new retrofit intervention or retrofit embodied carbon to the retention scenario. This is a boundary choice, not a claim that retrofit materials have zero emissions. ### 7.3 New-building energy use The Energy Code control affects both modeled efficiency and expected fuel choice. Base Code uses reviewer-calculated mean annual energy from matched U.S. Department of Energy and Pacific Northwest National Laboratory 2021 IECC Climate Zone 5A prototypes. Stretch Code, Specialized Code, and covered Fossil Fuel-Free Demonstration Program cases use the corresponding 2024 prototype means as a transferred stronger-code profile.[11][12][13] | Policy context | All-electric profile | Mixed-fuel profile | Expected all-electric share | | --- | ---: | ---: | ---: | | Base Code | 29.0631 electric | 16.0516 electric + 28.9670 gas | 20% | | Stretch Code | 26.3528 electric | 15.4387 electric + 25.9532 gas | 40% | | Specialized Code | 26.3528 electric | 15.4387 electric + 25.9532 gas | 75% | | Stretch/Specialized plus FFF | 26.3528 electric | not used | 100% | Energy values are kBtu/sf-year. The 2024 package lowers whole-building site EUI by about 9.3% in the all-electric prototypes and 8.1% in the mixed-fuel prototypes. This matched-model change is used as a conservative bridge to the stronger Massachusetts code context. It does not claim that the 2024 IECC is identical to Massachusetts Stretch or Specialized Code or convert HERS scores directly into EUI. The expected all-electric shares are project-selected planning estimates, informed by historical Massachusetts Base and Stretch observations, current program momentum, and current policy structure.[11][12][24][25] They are not measured current statewide or municipal shares. The central shares are 20% for Base, 40% for Stretch, 75% for Specialized, and 100% for a confirmed FFF-covered case. Specialized makes the mixed-fuel path less attractive but does not prohibit it. FFF uses the Specialized efficiency profile and treats covered modeled end uses as all-electric; projects subject to an exception or waiver require a different selection. Advanced Options can evaluate either an all-electric or mixed-fuel project in place of the expected mix. It can also apply a relative energy-use sensitivity of 24/29, 1, or 32/29 to the selected profiles. The lower and higher cases are transparent sensitivities, not measured bounds or guaranteed outcomes. No generic solar-production credit is assigned to Specialized Code. ### 7.4 New-building embodied carbon New-construction A1–A3 values are 180, 230, and 260 kg CO₂e/m², with 230 as the reference value. The Massachusetts 100-Home Study reports a 226.8 kg CO₂e/m² mean and a 179.2–257.9 first-to-third-quartile range for its sample; the calculator rounds those values.[4] These values are printed as labels in the box-and- whisker chart in Figure 5 on page 28. The chart labels were visually transcribed because they may not appear in extracted PDF text. The source covers 1–4-unit homes, is predominantly detached, includes garage materials, and uses a modeled MEP proxy. Applying it to 1–8-unit redevelopment is a transparent transfer, not a complete project assessment. The lower and higher cases do not add all excluded lifecycle modules. ### 7.5 Demolition C1 choices are 3, 5, and 10 kg CO₂e/m² of demolished existing-home area, with 5 as the reference value. The lower case is informed by an Oregon small-house mechanical-demolition scenario near 2.77 kg CO₂e/m².[14] RICS guidance provides additional boundary and magnitude context.[3] Because published studies differ in structure, equipment, worker travel, foundation treatment, and denominator, 5 is a planning value rather than a measured Massachusetts mean. C2 transport, C3 processing, C4 disposal, salvage, and reuse remain excluded. ### 7.6 Substitute housing For each additional household, the model uses the same 1,000 sf reference home with 50 kBtu/sf-year of whole-building site EUI. The 1,000-square-foot value is 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.[15] The 50 EUI value is 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.[6][7] The fuel profile is an author calculation from the U.S. Energy Information Administration State Energy Data System (SEDS), Massachusetts residential consumption estimates for 2022–2024.[16] The covered carriers are utility gas, oil/kerosene, propane/liquefied petroleum gas, and retail electricity. Their three-year totals are normalized to 100%: 45.27%, 26.56%, 2.99%, and 25.18%. They represent 804.367 of 853.419 trillion Btu. The excluded 5.75% consists mainly of wood (3.82%) and solar (1.91%). These are energy shares, not the prevalence of household heating fuels, and the electricity share is not split by end use. 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. The reference home uses actual Massachusetts and Boston-area housing-stock evidence to provide one consistent comparison for units 2..X. It does not assume that a brand-new code-built unit is constructed elsewhere. The same reference home is used for all three location choices; the location control changes only the expected driving difference. These are regional inputs, not a prediction of a specific address or building. This substitute-housing bundle is fixed in the current public controls. It is one of the most consequential assumptions in the model, so two alternative cases were calculated as sensitivity checks. Keeping the 1,000 sf and 50 kBtu/sf-year values but making the substitute housing all-electric leaves payback in Year 6 and reduces Year 30 cumulative net savings from 407.4 to 291.7 tCO₂e. Using a 900 sf all-electric unit at 26.3528 kBtu/sf-year moves payback to Year 8 and reduces Year 30 net savings to 228.7 tCO₂e. These checks illustrate the term's leverage; they are not selectable assumptions in this release. ### 7.7 Transportation and location The daily VMT differences are 12.5 miles for the transit-rich reference case, 5 miles for the walkable-town sensitivity, and 0 for the no-benefit case. They apply only to units 2..X. The reference value is informed by a reproducible project analysis of MassDOT Massachusetts Vehicle Census data normalized with American Community Survey household counts; quality-controlled inner and outer groups show a material geographic VMT difference.[17][18] Recorded Boston-region housing and location evidence provides additional directional context. The project adopts 12.5 as a cautious planning estimate, not as a causal relocation effect or a universal transit-oriented-development value. Five is an illustrative lower- benefit case, and zero assumes no transportation benefit. U.S. EPA passenger-vehicle evidence gives about 0.393 kg CO₂e per mile for an average gasoline-powered passenger vehicle.[19] The model uses 0.35 kg CO₂e per mile, about 11% lower, as a conservative direct-use planning factor. The lower value reduces the modeled transportation savings. It is not a Massachusetts fleet inventory or a lifecycle vehicle factor. ### 7.8 Direct fossil-combustion factors Natural gas, No. 2 fuel oil, and propane use 53.1145, 74.203, and 63.113 kg CO₂e/MMBtu, respectively, based on the U.S. EPA 2025 Greenhouse Gas Emission Factors Hub.[20] They represent direct stationary combustion and exclude upstream fuel-cycle emissions. Substitute-housing oil/kerosene uses the No. 2 fuel-oil factor as a simplification. ### 7.9 Green CCA attribute accounting Green CCA is off by default. 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. The result is shown 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 method is informed by the GHG Protocol distinction between location-based and market-based accounting, U.S. EPA guidance on RECs, and recorded Massachusetts municipal aggregation examples.[21][22][23] The selected generic pathway uses 0.020 kg CO₂e/kWh in 2025, 0.015 in 2030, 0.010 in 2040, and 0.005 from 2050 onward. These are project-selected attribute-accounting factors, not physical-grid factors or verified causal emissions reductions. Applicability depends on program design, product, participation, procurement, REC quality, and retirement. The scenario does not claim zero physical or marginal emissions, hourly matching, additionality, or validation of a municipal product. The current secondary calculation applies the attribute factor to 100% of modeled new-building electricity (`c = 1.0`). ## 8. Reference scenario and example results The reference scenario uses: - one 1,800 sf older gas home at 70 kBtu/sf-year; - four 900 sf all-electric new homes at 26.3528 kBtu/sf-year; - the Stretch/Specialized plus Fossil Fuel-Free policy label; - 230 kg CO₂e/m² of A1–A3 new-building emissions; - 5 kg CO₂e/m² of C1 demolition emissions; - three additional households, each with the 1,000 sf, 50 kBtu/sf-year substitute-housing bundle; - 12.5 avoided daily VMT per additional household and the fixed 0.35 kg CO₂e/mile vehicle factor; - the Mid future regional-grid pathway; and - no Green CCA result shown. The reference table uses one decimal place so that its values can be reproduced from the published method. The calculator uses adaptive display rounding: nearer-term values retain the ordinary result-tile precision, while Years 60 and 100 are labeled as long-term directional values and rounded to the nearest 10 tCO₂e. | Result | Displayed value | | --- | ---: | | Carbon payback | Year 6 | | Year 10 cumulative net savings | 76.0 tCO₂e | | Year 30 cumulative net savings | 407.4 tCO₂e | | Year 60 cumulative net savings | 913.2 tCO₂e | | Year 100 cumulative net savings | 1,587.6 tCO₂e | Before payback, a positive remaining balance means that some of the time-zero carbon debt has not yet been offset. After payback, the interface reports the same signed balance as positive cumulative net savings. These are example results for the stated inputs, not universal scientific conclusions. ## 9. Principal limitations and appropriate interpretation 1. Real homes vary in age, condition, systems, occupancy, and energy use. The existing-home choices are simplified examples, not parcel diagnoses. Their whole-building carrier splits are derived from modeled Massachusetts stock and preserve the adopted total site EUI; they are not metered end-use inventories for a particular building. 2. New-building profiles are transferred prototype results rather than measured Massachusetts outcomes. The expected electrification shares are policy-model assumptions, not observed current code-tier market shares. 3. The embodied-carbon benchmark is transferred from mostly detached 1–4-unit homes and includes source-specific garage and modeled-MEP treatment. 4. Demolition covers C1 only and uses values drawn from heterogeneous evidence. 5. Substitute housing is a regional counterfactual. The model does not locate households, predict housing prices, or represent a particular building. 6. SEDS carrier shares describe energy, not household heating-fuel prevalence, and exclude 5.75% of minor-fuel energy before normalization. 7. Transportation cases are evidence-informed location scenarios, not household travel predictions. Vehicle factors omit upstream and lifecycle effects. 8. Grid and vehicle pathways are conditional scenarios. Annual averages do not represent hourly or marginal emissions, and the final anchors are held constant after 2050. 9. Green CCA accounting does not prove physical displacement, marginal emissions reductions, additionality, enrollment, or product eligibility. 10. A4–A5, recurring embodied emissions, replacements, refrigerants, infrastructure, most parking effects, C2–C4, reuse, and final end of life are outside the calculation. 11. Occupancy, vacancy, rebound, induced travel, and market feedback are not dynamically modeled. 12. Municipal applicability still requires judgment about which inputs best describe the case being discussed. 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. ## 10. References 1. Massachusetts Executive Office of Energy and Environmental Affairs. *Massachusetts Clean Energy and Climate Plan for 2050*. 2022. [Policy page](https://www.mass.gov/info-details/massachusetts-clean-energy-and-climate-plan-for-2050). Accessed 2026-07-27. 2. Massachusetts Executive Office of Energy and Environmental Affairs. *Massachusetts Clean Energy and Climate Plan for 2025 and 2030*. 2022. [Policy page](https://www.mass.gov/info-details/massachusetts-clean-energy-and-climate-plan-for-2025-and-2030). Accessed 2026-07-27. 3. Royal Institution of Chartered Surveyors. *Whole Life Carbon Assessment for the Built Environment*, second edition, version 3. 2023/2024, especially sections 5.1.4 and 5.6.2. [Publisher PDF](https://www.rics.org/content/dam/ricsglobal/documents/standards/Whole_life_carbon_assessment_PS_Sept23.pdf). 4. NEHERS Alliance, Stephens & Company, Ekotrope, Builders for Climate Action, and NMR Group. *Massachusetts 100-Home Embodied Carbon Study: Final Report*. 2025-11-26, Figure 5 on p. 28. The box-and-whisker labels were visually transcribed because the chart annotations may not appear in extracted PDF text. [Publisher PDF](https://www.nehers.org/Data/Sites/1/media/embodied-carbon/the-massachusetts-100-home-embodied-carbon-study.pdf). 5. ISO New England. *2024 Electric Generator Air Emissions Report*, Annual Emissions and Emission Rates table. 2025. [Air Emissions page](https://www.iso-ne.com/about/key-stats/air-emissions). Accessed 2026-06-14. 6. National Renewable Energy Laboratory. *ResStock End-Use Load Profiles for the U.S. Building Stock, 2022 release*, Massachusetts baseline metadata and annual results. [Massachusetts CSV](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). 7. U.S. Energy Information Administration. *2020 Residential Energy Consumption Survey*, Table CE1.1.ST and related Northeast tables. 2020. [State energy-use table](https://www.eia.gov/consumption/residential/data/2020/state/xls/ce1.1.st.xlsx). 8. Massachusetts Energy Efficiency Advisory Council and Cadeo. *Massachusetts Income Eligible Single Family Impact Evaluation*. 2024. 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