How to read a material’s carbon.

Every EPD is registered with an organisation that publishes EPDs, called a programme operator, such as EPD International, EPD Hub or BRE, and given a number. With that number, anyone can find the original document and see who checked it.

For construction products the standard is EN 15804. Its current version, EN 15804+A2 (2019), splits carbon into three parts: fossil, biogenicbiogenic carbonCarbon that plants took out of the air as they grew and that is held in the material. EN 15804+A2 shows it as a negative number when the product is made. and land use. That split is what lets a timber EPD show the carbon the wood holds.

Every figure is for a set amount of product, called the declared unitdeclared unitThe amount of product every figure in a declaration refers to, such as one tonne, one square metre or one cubic metre.: a kilogram, a square metre or a cubic metre, for example. Check it before reading any number, because it changes what the number means.

Most EPDs are valid for five years. An expired one is still useful, but the product or the factory may have changed since it was written.

Further down is the results table: one column for each stage of the product’s life, and one row for each kind of impact. For carbon, read the row called GWP-totalGWPGlobal warming potential: the climate-change figure in an EPD, in kgCO₂e. Under EN 15804+A2, “GWP-total” is fossil, biogenic and land-use carbon added together..

Every bar on this site is drawn from that row, and nothing else.

An EPD cannot know where your site is. So for delivery and installation it assumes a typical journey, a type of lorry and an amount of site waste. These assumptions are called scenariosscenarioAn assumption an EPD makes for a stage it cannot know, such as the distance to site. Your project’s real figure will be different., and every EPD lists them so you can see what was assumed. The end of life and module D are also based on scenarios.

On this site, delivery and installation are drawn as grey bars with an asterisk.

If your site is further away, delivery produces more carbon than the EPD says; if it is nearer, less. For a project assessment, use your own distances and waste figures.

Under EN 15804+A2, most products must report stages A1–A3, C1–C4 and D. The use stage (B) is often left out, because maintenance and replacement depend on the building rather than the product.

If a stage is missing, the EPD simply does not report it. Treat it as a gap in the information, never as zero. That is why this site shows it as an empty, dashed space rather than a bar.

It is tempting to put two charts side by side and pick the shorter bar. Here are the two products you have just followed.

The brick’s figures are for one tonne. The timber’s are for one cubic metre. These are not equal amounts of anything, so the bars cannot be compared. That is why each chart on this site has its own scale.

Compare things that do the same job, such as a square metre of wall that performs the same. For that you need the quantities (how much of each product the wall uses), the same stages, and your own transport distances.

EN 15804 itself says EPDs can only be compared in the context of a whole building. Doing that is the job of a whole-life carbon assessment. This site gives you the evidence for one; it does not rank products.

A concrete strip footing spreads the weight of both leaves of the wall into firm ground, below the depth that frost reaches.

Masonry rises off the footing. Below ground the cavity is filled with lean-mix concrete so the soil cannot push the two leaves together. But the fill bridges the cavity, so water has to be stopped above it.

Hardcore, a damp-proof membrane, insulation and the slab. The finished floor sits level with the damp-proof course.

A damp-proof course in each leaf, at least 150 mm above the ground, stops damp rising. Over the filled cavity, a tray slopes down and out to catch water that gets into the cavity.

The inner leaf of blockwork carries the floors and roof. Stainless steel ties cross the cavity to hold the two leaves together, with a drip in the middle so water cannot run across.

Insulation goes against the inner leaf, leaving a clear cavity behind the brick. The facing brick keeps the weather out, but it is never completely watertight.

Any water that gets through runs down the back of the brick, onto the tray, and out through open joints left just above it, called weep holes. Without them, the tray would hold the water instead of draining it. Plasterboard finishes the inside.

This one small detail contains thirteen products, numbered on the drawing. Each can have its own EPD, and each EPD uses its own unit.

To work out the carbon of one, measure its quantity from the drawing, then convert that into the unit its EPD uses. The design decides how much of each product is used, which is why carbon only means something when it is counted for the whole wall, and the whole building.

Take the facing brick. A square metre of wall one brick thick contains about 59 bricks. This brick’s EPD is per tonne, so the bricks must first be converted into tonnes, which needs the weight of one brick.

Only then can the EPD figure be applied. It gives the carbon of making the bricks alone; every other product in the wall is counted in the same way.

It is tempting to compare one product with another: a brick with a timber stud, a figure per tonne with a figure per cubic metre. But nobody builds with a single material. They build walls.

The outside of a cavity wall is one product: facing brick, laid in mortar. The outside of a timber-frame wall is four: cladding, battens with an air gap behind, a breather membrane and sheathing board.

Behind the brick are a clear cavity, insulation, and steel ties holding the two leaves together. In the timber wall, the studs carry the load and insulation fills the space between them.

Inside the cavity wall: blockwork, adhesive dabs and plasterboard. Inside the timber wall: a vapour control layer, a gap for pipes and cables, plasterboard, and the nails, screws and plates that hold the frame together.

Each can have its own EPD, in its own unit: per tonne, per square metre, per cubic metre or per kilogram. Before anything can be added up, every quantity has to be converted into its EPD’s unit.

So which is lower carbon? Comparing a brick with a stud cannot tell you. Only the whole wall can: every layer counted in its own unit, over the same stages, for walls that perform the same.

That is the work of a whole-life carbon assessment, and it is why this site never ranks one material against another.