Compared to a standard new home built to code in Chelsea, Michigan. Adjust the parameters below to model your project.
| Scenario | Embodied (tons CO2) | Annual (tons/year) | 30-year total (tons) |
|---|
Every number on this page can be traced to an explicit formula, a published data source, and a defined unit conversion. The walkthrough below shows exactly how project inputs become operational and embodied carbon estimates. Each section corresponds to one engine module in src/.
The ZIP code resolves to a state via a zip_to_state.json crosswalk (~42,000 ZIPs). State drives four regional values:
climate_zones.json. Michigan resolves to CZ 5A.degree_days.json. CZ 5 uses 6,500 HDD / 900 CDD as the representative pair. This is a deliberate simplification: real ZIP-level NOAA normals vary within a zone (Chelsea is ~6,300 HDD, Marquette ~8,200), introducing roughly 1–3% spread on heating-dominated whole-life totals.electricity_rates.json and gas_rates.json, sourced from EIA Form 861.egrid.json, EPA eGRID 2022. Michigan's MROE subregion is ~0.47 kg/kWh.The house is modeled as a simple rectangular volume. From floor area and stories, the engine derives:
A 2,300 sf single-story house produces a 48 × 48 footprint, ~1,728 sf of gross walls, ~311 sf of windows, ~2,530 sf of roof, and ~20,700 cubic feet of volume.
Each envelope element's UA (heat loss coefficient, BTU/hr-°F) is area divided by R-value. The total UA includes a constant infiltration term derived from blower-door ACH50:
The ACH50/20 conversion approximates natural air change rate from blower-door ACH50 via the LBNL N-factor for typical US climates. The 0.018 factor is the volumetric heat capacity of air (BTU/cf-°F). Default R-values come from IECC 2021 for the climate zone unless overridden by the chosen assembly (Passive House paths use the actual derived R-value from the assembly stack).
Heating energy is computed via the classic degree-day method, modified to include ventilation losses and internal heat gain credits:
The 230 BTU/hr per person is ASHRAE metabolic heat for light office activity; 800 BTU/hr is the constant appliance/lighting baseline. The heating credit fraction is the share of the year that heating runs (heating_hours/8760), so summertime metabolic gains aren't subtracted from winter heating loads.
System efficiencies are: gas furnace AFUE 0.80, high-efficiency gas 0.95, standard heat pump HSPF 8.8 (COP ≈ 2.58), cold-climate heat pump HSPF 10.0 (COP ≈ 2.93), electric resistance 1.0, geothermal COP 4.0. HSPF is converted to COP by dividing by 3.412 (BTU/Wh).
Cooling load combines envelope, solar gain through windows, internal gains, and ventilation, then multiplies the sensible total by a latent uplift factor that depends on the climate's moisture zone:
The latent uplift is a lookup by climate zone and moisture zone — the dehumidification penalty for humid climates. Selected values:
| Zone | Humid (A) | Dry (B) | Marine (C) |
|---|---|---|---|
| 1 (Miami) | 0.40 | 0.10 | — |
| 3 (Atlanta) | 0.25 | 0.05 | 0.12 |
| 5 (Chicago) | 0.15 | 0.08 | 0.12 |
| 7 (Duluth) | 0.08 | 0.05 | — |
SEER ratings are converted to COP by dividing by 3.412. Code-minimum AC is SEER 14 (COP ≈ 4.1). The 200 × 4 in solar gain is 200 BTU/sf-hr average summer solar flux × 4 hours/day equivalent of direct exposure.
Hot water energy scales with occupants. Baseline electricity covers lighting, plug loads, and appliances:
Hot water energy factors (EF): gas tank 0.62, gas tankless 0.82, electric tank 0.92, heat pump water heater 3.5 (COP-equivalent). The baseline electricity floor of 5,500 kWh/year for a 2.5-person household is the EIA RECS 2020 median for non-HVAC residential electricity; it scales linearly with occupants and weakly with floor area.
PV is treated as both an operational offset and an embodied carbon cost:
The max(0, ...) clamp is intentional. Per ISO 21930 attributional LCA, exported electricity does not create negative operational carbon for the building. This means oversizing PV beyond actual consumption adds embodied carbon (1.8 t per kW) without further offset — so a 20 kW array on a leaky house has a higher whole-life total than a 10 kW array on the same house. The tool will not let users "buy negative carbon" with surplus PV.
Each envelope assembly (wall, roof, foundation) is built from a stack of materials drawn from the BEAM Tool v4.1 database (Builders for Climate Action). Each material has a per-thickness or per-mass embodied factor and, where applicable, a biogenic carbon storage value:
The bounding box is A1–A3 (cradle-to-gate): raw material extraction, transport to factory, and manufacturing. Excluded for now: A4 (transport to site), A5 (installation), B1–B7 (use and maintenance), C1–C4 (end of life), D (beyond system boundary). These omissions matter most for assemblies with short service life (cladding, roofing) and for biogenic materials whose net climate impact depends on end-of-life assumptions.
Windows use a placeholder of 80 kg CO₂e/m² of glazed area, drawn from industry-average aluminum-clad and vinyl windows. This is a flat assumption that does not yet vary by frame type or glazing performance.
Interior partitions, mechanical/electrical/plumbing equipment, floor finishes, and cabinetry are estimated via per-square-foot category presets, scaled by total floor area:
Sources: CLF MEP-LCA 2024, Athena Sustainable Materials Institute residential prototype outputs, Magwood (2022) Build Beyond Zero, BEAM v4.1. These are typical-range averages, not project-specific. Real residential non-envelope carbon varies by ±30% from these presets depending on actual equipment, finish brands, and interior configuration. For a 2,300 sf house this contributes 37–83 t of the total, making it the largest single source of absolute uncertainty in the whole-life total.
Each fuel stream is converted to CO₂e using a constant emissions factor; each is also priced at the state rate:
Whole-life carbon is the sum of embodied envelope + embodied non-envelope + embodied PV + 30 years of operational. The 30-year horizon is the IECC analysis period, not a building service life claim. The cost projection uses a 4% real discount rate per OMB Circular A-94 convention for long-horizon residential analysis.
Two distinct uncertainty calculations run alongside every result:
Absolute uncertainty is the realistic spread on a single scenario's total — typically ±20–30% of the central value. Comparative uncertainty is much narrower because shared modeling assumptions cancel between scenarios. This is why a result like "317 t vs 67 t = 79% reduction" can be very-high-confidence even though each absolute value has a ±15% range.
Approach follows ISO 14044 (LCA principles), EPA WBLCA Guide, and standard practice for comparative LCA where shared boundary conditions reduce relative uncertainty. The 1.65σ multiplier gives the 90% confidence interval assuming approximately normal error distribution on independent components.
The vertical bar at year 30 shows the 90% confidence interval on the absolute total. This is wider than the single number suggests because non-envelope materials, equipment, and finish choices have real product-level spread that this tool doesn't try to nail down precisely. A 200 t result might be 170–230 t in reality. That's not a flaw in the engine — it's an honest representation of what the data supports.
When comparing scenarios, the same modeling assumptions affect both — so the difference between them is much more reliable than the absolute values. "Very high confidence" means the gap is so large (3+ standard deviations) that the ranking is unambiguous. "Within modeling noise" means the difference falls within the inherent spread of the assumptions — the two scenarios are effectively a tie.