Environmental Product Declarations (EPDs) are essential for evaluating embodied carbon in glass, but interpreting them correctly requires understanding lifecycle stages, declared units, Global Warming Potential (GWP) and product specific disclosures. This guide explains how architects, façade consultants and sustainability teams can use a Glass EPD to make informed low embodied carbon glass specification decisions.
For background on architectural glass embodied carbon, see Glass and Embodied Carbon.
Sections:
- What Is an Environmental Product Declaration (EPD)?
- Why Are Glass EPDs Essential for Low Embodied Carbon Specification?
- How Do You Read a Glass EPD?
- Declared Unit
- System Boundary
- Global Warming Potential (GWP)
- Lifecycle Modules
- How Do You Compare Embodied Carbon Across Glass Products?
- Common EPD Interpretation Errors
- What Tools Help Compare Glass EPDs?
- What Questions Should You Ask a Glass Manufacturer?
- How Do You Use EPDs to Specify Lower Embodied Carbon Glass?
- Variables to Balance
- Key Takeaways
- Related Terms
- Frequently Asked Questions About Glass EPDs
What Is an Environmental Product Declaration (EPD)?
An EPD is a third‑party verified report that discloses a product’s environmental impacts using life cycle assessment (LCA) data. The LCA is the underlying study. The EPD is the standardized, public facing summary of that study. EPDs are prepared under Product Category Rules (PCRs) specific to each material type and follow international reporting standards including ISO 14025 and EN 15804.
A Type III EPD is an EPD that has been independently third‑party verified. Type III refers to the most rigorous category of environmental declarations under ISO 14025 and is the format most accepted for federal procurement, LEED sustainability credits and Buy Clean compliance.
Product Category Rules (PCRs) are standardized methodologies that tell manufacturers how to calculate and report environmental impacts for a specific product category, so EPDs can be compared on equal terms.
What an EPD Is
EPDs are a transparency tool, not a performance claim. The format and methodology are standardized, so data can be compared when assumptions match. An EPD is:
- A third‑party verified disclosure of environmental impacts
- Based on a life cycle assessment
- Standardized in format and reporting through PCRs and ISO 14025
- Used for procurement, design and compliance decisions
What an EPD Is Not
EPDs report data. They do not interpret or rank products.
- An EPD is not a certification of environmental superiority
- An EPD is not a performance ranking
- An EPD is not a guarantee of low embodied carbon products
- An EPD is not directly comparable across different PCRs, boundaries or declared units
Why Are Glass EPDs Essential for Low Embodied Carbon Specification?
A Glass EPD is the only widely accepted way to compare embodied carbon between architectural glass products on standardized terms. EPDs enable defensible, evidence‑based design decisions when choosing between products with similar performance.
EPDs support five key functions across modern projects:
- Procurement requirements. Federal and state Buy Clean building materials policies use EPDs, including the Federal Buy Clean Initiative, GSA low embodied carbon standards and Inflation Reduction Act embodied carbon requirements.
- LEED documentation. LEED v4 and v4.1 award credits for EPD use on LEED glass products.
- Buy Clean compliance. California, Oregon, Minnesota and other states require EPDs for specified materials.
- Material comparison. EPDs allow like‑for‑like comparison when declared units, boundaries and PCRs match.
- Carbon benchmarking. The EC3 tool by Building Transparency EC3 aggregates EPDs for benchmarking.
How Do You Read a Glass EPD?
A Glass EPD is organized into standardized sections that describe the product, define the scope of the assessment and report environmental impact data. Each section helps confirm relevance and comparability.
Product Description
This section identifies exactly which product the EPD covers. Confirm that the EPD applies to the product being specified.
- Glass type (flat glass, low iron, tinted, processed glass, IGU): confirm the EPD covers the same type of glass you are comparing. Different glass types have different manufacturing processes and different embodied carbon impacts.
- Manufacturer and plant locations: check where the glass is produced. Manufacturing location affects energy sources, transportation distance and overall carbon footprint.
- Product-specific, plant-specific or industry-average scope: reflects a single manufacturer’s product and is the most accurate for comparison. Industry-average EPDs are useful for benchmarking but less precise for specification decisions.
- Thickness, configuration and attributes: make sure the glass thickness and configuration match your project. Thicker glass or more complex assemblies typically have higher embodied carbon.
Declared Unit
The declared unit is the quantity to which impacts are normalized. If declared units differ, comparisons are not valid.
Glass EPDs follow two main PCRs:
- Flat glass EPD. Typically, 1 metric ton (1,000 kg) maintained for a 30‑year service life, per the GANA PCR for Flat Glass (Glass Association of North America).
- Processed glass EPD. Typically, 1 square meter (m²), per the UL Environment PCR for Processed Glass.
System Boundary
The system boundary defines which lifecycle stages are included. Different boundaries produce different GWP values.
| Boundary | Includes | Typical Use |
|---|---|---|
| Cradle‑to‑gate | Raw material extraction through factory gate | Most common in North American EPDs |
| Cradle‑to‑site | Adds transport to project site | Project‑specific assessments |
| Cradle‑to‑grave | Full life cycle through end of life | Common in European EPDs |
Global Warming Potential (GWP)
GWP is the headline embodied carbon metric reported in every EPD. It is expressed in kg CO₂eq per declared unit and converts all greenhouse gas emissions across the included stages into a single value.
- Includes carbon dioxide, methane, nitrous oxide and other regulated gases
- Lower GWP indicates lower embodied carbon
- Meaningful only when declared unit, boundary and PCR match
Lifecycle Modules
EPDs follow the EN 15804 framework, which isolates impacts by stage. Most North American glass EPDs report cradle‑to‑gate (A1 to A3).
| Module | Stage |
|---|---|
| A1–A3 | Raw material supply, transport, manufacturing |
| A4–A5 | Transport to site, installation |
| B1–B7 | Use stage, maintenance, replacement |
| C1–C4 | End of life |
| D | Reuse, recovery and recycling benefits |
How Do You Compare Embodied Carbon Across Glass Products?
Reliable comparison requires aligning assumptions before comparing numbers.
Step‑by‑Step Comparison Checklist
Each variable below changes the assumptions behind GWP. If any differ, the comparison is not apples‑to‑apples.
- Product type (flat glass to flat glass, IGU to IGU)
- Thickness or normalized basis
- Coating configuration (coated to coated, uncoated to uncoated)
- Functional performance level (similar U‑factor, SHGC, VLT)
- PCR version
- System boundary
- Declared unit
- Geographic scope
Common EPD Interpretation Errors
These are the most frequent and consequential mistakes.
- Comparing different declared units. Normalize before comparing.
- Comparing coated vs uncoated. Coatings add embodied carbon and affect performance.
- Ignoring manufacturing geography. PCRs and energy mixes differ by region.
- Looking only at the total GWP. Other indicators may matter.
- Ignoring operational performance. Lower embodied carbon with worse performance can increase total building carbon over time.
- Comparing across PCR versions. Methods change over time.
- EC3 tool. A free platform to benchmark GWP against industry averages.
- Vitro emissions™ Carbon and Energy Calculator. Models operational and embodied carbon for glazing assemblies. Access the calculator.
- Whole building LCA tools. Tally, One Click LCA and Athena Impact Estimator use EPD data for compliance and certification.
- Is the EPD a product specific EPD or an industry average?
- Is it a third‑party verified Type III EPD?
- What PCR was used?
- What manufacturing plant or plants are represented?
- What recycled glass cullet assumptions are included?
- What system boundary applies?
- What is the declared unit?
- How current is the data? Most EPDs are valid for five years.
- Embodied carbon. GWP per declared unit
- Thermal performance. U‑factor and operational energy from energy‑efficient glazing
- Solar control. SHGC and cooling load impact
- Durability. Expected service life
- Cost. First cost and lifecycle cost
- Availability. Regional manufacturing and lead time
What Tools Help Compare Glass EPDs?
Several tools support benchmarking and project workflows:
What Questions Should You Ask a Glass Manufacturer?
These questions determine whether the EPD is appropriate and comparable.
How Do You Use EPDs to Specify Lower Embodied Carbon Glass?
EPDs inform decisions when balanced against performance, durability and cost across the building’s service life.
Variables to Balance
Workflow
- Define performance requirements (U‑factor, SHGC, VLT).
- Identify candidate products.
- Request a product specific Glass EPD for each.
- Normalize data (declared unit, boundary, PCR).
- Compare GWP values.
- Model operational carbon.
- Evaluate operational carbon vs embodied carbon over service life.
- Document decisions for LEED, Buy Clean and other carbon reporting for buildings.
Key Takeaways
- An EPD is a standardized, third‑party verified disclosure based on an LCA.
- A Type III EPD is the accepted format for procurement and LEED credits.
- GWP per declared unit is the headline embodied carbon metric.
- Declared unit, boundary and PCR determine comparability.
- Most North American glass EPDs report cradle‑to‑gate (A1–A3).
- The EC3 tool supports benchmarking.
- The best specifications balance embodied and operational carbon over whole life.
Related Terms
- Environmental Product Declaration (EPD): A third-party verified report that discloses a product’s environmental impacts based on life cycle assessment (LCA) data in a standardized format.
- Type III EPD: A rigorously verified EPD that follows ISO 14025 standards and is widely accepted for LEED, Buy Clean and procurement requirements.
- Life Cycle Assessment (LCA): A method used to evaluate environmental impacts across a product’s life cycle, from raw material extraction through manufacturing and beyond.
- Embodied Carbon: The total greenhouse gas emissions associated with a material’s production, transport and lifecycle stages before use.
- Global Warming Potential (GWP): The primary metric in an EPD, expressed in kg CO₂eq, used to measure a product’s contribution to climate change.
- Declared Unit: The standardized quantity used in an EPD to report impacts, such as 1 metric ton of flat glass or 1 square meter of processed glass.
- Product Category Rules (PCR): The standardized methodology that defines how environmental data must be calculated and reported for a specific product type.
- System Boundary: Defines which life cycle stages are included in an EPD, such as cradle-to-gate or cradle-to-grave, affecting how GWP is reported.
- Operational Carbon: Emissions associated with the energy used to operate a building over time, considered alongside embodied carbon for full impact.
- Whole Life Carbon: The combined total of embodied carbon and operational carbon across a building’s full life cycle.
Frequently Asked Questions About Glass EPDs
What is the declared unit in a glass EPD?
Typically, 1 metric ton for flat glass and 1 square meter for processed glass.
What does A1 through A3 mean?
The cradle‑to‑gate stages: raw material supply, transport and manufacturing.
How do I compare two glass EPDs?
Match product type, declared unit, system boundary, PCR version and geography.
Are all glass EPDs directly comparable?
No. Differences in assumptions can make values look comparable when they are not.
Does an EPD prove a product is low embodied carbon?
No. Low embodied carbon depends on GWP meeting an established threshold.
What is GWP in architectural glass?
Global Warming Potential expressed in kg CO₂eq per declared unit.
How long is an EPD valid?
Typically five years from publication.
Updated on July 6, 2026
