The carbon case for timber
Timber is fundamentally different from other structural materials because trees absorb CO₂ as they grow. A cubic metre of softwood stores approximately 250 kgCO₂e of biogenic carbon. When that timber is used in construction, this carbon remains sequestered for the life of the building.
Cross-Laminated Timber (CLT) has an embodied carbon of approximately 0.437 kgCO₂e/kg (A1–A3), while glulam sits at around 0.512 kgCO₂e/kg. When biogenic carbon sequestration is credited (Module D), both can achieve net-negative embodied carbon at the product stage - a claim no steel or concrete product can make.
Steel's strengths and carbon challenges
Structural steel has an embodied carbon of approximately 1.55 kgCO₂e/kg for virgin material. However, recycled steel dramatically improves this to around 0.44 kgCO₂e/kg - making steel specification heavily dependent on the recycled content percentage.
Steel offers clear advantages in long spans, high-rise structures, and speed of erection. Hot-rolled steel sections are readily available and the industry has deep expertise. The recyclability of steel is also excellent - virtually 100% of structural steel is recycled at end of life, which provides significant Module D credits.
Comparing real-world performance
For a typical 4–8 storey residential building, the structural frame carbon comparison looks like this:
Steel frame (50% recycled content): approximately 80–120 kgCO₂e/m² GIA for the structural frame alone.
CLT frame: approximately 40–70 kgCO₂e/m² GIA before biogenic carbon credits, potentially net-negative with sequestration credits.
Reinforced concrete frame: approximately 100–150 kgCO₂e/m² GIA.
However, the comparison isn't purely about carbon numbers. Fire strategy, acoustic performance, moisture management, and insurance requirements all influence material selection. CLT buildings above 18m require sprinklers and may face higher insurance premiums.
Hybrid approaches
Increasingly, the most carbon-efficient approach combines materials where each performs best. A concrete or steel core for stability, CLT floor plates for carbon reduction, and steel connections for robustness creates a system that optimises structural performance and carbon simultaneously.
This hybrid approach also manages supply chain risk - CLT availability in the UK is growing but still limited compared to steel and concrete. Projects specifying 100% timber structures may face lead time challenges.
Future Homes Standard implications
The Future Homes Standard doesn't directly mandate structural material choices, but the emerging Part Z requirements will make embodied carbon reporting mandatory. Projects that can demonstrate lower embodied carbon through timber specification will have a competitive advantage.
Specifiers should also consider the whole-building picture: timber structures are typically lighter, enabling smaller foundations (less concrete), and timber's thermal performance can reduce insulation requirements in some wall build-ups.
Key Takeaways
- CLT can achieve net-negative embodied carbon when biogenic sequestration is credited
- Recycled steel (0.44 kgCO₂e/kg) dramatically outperforms virgin steel (1.55 kgCO₂e/kg)
- Hybrid timber-steel-concrete structures often give the best carbon outcome
- Fire strategy and insurance are key non-carbon factors for timber above 18m
- Lighter timber structures reduce foundation requirements - consider the whole building
Ready to calculate your project's carbon impact?
Use our Carbon Calculator to specify materials and discover lower-carbon alternatives.
Open Carbon Calculator