Open any Environmental Product Declaration (EPD), and you'll find a long table of impact results. Most people skip straight to one row: Global Warming Potential, or GWP.

That's no accident. GWP is the carbon footprint of the product. It's the number architects plug into carbon models, the number buyers compare in tenders, and the number that green building assessors check.

But GWP in an EPD can confuse people. Newer EPDs split it into four parts. Some values even turn negative. So this guide explains what GWP means, how to read each version and how to compare products without falling into common traps.

What Is Global Warming Potential?

Global Warming Potential measures how much a product adds to climate change. It rolls all greenhouse gases into one figure, shown in kilograms of carbon dioxide equivalent (kg CO₂e).

Different gases trap different amounts of heat. So scientists convert each gas into the amount of CO₂ that would cause the same warming. For example:

  • Carbon dioxide (CO₂) counts as 1, the baseline.
  • Methane (CH₄) traps far more heat. Over 100 years, it counts as roughly 28 times CO₂, depending on the method used.
  • Nitrous oxide (N₂O) counts as nearly 300 times CO₂.

So one kilogram of methane adds far more to the GWP figure than one kilogram of CO₂. Most EPDs use this 100-year time frame, often written as GWP100.

Where You Find GWP in an EPD

GWP sits in the main results table, usually the first row under "core environmental impact indicators". Each column shows a life cycle module:

  • A1–A3: product stage, from raw materials to the factory gate
  • A4–A5: transport to site and installation
  • B1–B7: use stage, such as repair and replacement
  • C1–C4: end of life, from demolition to disposal
  • D: possible benefits from reuse or recycling

The figure always relates to the EPD's declared unit, such as 1 tonne, 1 m³ or 1 m² of product. So a GWP of 250 means 250 kg CO₂e for that one unit, not for the whole project.

The Four Types of GWP in Newer EPDs

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Older EPDs, built to EN 15804+A1, show one simple GWP row. Newer EPDs, built to EN 15804+A2, split it into four rows. Here's what each one means.

GWP-fossil

This covers greenhouse gases from fossil sources, such as burning gas, oil or coal for energy and heat. For most products, like steel, cement, glass and aluminium, this forms the biggest share.

GWP-biogenic

This covers carbon from living things, such as trees and plants. Wood stores carbon as it grows. So timber products often show a negative biogenic value at the product stage, because the carbon gets locked inside the material. That carbon is later released, usually shown at end of life in module C.

GWP-luluc

This stands for "land use and land use change". It covers emissions from changing land, such as clearing forest for farming or mining. For most building products, this value stays small.

GWP-total

This is the sum of the three: fossil plus biogenic plus luluc. Most green building tools and carbon models now use GWP-total as the main figure.

GWP indicatorWhat it coversTypical sizeGWP-fossilFossil fuels and industrial processesLargest share for most productsGWP-biogenicCarbon stored or released by plants and woodOften negative for timber at A1–A3GWP-lulucLand use changeUsually smallGWP-totalAll three added togetherMain figure for most comparisons

Why Negative GWP Values Need Care

A negative number can look like great news. And for wood products, it does reflect real carbon storage. But it can also mislead if you read it alone.

Here's why. A timber product may show negative GWP at A1–A3, because the tree took in carbon as it grew. However, that carbon usually returns to the air at end of life, when the wood gets burned or breaks down. So the stored carbon shows up again as a positive value in module C.

That's why experts suggest reading GWP across the whole life cycle, not just the product stage. If you compare a timber product with a steel product using A1–A3 alone, the timber may look better than it really is over its full life.

How to Compare GWP Between Two EPDs

GWP comparisons only work when the basics line up. So check these five points first:

  1. Same declared unit. Compare per tonne with per tonne, or per m² with per m².
  2. Same modules. Compare A1–A3 with A1–A3, or full life cycle with full life cycle.
  3. Same GWP indicator. Use GWP-total for both, rather than mixing GWP-total and GWP-fossil.
  4. Same EN 15804 version. A1 and A2 EPDs use different methods, so results can differ slightly.
  5. Same Product Category Rules (PCR). Different PCRs can set different calculation rules.

A simple example: Say Insulation A shows 3.2 kg CO₂e per m² and Insulation B shows 4.1 kg CO₂e per m². Both cover A1–A3 at the same thermal performance. So Insulation A has the lower product-stage carbon. But if B's EPD reported per kg instead, you couldn't compare them directly without converting first.

GWP in an EPD vs a Product Carbon Footprint

Both measure carbon, so people often mix them up.

A product carbon footprint (PCF) reports GWP only. It may follow ISO 14067, and it doesn't always carry third-party verification.

An EPD reports GWP plus many other impacts, such as ozone depletion, acidification, energy and water use. It follows ISO 14025 and EN 15804, and an independent verifier always checks it.

So a carbon footprint study works well as a first step or for internal goals. Many manufacturers start with carbon footprint consulting and then move to a full EPD when buyers and green building projects ask for it.

How Manufacturers Can Lower Their GWP

Once you know your GWP, you can work to reduce it. The biggest wins usually come from the product stage.

  • Switch to cleaner energy, such as solar power for the factory.
  • Use more recycled content, especially for metals.
  • Cut cement content with replacements like GGBS or fly ash.
  • Improve process efficiency to use less heat and fuel.
  • Source locally to cut transport emissions.

Then update your EPD to show the improvement. A lower, verified GWP gives your sales team a clear advantage in specs and tenders.

Manufacturers who want their GWP calculated and verified properly often work with specialists. Teams such as EnviroLink prepare Environmental Product Declarations for GCC manufacturers, with GWP reported to the latest EN 15804 rules.

Final Thoughts

GWP in an EPD tells you the carbon story of a product in one clear figure. Once you know the four types, the declared unit and the life cycle modules, the number becomes easy to read.

So look for GWP-total, check the unit and modules, and read negative values with care. Then you can compare products fairly and make choices that truly cut carbon on your projects.

Frequently Asked Questions

What does GWP mean in an EPD?

GWP stands for Global Warming Potential. In an EPD, it shows the product's carbon footprint in kilograms of CO₂ equivalent per declared unit. It combines all greenhouse gases into one figure. Architects, buyers and green building assessors use it as the main number when they compare products on carbon.

What is the difference between GWP-total and GWP-fossil?

GWP-fossil covers emissions from fossil fuels and industrial processes. GWP-total adds biogenic carbon and land use change to that figure. For most products, the two numbers sit close together. For wood and bio-based products, they can differ a lot because of stored carbon. Most comparisons use GWP-total.

Why is GWP negative in some EPDs?

Some EPDs, mostly for timber and bio-based products, show negative biogenic GWP at the product stage. That happens because plants absorb carbon as they grow. However, that carbon usually returns to the air at end of life. So you should read GWP across all modules, not just A1 to A3.

What unit is GWP measured in?

GWP uses kilograms of carbon dioxide equivalent, written as kg CO₂e. The figure always relates to the EPD's declared or functional unit, such as one tonne, one cubic metre or one square metre of product. So always check the unit before you compare two EPDs.

Can I compare GWP values from two different EPDs?

Yes, but only when they line up. Both EPDs should use the same declared unit, the same life cycle modules, the same GWP indicator and ideally the same EN 15804 version and PCR. If any of these differ, the comparison may mislead you, so flag the gap before you decide.

Is GWP the same as a product carbon footprint?

They measure the same thing, but they come in different documents. A product carbon footprint reports GWP only and may lack third-party verification. An EPD reports GWP alongside many other impacts, follows ISO 14025 and always needs independent verification. Most green building credits ask for the EPD.