What Is Life-Cycle Assessment?

A life-cycle assessment is a structured way of answering one deceptively simple question: over its whole life, what does this thing do to the environment? The power - and the discipline - is in the words "whole life". A naive comparison looks only at the object in front of you: this tile feels natural, that one feels synthetic. An LCA refuses that shortcut. It follows the product backwards to the cradle - the ore in the ground, the tree in the forest, the crude oil in the well - and forwards to the grave - the landfill, the incinerator, the recycling plant - and it counts what happens at every step in between: the energy burned, the raw materials consumed, the emissions released to air, water and land. Nothing that matters is allowed to hide off-stage.

This "cradle-to-grave" thinking is what stops the classic mistakes. A material can look clean at the point of use while being filthy to make; a product can be cheap to run while being ruinous to dispose of; a "green" swap can quietly shift the damage from one stage to another rather than removing it. Economists call that shifting the burden, and LCA is designed expressly to catch it, because it holds the whole life in view at once. If a low-carbon manufacturing process simply moves the emissions to transport, or a recyclable product is never actually recycled, a proper LCA sees it.

A second discipline is that LCA is, in principle, multi-criteria. Carbon - specifically global warming potential, measured in kilograms of CO2-equivalent - is the impact this course cares about most, and often the headline. But a full LCA can also track acidification, eutrophication, ozone depletion, water use, resource depletion and more, because a choice that cuts carbon might worsen water use or toxicity, and honest design should at least know when it is making such a trade. For most building-carbon work you will focus on the global-warming number, but never forget it is one column in a wider table - and that focusing on carbon alone is itself a scoping decision, not a law of nature. The point of the method is to make impact visible, complete and comparable rather than asserted - and everything else in this module builds on that.

An LCA is not one calculation but a defined sequence of four phases, and the order matters. The first phase is goal and scope definition, and it is where most of the honesty of an assessment is decided. Here you write down, before touching a single number, what question you are answering and for whom, what system you are studying, where its boundaries lie (2.2), what the functional unit is - the precise thing you are measuring impact per (one square metre of finished wall for sixty years, say), and what will be included and excluded. Get this phase wrong and every later number, however precise, answers the wrong question. Two "carbon footprints" of the same product can differ wildly simply because their goal and scope differed - and a reader who skips the scope is easily misled. This is why professionals read the scope before the result.

The second phase is the life-cycle inventory, or LCI, and it is the heavy lifting. Here you build a model of the whole system as a web of processes - mine the ore, make the cement, truck it to site, run the building, demolish it - and for each process you compile every input (energy, water, raw materials) and every output (emissions to air, water and land, plus waste). The result is a vast list of flows: so many kilograms of CO2, so many of methane, so many megajoules of energy, and so on, all scaled to the functional unit. Most of this data does not come from measuring your own project directly; it comes from databases and EPDs (2.3) that hold the inventory for standard materials and processes, which you assemble into your specific system. The inventory is where "garbage in, garbage out" bites hardest: an LCI built on weak or wrong data produces a confident-looking number that is quietly meaningless. A good assessor spends far more effort here - sourcing, checking and documenting data - than on the arithmetic, because the arithmetic is trivial and the data is everything. And because the inventory is scaled to the functional unit set in phase one, the two phases are joined at the hip: a shift in scope quietly re-weights the entire inventory, which is one more reason the goal-and-scope decisions come first and are written down before any number is trusted.

The inventory gives you a mountain of raw flows - kilograms of dozens of different substances - which by itself tells you little. The third phase, life-cycle impact assessment (LCIA), turns that mountain into meaning by grouping the flows into impact categories and weighting each by how much it actually harms. This is where global warming potential earns its keep: every greenhouse gas in the inventory is multiplied by its GWP (Module 1.1) and summed into a single figure in kilograms of CO2-equivalent, so that methane, refrigerants and CO2 become one comparable carbon number. Other categories - acidification, eutrophication, resource depletion and the rest - are built the same way, each with its own characterisation factors. LCIA is what lets you say "this option is 30% lower carbon" instead of drowning in a spreadsheet of raw substances; it is the translation from chemistry to consequence.

The fourth phase, interpretation, is where judgement returns. Here you ask what the results actually mean and whether you can trust them: which life-cycle stages and materials dominate (the hotspots you will design against); how sensitive the answer is to your assumptions; how good the data was; and whether the conclusion is robust enough to act on. Crucially, interpretation is not a box at the end - in a proper LCA it loops back, sending you to refine the scope, chase better data or test a different assumption. A single point number with no sense of its uncertainty is a red flag; a mature assessment says "about this much, dominated by the structure, and here is how confident we are."

Two cautions travel with these phases. First, LCA is powerful but not objective truth - it is a model, and every model embeds choices (boundaries, data vintage, allocation rules, the categories included) that a skilled practitioner can defend and an unskilled or motivated one can bend. Second, precisely because the method is demanding, binding results belong to the standards and a qualified specialist, not to a designer running a quick tool. Your job is to understand the four phases well enough to read an assessment critically, brief one intelligently, and know when a number deserves trust - which is exactly what carbon literacy means.

What stops LCA from being everyone's private opinion is that the method itself is standardised. The international standards ISO 14040 and ISO 14044 set out the principles, framework and requirements for conducting an LCA: they define the four phases, insist on a clear goal and scope and functional unit, require transparency about boundaries, data and assumptions, and - for any study that will be published as a comparison - demand independent critical review. They do not tell you the answer for your building; they tell you how to reach an answer that others can trust and check. That is the difference between a number you can defend and a number you merely assert.

For the built environment, this general method is made specific by a family of standards built on top of ISO 14040/44 - most importantly EN 15804, which sets the rules for construction-product EPDs so that different products' declarations are actually comparable, and EN 15978, which sets the method for assessing a whole building (the backbone of whole-life carbon assessment, 2.4). You will meet these properly in 2.2, 2.3 and Module 9; for now the point is that a building carbon assessment is not improvised - it sits inside a nested rulebook, from the global ISO framework down to construction-specific methods, precisely so that results mean the same thing across products, projects and countries.

It is equally important to be clear about what LCA is not. It is not a certification or a label - it is a method that certifications and labels rely on. It is not a single fixed number for a material - the same material has different figures depending on scope, data and boundaries, which is why context always matters. And it is not a substitute for design judgement - it measures the consequences of decisions you still have to make. Think of LCA as the instrument that makes the invisible visible: a disciplined way to see where a building's impact really is, so you can act on evidence instead of intuition or marketing. Master how to read it, and you can tell a real carbon claim from a hollow one - the foundation this whole module is built to give you.