The construction industry accounts for approximately 40% of global carbon emissions, with embodied carbon — the emissions locked into materials before a building is even occupied — representing a growing proportion as operational energy efficiency improves. Timber construction offers a unique proposition in this context: it is the only mainstream structural material that stores atmospheric carbon rather than emitting it during production.
Understanding how to calculate, report, and optimise embodied carbon in structural timber construction is now essential knowledge for architects pursuing net-zero design. This guide explains the methodology, data sources, and practical strategies for minimising whole-life carbon in timber buildings.
The EN 15978 Framework
Life Cycle Modules
EN 15978 (Sustainability of construction works — Assessment of environmental performance of buildings) provides the standardised framework for whole-life carbon assessment. It divides the building life cycle into discrete modules:
| Module | Stage | Description |
|---|---|---|
| A1 | Product | Raw material extraction and processing |
| A2 | Product | Transport to manufacturer |
| A3 | Product | Manufacturing |
| A4 | Construction | Transport to site |
| A5 | Construction | Construction/installation process |
| B1 | Use | Use (emissions from installed products) |
| B2 | Use | Maintenance |
| B3 | Use | Repair |
| B4 | Use | Replacement |
| B5 | Use | Refurbishment |
| B6 | Use | Operational energy use |
| B7 | Use | Operational water use |
| C1 | End of life | Deconstruction/demolition |
| C2 | End of life | Transport to waste processing |
| C3 | End of life | Waste processing |
| C4 | End of life | Disposal |
| D | Beyond life cycle | Reuse, recovery, recycling potential |
For embodied carbon assessment, the focus is on Modules A1–A5, C1–C4, and D. Modules B6 and B7 (operational carbon) are assessed separately.
Calculation Methodology
The basic calculation for each material is:
Embodied carbon = Quantity × Carbon factor
Where:
- Quantity is measured in kg, m³, or m² (matching the EPD functional unit)
- Carbon factor is from an EPD or database, in kgCO₂e per functional unit
For a complete building assessment:
Total embodied carbon = Σ (Quantity_i × CF_i) for all materials i, across all modules
Reporting Requirements
Current best practice (RICS Whole Life Carbon Assessment, 2nd edition) requires reporting:
- A1–A5 (upfront carbon) — always reported
- B1–B5 (in-use embodied carbon) — reported where data available
- C1–C4 (end-of-life carbon) — reported with stated assumptions
- D (beyond life cycle) — reported separately, never netted off A–C
- Biogenic carbon — reported separately from fossil carbon
Timber-Specific Carbon Accounting
Biogenic Carbon: The Unique Timber Advantage
Trees absorb CO₂ from the atmosphere through photosynthesis, converting it to cellulose, hemicellulose, and lignin — the structural components of wood. This biogenic carbon remains stored in timber products for their entire service life.
Carbon storage in timber products:
| Product | Density (kg/m³) | Carbon content | CO₂ stored (kg/m³) |
|---|---|---|---|
| Softwood (C24) | 420 | 50% | 770 |
| Glulam (GL24h) | 430 | 50% | 789 |
| CLT (5-layer) | 470 | 50% | 862 |
| LVL | 510 | 50% | 936 |
| Hardwood (oak) | 690 | 50% | 1,265 |
Calculation: CO₂ stored = density × carbon content × (44/12), where 44/12 is the molecular weight ratio of CO₂ to C
How Biogenic Carbon Is Reported
Under EN 15804+A2 (the standard for EPDs of construction products), biogenic carbon is reported using the "−1/+1" approach:
- Module A1: −1 kgCO₂ for each kg of biogenic CO₂ stored in the product
- Module C3/C4: +1 kgCO₂ for each kg of biogenic CO₂ released at end of life
This means that over the full life cycle (A–C), biogenic carbon is carbon-neutral if the timber is incinerated or decomposes. The climate benefit comes from:
- Temporal storage: Carbon is locked away for 60–100+ years (the building's life)
- Displacement: Timber replaces carbon-intensive materials (steel, concrete)
- Module D credits: If timber is reused or recycled, the stored carbon extends beyond one building life
- Sustainable forestry: If the harvested forest regrows (as required by FSC/PEFC), the carbon cycle is genuinely circular
Fossil Carbon in Timber Production
While timber stores biogenic carbon, its production still generates fossil carbon emissions from:
| Source | Typical Contribution | Notes |
|---|---|---|
| Forestry operations | 5–10% | Harvesting machinery, road building |
| Log transport | 5–15% | Distance-dependent |
| Sawmilling | 15–25% | Electricity for saws, handling |
| Kiln drying | 30–50% | Largest single source — heat energy |
| Planing/processing | 5–10% | Electricity for machines |
| Adhesives (glulam/CLT) | 5–15% | Petrochemical-derived resins |
| Packaging and delivery | 5–10% | Transport to customer |
Kiln drying is typically the largest emission source. Mills using biomass (wood waste) for kiln heat achieve significantly lower carbon factors than those using natural gas.
EPD Data for Timber Products
Understanding EPDs
An Environmental Product Declaration (EPD) is a standardised document (to EN 15804) that reports the environmental impacts of a product across its life cycle. EPDs are:
- Third-party verified — independently audited
- Quantitative — specific numbers, not qualitative claims
- Comparable — same methodology allows product comparison
- Time-limited — typically valid for 5 years
Key EPD Values for Common Timber Products
| Product | A1–A3 (fossil) | A1–A3 (biogenic) | Net A1–A3 | Source |
|---|---|---|---|---|
| Sawn softwood (C24) | 0.26 kgCO₂e/kg | −1.58 kgCO₂e/kg | −1.32 kgCO₂e/kg | Generic EPD |
| Glulam (GL24h) | 0.45 kgCO₂e/kg | −1.56 kgCO₂e/kg | −1.11 kgCO₂e/kg | Generic EPD |
| CLT (5-layer) | 0.42 kgCO₂e/kg | −1.54 kgCO₂e/kg | −1.12 kgCO₂e/kg | Generic EPD |
| LVL | 0.52 kgCO₂e/kg | −1.52 kgCO₂e/kg | −1.00 kgCO₂e/kg | Generic EPD |
| Plywood (softwood) | 0.65 kgCO₂e/kg | −1.40 kgCO₂e/kg | −0.75 kgCO₂e/kg | Generic EPD |
| OSB | 0.45 kgCO₂e/kg | −1.35 kgCO₂e/kg | −0.90 kgCO₂e/kg | Generic EPD |
Note: Values are indicative — always use project-specific or manufacturer-specific EPDs where available.
EPD Databases
| Database | Coverage | Access | Notes |
|---|---|---|---|
| ECO Platform | Pan-European | Free | Aggregates national programmes |
| IBU | Germany | Free | Extensive timber product coverage |
| EPD Norge | Scandinavia | Free | Nordic timber products |
| BRE Environmental Profiles | UK | Subscription | Used in BREEAM assessments |
| INIES | France | Free | French market products |
| ICE Database (Bath) | Generic | Free | Conservative generic values |
| One Click LCA | Global | Subscription | Software with integrated database |
Whole-Life Carbon Assessment: Step by Step
Step 1: Define the Assessment Scope
Establish:
- Building area (GIA in m²)
- Assessment boundary (which modules to include)
- Reference study period (typically 60 years for buildings)
- Biogenic carbon reporting approach (separate or integrated)
- Target benchmark (LETI, RIBA, GLA, or project-specific)
Step 2: Quantify Materials
For the timber structure, quantify:
| Element | Measurement | Example |
|---|---|---|
| Floor joists | Linear metres × section size | 450 lm of 47 × 225 C24 |
| Wall studs | Linear metres × section size | 680 lm of 38 × 140 C24 |
| Roof rafters | Linear metres × section size | 320 lm of 47 × 200 C24 |
| Glulam beams | Volume (m³) | 2.8 m³ GL24h |
| CLT panels | Volume (m³) or area × thickness | 145 m² × 160 mm |
| Sheathing (OSB) | Area × thickness | 380 m² × 15 mm |
| Battens | Linear metres × section size | 520 lm of 25 × 50 |
Convert all quantities to mass (kg) using the product density from the EPD.
Step 3: Apply Carbon Factors
For each material, apply the appropriate carbon factor for each life cycle module:
Example: C24 structural timber floor joists
- Quantity: 450 lm of 47 × 225 mm = 4.76 m³ = 2,000 kg (at 420 kg/m³)
- A1–A3 (fossil): 2,000 × 0.26 = 520 kgCO₂e
- A1–A3 (biogenic): 2,000 × (−1.58) = −3,160 kgCO₂e
- A4 (transport, 500 km by road): 2,000 × 0.005 = 10 kgCO₂e
- A5 (construction waste, 5%): 100 kg × 0.26 = 26 kgCO₂e
- C3 (incineration with energy recovery): 2,000 × 0.02 = 40 kgCO₂e
- C3 (biogenic release): 2,000 × 1.58 = +3,160 kgCO₂e
- D (energy recovery credit): 2,000 × (−0.45) = −900 kgCO₂e
Step 4: Sum and Report
Aggregate results across all materials and report per m² of gross internal floor area:
| Module | Timber Structure | Concrete Alternative | Saving |
|---|---|---|---|
| A1–A3 (fossil) | 85 kgCO₂e/m² | 180 kgCO₂e/m² | 53% |
| A1–A3 (biogenic) | −120 kgCO₂e/m² | 0 kgCO₂e/m² | — |
| A4–A5 | 15 kgCO₂e/m² | 25 kgCO₂e/m² | 40% |
| Upfront (A1–A5, fossil) | 100 kgCO₂e/m² | 205 kgCO₂e/m² | 51% |
| C1–C4 | 45 kgCO₂e/m² | 35 kgCO₂e/m² | −29% |
| D | −65 kgCO₂e/m² | −15 kgCO₂e/m² | 333% |
LETI and Industry Targets
LETI Embodied Carbon Targets
The London Energy Transformation Initiative provides the most widely referenced embodied carbon targets in the UK:
| Building Type | 2020 Target | 2025 Target | 2030 Target |
|---|---|---|---|
| Residential | <600 kgCO₂e/m² | <500 kgCO₂e/m² | <350 kgCO₂e/m² |
| Commercial (office) | <600 kgCO₂e/m² | <500 kgCO₂e/m² | <350 kgCO₂e/m² |
| Education | <500 kgCO₂e/m² | <400 kgCO₂e/m² | <300 kgCO₂e/m² |
Modules A1–A5, excluding biogenic carbon, excluding sequestration
RIBA 2030 Climate Challenge
RIBA sets progressive targets for embodied carbon:
- 2020: <1,000 kgCO₂e/m² (A1–A5)
- 2025: <800 kgCO₂e/m²
- 2030: <625 kgCO₂e/m²
How Timber Buildings Perform Against Targets
| Building Type | Typical A1–A5 (fossil) | LETI 2030 Target | Margin |
|---|---|---|---|
| Timber frame house | 200–350 kgCO₂e/m² | 350 kgCO₂e/m² | Meets today |
| CLT apartment building | 250–400 kgCO₂e/m² | 350 kgCO₂e/m² | Close/meets |
| Hybrid timber-concrete | 350–500 kgCO₂e/m² | 350 kgCO₂e/m² | Challenging |
| Mass timber office | 200–350 kgCO₂e/m² | 350 kgCO₂e/m² | Meets today |
Timber construction is one of the few structural systems that can meet 2030 embodied carbon targets with current technology and supply chains.
Module D: Beyond the Building Life Cycle
Why Module D Matters for Timber
Module D reports the environmental benefits that occur after the building's life ends. For timber, these benefits can be substantial:
Scenario 1: Timber reused in new building
- Stored biogenic carbon continues to be sequestered
- Avoided production of new timber (saving A1–A3 emissions)
- Module D credit: −0.8 to −1.5 kgCO₂e/kg
Scenario 2: Timber recycled into particleboard
- Stored biogenic carbon continues to be sequestered (in new product)
- Avoided production of virgin particleboard
- Module D credit: −0.4 to −0.8 kgCO₂e/kg
Scenario 3: Timber used as biomass fuel
- Stored biogenic carbon released (reported in C3)
- Avoided fossil fuel use for energy generation
- Module D credit: −0.3 to −0.6 kgCO₂e/kg (depends on displaced fuel)
Designing for Module D
To maximise end-of-life benefits, design timber buildings for:
- Disassembly: Use mechanical connections (bolts, screws) rather than adhesives
- Material identification: Label timber elements with species, grade, and treatment
- Contamination avoidance: Minimise chemical treatments that prevent recycling
- Standardisation: Use standard sections that can be reused without re-machining
- Documentation: Maintain a materials passport recording all timber specifications
Practical Strategies to Reduce Embodied Carbon
Material Selection
| Strategy | Carbon Saving | Practicality |
|---|---|---|
| Use sawn timber instead of glulam (where possible) | 40–50% per kg | High — for standard spans |
| Specify locally sourced timber (reduce A4) | 5–15% | Medium — depends on availability |
| Choose manufacturers using biomass energy | 20–40% of A1–A3 | Medium — check EPDs |
| Use FSC/PEFC certified timber (sustainable forestry) | Ensures carbon neutrality | High — standard practice |
| Avoid over-specification (right-size members) | 10–20% | High — engineering optimisation |
| Minimise waste (DfMA, CNC cutting) | 5–15% | High — design stage decision |
Design Optimisation
For C24 structural construction timber, optimise the structural design to minimise material use:
- Use efficient structural forms (trusses vs solid beams for long spans)
- Optimise joist spacing (400 mm vs 600 mm centres — assess which uses less total timber)
- Consider composite action (timber-concrete composite floors)
- Use higher-grade timber for critical members (C30 or GL28h) to reduce section sizes
- Eliminate redundant members through careful engineering
Construction Process (Module A5)
Reduce construction-phase carbon by:
- Minimising waste: target <5% timber waste (vs industry average 10–15%)
- Using prefabrication to reduce site energy use
- Specifying electric site equipment where possible
- Recycling all timber offcuts (biomass or reuse)
- Avoiding unnecessary temporary works
Carbon Assessment Tools
Software Options
| Tool | Cost | Features | Best For |
|---|---|---|---|
| One Click LCA | Subscription | Comprehensive, BIM integration | Large projects, BREEAM |
| eTool | Subscription | User-friendly, good timber data | Medium projects |
| RICS Carbon Calculator | Free | Basic, spreadsheet-based | Small projects, early stage |
| EC3 (Building Transparency) | Free | EPD comparison tool | Material selection |
| PHribbon (Passive House) | Free | Integrated with PHPP | Passive House projects |
BIM-Based Carbon Assessment
For BIM-enabled projects, embodied carbon can be calculated directly from the model:
- Assign materials to all model elements (with density and EPD reference)
- Extract material quantities from the model (by volume or mass)
- Apply carbon factors from EPD database
- Generate carbon report by element, material, and life cycle module
- Iterate design to reduce carbon (structural optimisation)
This workflow enables real-time carbon tracking as the design develops, allowing architects to make informed material choices at every stage.
Reporting and Certification
BREEAM Credits for Embodied Carbon
BREEAM New Construction awards credits under Mat 01 (Life Cycle Impacts) for:
- Conducting a whole-life carbon assessment to EN 15978
- Achieving embodied carbon below benchmark values
- Using EPD data for >80% of materials by mass
- Reporting biogenic carbon separately
GLA (Greater London Authority) Requirements
The GLA requires all referable planning applications to submit a Whole Life Carbon Assessment:
- Modules A1–A5, B1–B5, C1–C4 reported
- Module D reported separately
- Results benchmarked against GLA targets
- Reduction strategies demonstrated
Voluntary Carbon Reporting
Beyond regulatory requirements, voluntary reporting frameworks include:
- WLCN (World Green Building Council) Net Zero Carbon Buildings Commitment
- SBTi (Science Based Targets initiative) for construction companies
- CDP (Carbon Disclosure Project) for corporate reporting
Related Resources
- Structural Timber Guide — comprehensive reference for timber construction
- C24 Structural Construction Timber — product specifications with sustainability credentials
- BSH Glued Laminated Beams — engineered timber with EPD data
- FSC vs PEFC Certification — understanding forest certification schemes