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1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsCompare embodied carbon only after you have made the alternatives equivalent. Define the same building function and performance, use compatible Environmental Product Declarations (EPDs), align units and life-cycle modules, and disclose geography, technology and data period. A lower number for one product is not a universal ranking if the products provide different service or were assessed under different rules.
Start with the service, not the material
Embodied carbon is the greenhouse-gas impact associated with producing, transporting, installing, maintaining, replacing and disposing of construction products and assets. The useful comparison is therefore not “kilograms of carbon per kilogram of material.” It is the impact of delivering a defined function at a defined level of performance.
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Write the decision statement before collecting numbers. Examples include:
- Two structural systems that carry the same load over the same span for the required design life.
- Two insulation products that provide the same thermal resistance at the same location for a stated service period.
- Two facade systems that meet the same fire, weather, acoustic and durability requirements.
The quantity of each option must be sufficient to provide that service. A material with a low factor per kilogram can require more mass, thickness, replacements or supporting structure, eliminating an apparent advantage. Conversely, a higher product-stage factor may be justified if the product lasts longer or avoids replacement.
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What an EPD tells you—and what it does not
An EPD is a verified declaration of environmental impacts for a defined product, configuration, production route and life-cycle scope. ISO 21930:2017 provides core rules for construction-product EPDs, including modules, scenarios, reporting and the conditions under which products can be compared. ISO reports that this edition was reviewed and confirmed in 2023.
An EPD number is not automatically comparable with every other EPD number. Comparison requires compatible product-category rules (PCRs), equivalent service and performance, matching indicators and aligned boundaries. Two declarations can both be valid while answering different questions.
Before using a value, record the following fields from each declaration:
- Product name, manufacturer, plant and production route.
- EPD programme operator, verifier and declaration date or validity period.
- PCR and standard edition, such as the applicable EN 15804 version or another regional rule set.
- Declared unit or functional unit, including dimensions, density, thickness or product configuration.
- Life-cycle modules included and any excluded processes.
- Global-warming-potential (GWP) indicator and whether fossil, biogenic and land-use components are reported separately.
- Geography, electricity mix, raw-material sources and manufacturing technology.
- Data period, share of primary data and use of sector-average or generic datasets.
- Service-life, maintenance, replacement, transport, end-of-life and module D scenarios.
Keep a copy of every declaration and a traceable calculation sheet. If a field is missing, mark the comparison as limited rather than filling the gap with an assumed value.
Use the right unit and functional basis
EPDs commonly report impacts per kilogram, cubic metre, square metre, metre, item or another declared unit. A conversion is meaningful only when the physical and performance information needed for that conversion is known. Converting a board from kilograms to cubic metres requires density; converting insulation from kilograms to square metres also requires thickness and thermal performance.
For a product-stage calculation, RICS expresses the basic relationship as:
A1–A3 impact = material quantity × material embodied-carbon factor.
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Use a single metric throughout the calculation. If one EPD is per cubic metre and the project quantity is in square metres, determine the actual installed thickness and volume before multiplying. Do not multiply a mass factor by a volume quantity without a documented density. Do not compare one product per kilogram with another per square metre and imply equal performance.
When products provide different performance levels, normalize to a functional service. For insulation, for example, calculate the area and thickness needed to achieve the same thermal resistance, then include the resulting quantities. For structure, design each option to the same load, span, stability, fire and durability requirements before comparing impacts. The functional basis should state assumptions clearly enough that another assessor can reproduce the quantity.
Read life-cycle modules consistently
A1–A3: product stage
A1–A3 covers raw-material extraction and processing, inbound transport and manufacturing. It is the usual starting point for a product-stage comparison. It is not whole-life carbon and does not include every impact associated with using the product in a building or infrastructure asset.
A4 and A5: transport and construction
A4 covers transport to the construction site; A5 covers construction or installation activities, including relevant losses, energy and waste treatment. EPD default scenarios may not match your project. Replace them with project-specific distances, vehicle types, load factors, delivery frequency, site energy and installation waste when those details are known.
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B modules: use, maintenance and replacement
Use-stage modules can include operational impacts, maintenance, repair, refurbishment and replacement. A product that must be replaced twice during the reference study period cannot be compared fairly with one that remains in service using A1–A3 alone. State the reference study period and the service-life source for every replacement assumption.
C modules: end of life
C1–C4 represent deconstruction, transport, waste processing and final disposal. The scenarios are assumptions, not guaranteed outcomes. Recycling rates, landfill conditions, incineration energy recovery and transport distances should reflect the project region and the available waste infrastructure.
Module D: benefits beyond the system boundary
Module D reports potential benefits or loads from reuse, recycling or recovered energy beyond the assessed product system. It is not an automatic credit against A1–A3. Keep D separate, show the allocation and recovery assumptions, and never subtract a distant or uncertain benefit from the product-stage result without following the applicable PCR and whole-life method.
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Keep GWP components and biogenic carbon visible
Global warming potential can be reported as a single total or separated into fossil, biogenic and land-use-change components. EN 15804+A1 and EN 15804+A2 do not present these components in the same way; A2 introduced a more explicit breakdown. Identify the applicable edition before comparing totals.
Biogenic carbon requires particular care. Timber and other bio-based products can show a negative A1–A3 GWP-total because carbon absorbed during growth is reported as stored in the product. The eventual release, continued storage, reuse and disposal pathway depend on the end-of-life scenario. A negative product-stage figure is therefore not, by itself, proof of negative upfront carbon or a lower whole-life outcome.
Australian Government infrastructure guidance illustrates the issue with worked EPD examples. One example concrete declaration reports 255 kg CO₂e/m³ GWP-total for A1–A3 under an assumed 100% recycling end-of-life scenario. A separate timber example reports −715 kg CO₂e/m³ GWP-total for A1–A3 under an assumed 100% landfill scenario, reflecting reported biogenic storage. Neither value is a generic factor for all concrete or timber, and neither should be quoted as a standalone material ranking. The examples demonstrate why the end-of-life scenario and biogenic accounting must travel with the number.
A defensible comparison workflow
- Define the decision. Name the element, location, alternatives, required performance, reference study period and decision stage. Record constraints such as fire rating, structural capacity, moisture exposure, acoustic rating, thermal target and maximum thickness.
- Set the functional basis. Specify the amount of service being compared: for example, one square metre of envelope meeting a stated U-value for 60 years, or a structural bay carrying a stated load for the design life. Include ancillary materials that differ between options, such as fixings, finishes, reinforcement or additional framing.
- Gather appropriate EPDs. Prefer product-specific declarations for the product and supply chain expected on the project. If none is available, use a sector-average or generic dataset or a documented proxy, and label it as such. RICS distinguishes manufacturer-specific, sector-average and generic data and expects representativeness to improve as design develops.
- Check PCR and verification context. Confirm that declarations follow compatible category rules, programme procedures and standard editions. Record the verifier, validity dates and any stated comparability conditions. A declaration prepared under one PCR may not be directly comparable with one prepared under another.
- Align units and quantities. Convert each factor and project quantity to the same basis, using documented density, dimensions, thickness and performance data. Keep unit conversions separate from design assumptions so errors can be audited.
- Compare A1–A3 when the question is product-stage carbon. Present the result with its unit, product configuration, geography, data period and GWP breakdown. Do not label this result whole-life carbon.
- Add project stages. Calculate A4 transport and A5 installation with project information where available. Include realistic waste and site-energy assumptions rather than silently retaining a default EPD scenario.
- Model the asset life. Add maintenance, repair, replacement and operational effects where relevant. Apply the same reference study period and service-life assumptions to every option.
- Model end of life and module D. Use regional collection, processing and disposal assumptions. Report C modules and D separately, with allocation and recovery rules stated.
- Document uncertainty. Identify proxies, regional mismatches, old data, missing modules, uncertain quantities and scenario choices. Use ranges or sensitivity cases when a single point estimate would imply unjustified precision.
How geography, technology and data age change the result
Manufacturing location affects electricity generation, fuels, transport, raw-material sourcing and waste treatment. The same nominal product made in two plants can have materially different impacts. Select data representative of the expected plant and supply chain, and state when that is not possible.
Production technology also matters. Cement chemistry, kiln fuel, recycled aggregate, steel furnace route, timber drying, insulation blowing agents and recycled-content accounting can all change GWP. A sector-average factor may be suitable during concept design but should be replaced by a more specific declaration when procurement is settled.
Data age is another comparability axis. The historical 2017 RICS edition cited EN 15804 recommendations of maximum ages of five years for product-specific data and ten years for generic data. Those figures are historical guidance, not a blanket current requirement. Check the applicable current PCR, regional policy and assessment standard instead of copying an old threshold.
RICS’s Whole Life Carbon Assessment for the Built Environment, second edition, took full effect on 1 July 2024 for RICS members’ assessments. It covers buildings and infrastructure, includes carbon-contingency reporting and discusses integration with assessment software. The current RICS context and any local regulation should govern a project rather than an out-of-date template.
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Why two apparently similar EPDs disagree
| Cause | What changes | How to handle it |
|---|---|---|
| Different PCR or standard edition | Impact categories, allocation rules and GWP presentation can differ. | Use compatible rules or explain the limitation; do not compare bare totals. |
| Different declared units | One factor may be per kilogram, another per cubic metre or square metre. | Convert with verified density, dimensions and performance, then normalize to the same function. |
| Different plant or energy mix | Electricity, fuels, sourcing and manufacturing efficiency alter A1–A3. | Choose data for the expected supply chain or disclose the proxy. |
| Different boundaries | One declaration may cover A1–A3, another A1–A5 or cradle-to-grave. | Compare identical modules first; expand both using the same whole-life method. |
| Different scenarios | Transport, replacement, disposal, recycling and module D assumptions vary. | Replace defaults with common project scenarios and report them. |
| Biogenic accounting | Stored carbon can make product-stage GWP negative. | Show fossil, biogenic and land-use components and follow the carbon through end of life. |
Reporting results so they can be audited
A useful comparison report lets a reviewer reproduce every number. Include:
- The decision statement and functional unit.
- Product descriptions, quantities, dimensions and performance assumptions.
- EPD identifiers, PCR, programme operator, standard edition and validity dates.
- GWP indicator definition and fossil, biogenic and land-use components.
- Modules included, excluded and separately reported.
- Geography, plant, energy mix, raw-material source and data period.
- Calculation factors and unit conversions, including density or thickness sources.
- Reference study period, service lives, maintenance and replacement intervals.
- Transport, installation, waste, end-of-life and module D scenarios.
- Proxy, generic-data and uncertainty statements, with sensitivity results where material.
Present at least two views when they answer different decisions: an A1–A3 comparison for product selection and a whole-life result for the asset decision. Keep the views clearly labelled so a reader cannot mistake a cradle-to-gate value for a cradle-to-grave total.
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Ranking materials per kilogram
Problem: Mass-based factors ignore density, thickness, quantity and performance.
Correction: Normalize each option to the same functional service and include all materials needed to deliver it.
Calling A1–A3 whole-life carbon
Problem: Product-stage figures omit transport, installation, use, replacement and end of life.
Correction: Label A1–A3 explicitly and add later modules only when the same scenarios and reference period are used.
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Problem: Different rules can change boundaries, allocation and GWP components.
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Correction: Use compatible declarations or explain why a screening comparison remains provisional.
Treating a generic dataset as product-specific
Problem: Sector averages may not represent the selected plant, formulation or supply chain.
Correction: Label the dataset honestly, apply a justified proxy and update the assessment as procurement information improves.
Silently taking module D credit
Problem: Future reuse or recycling benefits may be uncertain and outside the assessed system boundary.
Correction: Report module D separately with its allocation and recovery assumptions.
Using a negative timber result as an upfront-carbon claim
Problem: Stored biogenic carbon can be released later under the stated end-of-life scenario.
Correction: Show the biogenic component and assess the complete relevant life cycle.
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Choosing the right level of detail
At concept stage, use consistent generic or sector-average data to compare design directions, but flag the result as provisional. During developed design, replace proxies with declarations matching the specified product, plant and performance. At procurement and construction, update quantities, transport and installation assumptions with supplier and contractor information. At handover, preserve the evidence trail so future refurbishment or replacement assessments can use the actual installed products.
For regulated or certified projects, follow the regional methodology required by the client, authority, rating system or professional standard. EPD programmes and available datasets vary by country, and a database developed for one electricity grid or waste system may not represent another. A transparent regional mismatch is more defensible than false precision.
A practical decision rule
Choose the option with the lower impact only after it passes four tests: it provides the same required service; its EPDs follow compatible rules; the calculation uses the same units and life-cycle modules; and its geography, technology, data period and scenarios are representative enough for the decision. If any test fails, report the result as a screening indication, not a definitive ranking, and identify what information would resolve the uncertainty.
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