Doing an Embodied Carbon Assessment

Strip away the software and the jargon and an embodied carbon assessment is one repeated multiplication. For every material or product in the building you need two numbers. The first is the quantity - how much of it there is, in the unit its carbon factor uses: tonnes or cubic metres of concrete, tonnes of reinforcement steel, square metres of glazing or plasterboard, kilograms of aluminium. The second is the carbon factor (sometimes called an emission factor or carbon coefficient) - the mass of carbon dioxide equivalent released to produce one unit of that material, typically expressed in kgCO2e per kg or per m3. Multiply the two and you get the embodied carbon of that material; add every line together and you get the building's total, usually then divided by floor area to give a kgCO2e/m2 figure you can compare against a benchmark.

This is why the equation is often written as embodied carbon = the sum of (quantity x carbon factor) across all materials. The elegance is that it scales from a back-of-envelope estimate of three materials to a line-by-line model of thousands, and the logic never changes. It also tells you immediately where your effort should go: because the total is a sum of products, the lines that dominate are those where a large quantity meets a high factor. In most buildings that means the structure - concrete, cement and steel are both voluminous and carbon-intensive - which is why they deserve your best quantities and best factors while a small run of ironmongery can be estimated crudely without hurting the total.

Two cautions travel with the sum. First, quantities and factors must share a boundary: a factor that covers only cradle-to-gate (raw material extraction to the factory gate, the stages labelled A1-A3) must not be silently compared with one that includes transport and installation. Second, a carbon factor is never a universal constant - it varies by product, region, manufacturing route and data vintage, so the number you multiply by is itself an estimate carrying uncertainty. Get the quantities and the factors from sound sources, keep their boundaries consistent, and the sum does the rest.

The instinct is to assess at the end, when the drawings are complete and the quantities are certain - and that instinct is exactly wrong. An assessment done only at the end is a post-mortem: it can report the carbon but it is far too late to change it, because the structural system, the material palette and the decision to build at all are long settled. The value of an assessment comes from doing it early, when the design is still soft and the biggest levers - reuse versus new-build, concrete versus timber, how much floor area, how deep the frame - are still in your hands. A rough number at concept stage that redirects the structure is worth more than a precise number at completion that changes nothing.

So the discipline is to assess at every stage, accepting that early assessments are coarse. At concept, you may have only floor areas and a structural intent; you estimate quantities from rules of thumb (so many kilograms of concrete and steel per square metre for this frame type) and apply generic factors. At scheme design, elements are firmer and you refine quantities from the developing model. At detailed design, you can take quantities from a proper bill and factors from specific products or EPDs. Each stage sharpens the number - but each stage can still change the design, which is the point. Carbon should sit in the design conversation from the first week, not arrive as a report at the end.

This early-and-often habit also changes what the number is for. An end-stage figure answers "how much carbon did we spend?" An early figure answers the far more useful "how much carbon will this decision cost, and is there a lighter option?" It turns the assessment from an accounting exercise into a design instrument. In India, where product-specific carbon data is still thin, this matters even more: you will rarely have precise factors early, so the skill is to make confident directional decisions from rough numbers, and reserve precision for where it will actually change an outcome. The aim is not a perfect number but a better building, and a better building is won at the stages where the design is still soft.

Because you assess early and often, an embodied carbon assessment is not a single event but a model that lives alongside the design, coarse at first and sharpening over time. The trick to starting is to resist completeness. You do not need every material to begin; you need the few that dominate. Model the structure and the envelope first - frame, floors, foundations, external walls, roof - with rough quantities and generic factors, and you will typically capture the large majority of the up-front carbon in an afternoon. That first pass, however crude, tells you where the carbon is and gives you a baseline to improve against. Chasing the ironmongery and the paint before you have the frame is a classic beginner's error: precision on the small lines while the big ones are guesses.

As the design develops, you refine along two axes. Quantities get better as the model and drawings firm up - estimated concrete volumes become scheduled ones, assumed slab depths become designed ones. Factors get better as generic database values are replaced by product-specific or EPD-based ones for the materials that matter. You do not refine everything equally; you spend refinement effort where it moves the total, which is why a sensitivity check - which lines, if wrong, would most change the answer? - is worth doing early. The figure narrows from a wide range at concept to a tighter one at detailed design, and reporting that narrowing honestly is part of the craft.

Crucially, refinement is not just about a more accurate number - it is about acting on each iteration. The loop is: estimate, find the hotspots, change the design to cut them, re-estimate, repeat. A model that only ever gets more precise without ever driving a reduction has missed its purpose. The best assessors treat each pass as a prompt: the slab is the biggest line, can it be thinner or a different system; the facade is heavy, can it be simpler. The number is a means; a lower-carbon building is the end. This is also why the living model belongs with the designer and not only with a distant consultant: the person who can change the slab is the person who should see, at the moment of deciding, what the slab costs in carbon. A model that updates as the design moves keeps that feedback immediate, and turns carbon from an after-the-fact audit into a live constraint the design is shaped around.

A total is only meaningful once you can say what it includes, so every assessment must state its boundary before it states its number. Which life-cycle stages are counted - only the up-front product and construction carbon (A1-A5), or also use-stage replacement and end-of-life? Which elements - just structure, or facade, finishes, services and external works too? What building area is the denominator? Two figures for the same building can differ by a factor of two purely because they draw the line differently, so a kgCO2e/m2 quoted without its boundary is close to meaningless. Naming the boundary is not bureaucracy; it is what makes the number honest and comparable.

With the boundary fixed, the most useful thing the result gives you is not the total but the breakdown. Sort the lines by carbon and the picture is almost always the same shape: a handful of elements carry most of the impact, structure usually first, then the envelope, with finishes, services and the long tail behind. This is the hotspot map, and it is where design attention pays off - a ten per cent cut on the biggest line beats eliminating several small ones. A good assessment output is therefore a ranked list, not a single figure, because the ranking is what tells you where to design.

Finally, read the number with appropriate humility. Even a careful assessment carries real uncertainty from the factors, the quantities and the assumptions, so treat the result as an informed estimate with a range, useful for comparing options and tracking progress, not as a precise fact. And keep the firm boundary of this course in view: the method here lets you steer the design and understand the drivers, but the binding, reported figure a project declares - the audited whole-life number to EN 15978, with verified EPD data and defined boundaries - belongs to the recognised standards and a qualified LCA or carbon specialist. Your job is to design the carbon down and to understand the assessment well enough to interrogate it; their job is to certify the number. Read this way - boundary first, breakdown before total, result as a range with hotspots named - an assessment stops being a verdict handed down at the end and becomes a map you navigate by from the start.