Embodied Carbon Screening: A Practical A vs B Method
This guide explains one disciplined way to compare two design options on embodied carbon without assuming the numbers: the factors come from you, the math stays transparent, and the result is a screening aid rather than a certification. The same method runs live in the Embodied Carbon Screening & Comparison calculator.
1. Know what "embodied carbon" covers
Embodied carbon is the greenhouse-gas impact that comes with materials and construction, not with building operation. In European lifecycle practice it is usually labelled by module: A1–A3 (raw material supply, transport to the gate, and manufacturing) form the "product stage" that dominates most material comparisons. Transport and on-site work are A4–A5, use phase B, and end-of-life C. A screening comparison should pick one boundary and hold it for both options — mixing an A1–A3 factor with a cradle-to-grave factor reverses conclusions.
2. Collect your numbers honestly
For each material in each option you need three things:
- Quantity in a usable unit (kg, t, m³, m, each).
- Emission factor as kg CO₂e per one unit of that unit.
- Provenance: the lifecycle scope, the source of the figure, and the geography it applies to.
Prefer project-specific Environmental Product Declarations, supplier figures, or a published national/regional dataset for the project country and year. Record the source. If you cannot source a factor, say so — an unsourced factor is a guess, and the screening is only as good as the worst factor in the list.
3. Do the arithmetic
Each material total is simply quantity × factor. Sum the option, convert to tonnes
(÷ 1,000), and, when the two options serve the same floor area, divide by that area for a
per-m² intensity. There is no allocation, no conversion, and no process model built in — the
engine reproduces exactly what you entered.
Worked example (250 t concrete + rebar, user factors)
Option A (baseline): concrete 250 t × 130 kgCO₂e/t = 32,500 kg; rebar 30 t × 1,200 = 36,000 kg → total 68,500 kg (68.5 t).
Option B (low-carbon mix, 28 t rebar): concrete 250 t × 95 = 23,750 kg; rebar 28 t × 1,200 = 33,600 kg → total 57,350 kg (57.35 t).
Difference: B − A = −11,150 kg CO₂e (−16.3%). At 1,000 m²: A = 68.5 kg/m², B = 57.35 kg/m².
Factors above are example values only — replace them with your own sourced numbers.
4. Read the result as a screen, not a verdict
A comparison is valid only when the functional unit matches (same quantities of comparable materials), the scope matches (same modules), and the factors are contemporaneous for the same geography. The calculator highlights the lower option and the percentage gap precisely so the decision stays proportionate: a 2% difference is easily swallowed by factor uncertainty, while a 16% gap is a real signal worth engineering action.
5. Document and decide
Save the inputs with the results so the comparison survives review: quantities, factors, scope, source, and geography per row. Then treat the outcome as one input to the design decision — alongside cost, constructability, and schedule. A screening comparison is the start of a low-carbon conversation, not the end of it. Formal reporting follows an adopted methodology (for example EN 15978 or ISO 14067) with verified data.
Limits. This is a screening model on user-supplied factors: no factor database, no lifecycle modelling, and no verification of the figures you enter are implied anywhere in the tool.