The Carbon in Your Walls

The Hidden Footprint

Heat a home and you burn energy. Insulate it well and you burn less. Over decades of policy and regulation, we’ve become good at reducing this operational energy. So good, in fact, that in a well-insulated modern home, the energy used to make the building can now exceed the energy used to run it over a 60-year lifespan.

That’s the shift. As homes become more efficient to heat and cool, the carbon locked into the materials themselves becomes the dominant share of their lifetime climate impact.

Concrete production alone accounts for roughly 8% of global CO₂ emissions. Steel adds another 7%. These two materials form the backbone of conventional construction, and their carbon intensity is enormous. A cubic metre of reinforced concrete carries approximately 200–400 kgCO₂e of embodied carbon. A cubic metre of hemp-lime stores approximately 110 kgCO₂e. One adds carbon to the atmosphere. The other removes it.

How Materials Store Carbon

Plants grow by pulling CO₂ from the air and converting it into biomass through photosynthesis. When those plants become building materials (timber, hemp, straw, wood fibre, cork), the carbon stays locked in the structure for as long as the material exists. A timber beam. A hemp-lime wall. A wood fibre insulation board. Each is a carbon store.

This is biogenic carbon, and it changes the equation. A material that stores more carbon during its growth than is emitted during its processing and installation has a negative carbon footprint. It’s taken more from the atmosphere than it’s put in.

Hemp is the strongest example. Four months of growth. Three metres of height. No pesticides or irrigation. When that hemp becomes a wall, it stores roughly 110 kgCO₂e per cubic metre. The lime binder also absorbs CO₂ as it carbonates over months and years, adding to the total.

Timber stores around 700–900 kgCO₂e per cubic metre of carbon absorbed during its growth, though processing, kiln-drying, and transport reduce the net figure. The balance is still strongly positive: a timber-framed house is a significant carbon store.

Cork, harvested from living trees that continue growing after harvest, is carbon-negative by a wide margin. The tree absorbs more CO₂ after each harvest than was emitted in processing the bark.

What About Conventional Materials?

Cement, the binder in concrete, requires heating limestone to over 1,400°C. This process both burns fuel and releases CO₂ chemically from the limestone itself. Alternatives are emerging (geopolymer cements, supplementary cementitious materials), but they’re not yet mainstream for most construction.

Steel requires temperatures above 1,500°C and is primarily produced from iron ore using coal as a reducing agent. Recycled steel has a significantly lower footprint, but most structural steel still carries a heavy carbon burden.

Synthetic insulation materials like expanded polystyrene (EPS) and extruded polystyrene (XPS) are petroleum-based. Their embodied carbon is moderate, but they are neither biodegradable nor easily recyclable at end of life, which extends their environmental impact beyond the carbon calculation alone.

Comparing these with natural alternatives reveals clear differences. We pulled EPDs for two insulation boards used in similar wall assemblies: a 140mm wood fibre board from Steico and a 100mm EPS board from a major European manufacturer, adjusted to equivalent thermal resistance. The wood fibre board showed a net embodied carbon of −17 kgCO₂e per square metre (it stored more than it emitted). The EPS board showed +8.4 kgCO₂e per square metre. Same function, opposite direction.

Reading the Numbers

EPDs (Environmental Product Declarations) are the tool for comparing embodied carbon between materials. An EPD follows the EN 15804 standard and presents lifecycle data in a structured format. The key figure for carbon is GWP, Global Warming Potential, expressed in kgCO₂e.

EPDs divide the lifecycle into stages:

A1–A3 covers raw material extraction through manufacturing. This is the most commonly reported range and the one most people compare.

A4–A5 adds transport to site and installation.

B1–B7 covers the use phase, including maintenance and replacement.

C1–C4 covers end of life: demolition, transport, waste processing, and disposal.

D captures benefits beyond the system boundary, such as energy recovery from incineration or credits for recycling.

When comparing EPDs, check that you’re comparing the same stages. A1–A3 is the minimum. Cradle-to-grave (A–C) gives the complete picture. Module D adds important context for materials that can be reused, recycled, or composted.

Not every product has an EPD. Smaller manufacturers and artisan producers may not have invested in the process. That’s fine. An EPD isn’t the only way to assess environmental impact, but it is the most transparent.

What You Can Do With This

Knowing about embodied carbon doesn’t mean you need to calculate the lifecycle of every purchase. A few principles carry most of the weight.

Choose materials grown over materials mined. Timber, hemp, cork, wool, linen, and straw all store carbon. Stone, clay, and lime have low processing energy compared with concrete and steel. Choose these when you can.

Favour local where possible. A Latvian birch plywood shipped 1,500 km has a lower transport footprint than you might assume (bulk shipping is efficient), but a locally sourced oak beam has less still.

Think about longevity. A material that lasts 100 years has one-fifth the embodied carbon per year of service as one that lasts 20. Solid oak outlasts laminate. Lime plaster outlasts emulsion paint. Durability is itself a carbon strategy.

Use EPDs when they’re available, particularly for big-ticket items like insulation, flooring, and structural materials. They turn vague claims into comparable data.

Products to Explore

Wood fibre insulation boards from Steico or Gutex, with published EPDs showing carbon-negative performance. Hemp-lime building systems from Tradical or UK Hempcrete. FSC-certified structural timber and birch plywood from Finnish and Baltic producers. Cork insulation and flooring from Amorim, whose EPDs demonstrate strong carbon-storage figures.

Common Questions

What is embodied carbon in simple terms?

The total CO₂ released to extract, manufacture, transport, and install a building material. It’s the carbon cost paid before the material does anything useful in your home.

Is timber always better than concrete?

For most residential and interior applications, timber has a vastly lower (often negative) carbon footprint. Concrete has structural properties that timber can’t always replace, particularly in foundations and load-bearing ground-floor slabs. Where you have a choice, timber is the lower-carbon option.

How do I find a product’s EPD?

Many manufacturers publish EPDs on their websites. The European EPD databases (EPD Norway, IBU in Germany, the International EPD System) are searchable. For products without an EPD, ask the manufacturer about their environmental data.

Does transport cancel out the carbon benefits of natural materials?

Rarely. Manufacturing energy dominates the lifecycle footprint of most building materials. Transport typically accounts for 5–15% of total embodied carbon, even for materials shipped across Europe. A wood fibre insulation board from Germany used in the UK is still vastly lower-carbon than locally produced EPS.

What about carbon from heating — doesn’t that matter more?

It used to. In poorly insulated older homes, operational energy still dominates. But as insulation standards improve, embodied carbon becomes a larger share of the total. For new builds and deep retrofits, embodied carbon is already the bigger number. Both matter; addressing embodied carbon means choosing materials that contribute to the solution while they keep you warm.