Environmental Product Declarations for Wood, Concrete, and Steel: What the Numbers Mean

Structural materials get more environmental scrutiny than almost anything else in a building. Cement production releases large volumes of carbon dioxide, steel carries high embodied energy, and wood raises questions about forestry and land use. Manufacturers answer with different strategies: recycled content, alternative fuels, certified fiber, and carbon offsets. Because every material has a complicated life cycle, an environmental product declaration (EPD) that narrows the picture to a few parameters will not give the full environmental profile. Designers who specify steel reinforcement in structural concrete already check strength, ductility, and cover requirements; the environmental record of that steel deserves the same attention.

An EPD is a third-party verified document that reports the environmental impacts of a product, calculated with a life cycle assessment (LCA) and organized under a defined set of rules. The verification step matters: an independent reviewer checks that the underlying data, the methodology, and the reporting follow the rules before the declaration is published. This article explains what those documents contain, why the numbers for different materials resist direct comparison, and how a designer can use them without being misled.

What an Environmental Product Declaration Contains

An EPD reports impact categories measured over the stages of a product’s life. The most quoted figure is global warming potential (GWP), expressed in kilograms of carbon dioxide equivalent, but a complete declaration also covers ozone depletion, acidification, eutrophication, smog formation, and primary energy demand. A typical modern home brings concrete, wood, and steel together in one structure, and each material’s declaration uses the same vocabulary with different assumptions underneath.

Product Category Rules

Every EPD sits on a foundation of product category rules (PCRs), which define the product group, the functional or declared unit, the system boundary, and the allocation methods. The PCR is what makes two EPDs from the same product group speak the same language. When the PCRs differ, the numbers do not line up.

Declared Units and Functional Units

A declared unit is a simple quantity of product, such as one tonne of steel rebar or one cubic meter of concrete. A functional unit describes the function delivered, such as one square meter of wall assembly with a stated thermal performance for 50 years. Functional units allow comparison across systems; declared units do not.

The impact categories in a typical EPD look like this:

• Global warming potential (GWP), in kg CO2 equivalent
• Ozone depletion potential, in kg CFC-11 equivalent
• Acidification potential, in kg SO2 equivalent
• Eutrophication potential, in kg PO4 equivalent
• Smog formation potential, in kg O3 equivalent
• Primary energy demand, renewable and non-renewable, in MJ

An EPD is only as good as the data behind it. Manufacturers can commission an LCA of their own production, or they can rely on industry-average data compiled by their trade association, and the declaration states which one was used. A manufacturer-specific EPD for a single plant reflects that plant’s energy mix, raw material sources, and process efficiency; an industry-average EPD represents a fleet of facilities with different profiles. Both are legitimate, and both answer different questions.

Why EPDs Resist Direct Comparison

The phrase “apples to pineapples” is common in the EPD world for good reason. A concrete EPD may report per cubic meter while a steel EPD reports per tonne, and the two products occupy very different roles in a structure. A designer comparing a concrete column with a steel column must calculate steel quantity for the actual member, then compare whole assemblies on a common basis.

Five Reasons EPD Comparisons Go Wrong

1. Different declared units: per tonne, per cubic meter, per square meter, or per piece.
2. Different system boundaries: cradle to gate, cradle to grave, or with module D included.
3. Different data vintages and validity periods; a five-year-old EPD may not reflect current production.
4. Industry-average versus manufacturer-specific data; regional grids and fuel mixes shift the results.
5. Different treatment of biogenic carbon, recycled content, and allocation between co-products.

Manufacturer-specific EPDs deserve the same skepticism as marketing claims: the manufacturer chooses the product, the plant, and the time window. Industry-average EPDs smooth out those choices but hide the spread between the best and worst performers in a sector. The practical move is to ask which type a declaration is before comparing it with anything else.

These differences do not make EPDs useless. They mean a responsible comparison starts by checking that the declarations were built on the same rules and that the boundaries match the question being asked.

Life Cycle Stages and System Boundaries

Life cycle assessments divide a product’s life into modules labeled A through D. Most structural EPDs are cradle to gate, covering modules A1 through A3: raw material supply, transport to the factory, and manufacturing. Some add A4 and A5 for transport to site and installation, and a few carry the analysis through use (B), end of life (C), and the benefits of recycling (D). Structural systems such as steel-concrete composite beams show why boundaries matter: fabrication dominates steel’s footprint, while concrete’s impacts spread across the cement kiln, the ready-mix plant, and the curing process.

Reading the Module Labels

What Cradle to Gate Really Covers

A cradle-to-gate EPD stops at the factory gate. It reports what it costs to make the product, but nothing about transport, erection, maintenance, or demolition. Structural materials are almost never replaced during the building’s life, so the omitted stages are often minor, but they are not zero, and the difference between two products can change once transport distances and end-of-life fate are included.

When two EPDs carry the same module labels, the declared unit and the PCR still decide whether the numbers line up. When the labels differ, the comparison needs adjustment before it means anything.

How to Use EPDs in Material Selection

The practical workflow treats EPDs as design input, not as a scorecard. Just as the placement of steel reinforcement in concrete footings depends on the loads and detailing of a specific project, the right material depends on function, climate, and local supply chain.

A Step-by-Step Selection Workflow

1. Define the function: the span, load, fire rating, and service life the element must deliver.
2. Fix a functional unit for the comparison, such as one bay of structure for 50 years.
3. Check that the EPDs share the same PCR, declared unit, and modules.
4. Compare whole assemblies, including connections, finishes, and protection systems.
5. Bring in regional data for transport distances and grid emissions.
6. Record the assumptions so the comparison can be revisited at procurement.

Rating systems reward the practice. LEED v4 and v4.1 grant credit for building product disclosure and optimization, with points for the number of products that carry EPDs and for products that beat industry averages. Whole-building life cycle assessment goes further, comparing complete structural designs rather than individual products.

Suppliers can usually produce an EPD on request, and many publish them on their websites. The useful questions to ask at bid time are short: which PCR does this follow, what declared unit does it use, and when was the underlying data collected? The answers sort the declarations into comparable groups faster than any spreadsheet.

Limits Every Specifier Should Know

A single parameter such as GWP tells a partial story. Two products can report the same carbon footprint and differ widely in toxicity, water use, or recycled content. The long-running structural debate between reinforced concrete structures and steel structures will not be settled by EPDs alone, because each declaration answers a slightly different question and each material keeps improving its numbers over time.

Where the Data Falls Short

• Regional grids change the picture; the same steel mill looks different on a cleaner electricity mix.
• Concrete carbonation absorbs CO2 slowly over the structure’s life, and few EPDs capture it.
• Wood’s biogenic carbon accounting varies between methodologies.
• Recycled content and end-of-life allocation rules differ between PCRs.
• Manufacturer EPDs cover only the products their makers choose to declare.

EPDs also age. A validity period of five years is common, and the underlying production data can change faster than that, especially for energy-intensive materials where the fuel mix shifts year to year. Checking the issue date is part of reading the document.

Putting EPDs to Work on Real Projects

The habit of asking for an EPD can start at the material library, before any design decision is made. The same declaration system that covers structural members now extends to finishes such as decorative concrete floor and wall tiles, to insulation, glazing, and roofing, so the request can be made on every product category from the first day of a project.

Making the Request Routine

On real projects the process comes down to five habits:

1. Write the EPD requirement into the specification for every major product category.
2. Request declarations at bid time and reject incomplete responses where feasible.
3. Store the EPDs in a shared library with issue dates visible.
4. Compare candidates on a common functional unit before the design is fixed.
5. Update the comparison when the procured product differs from the design assumption.

The goal is not to crown a single champion material. Concrete, steel, and wood all have a place, and each has a path to lower impacts. The discipline is making sure the numbers you read answer the question you asked, so the choice rests on evidence instead of habit.