On our website we show how much carbon dioxide is stored in the timber installed in the buildings we have designed. In this article we describe exactly how we calculate it – so that the figure can be checked.
What does the figure show – and what not?
As it grows, a tree takes carbon dioxide from the air and builds the carbon into its own material. As long as the timber is in the building, this carbon stays there. The figure shows this stored amount, converted to carbon dioxide.
It does not show how much emission was “saved” compared with another structure. That can only be determined with a life-cycle assessment (EN 15978), because it requires a comparison building – see our article on the EPBD. Nor do we deduct the emissions from production and transport.
The formula
The calculation follows the European standard EN 16449:
CO₂ = 44/12 × cf × ρω × V / (1 + ω/100)
| Symbol | Meaning | Value we use |
|---|---|---|
| 44/12 | ratio of the molar masses of carbon dioxide and carbon | 3.67 |
| cf | carbon fraction of dry wood | 0.5 (the standard value) |
| ρω | density of the wood at moisture content ω | 440 kg/m³ |
| ω | moisture content of the wood | 12% |
| V | volume of installed timber | from the fabrication drawings, in m³ |
We use a cautious density value: spruce has a density of 440–470 kg/m³, and we calculate with the lower value so that the figure tends to be lower rather than higher. If the manufacturer’s environmental product declaration (EPD) for a project gives a different value, we use that.
With these values, 1 m³ of installed softwood stores about 720 kg of carbon dioxide: 3.67 × 0.5 × 440 / 1.12 ≈ 720 kg.
What do we include?
- The structural timber we designed in the project: CLT, glulam and other structural timber.
- The exact volume: based on the bill of materials extracted element by element from the 3D model prepared for the fabrication drawings.
- Only elements that have been or are being installed: the figure covers completed projects and projects under construction.
An example: the timber of a house built from 100 m³ of CLT stores about 72 tonnes of carbon dioxide according to the formula above.
How long is the carbon stored?
As long as the timber is in the building. If the timber is burned after demolition, the carbon is released; if it is reused – for example as furniture board or another product – storage continues. That is another reason why it matters that the structure can be dismantled and the elements reused.
How is the figure updated?
We keep the data of every project in a shared register: the timber volume, the CLT area designed and the number of fabrication drawings prepared. For each new project, the figures are updated from this register.
Why does this matter to you?
In the EU, from 2028 for new buildings over 1000 m² and from 2030 for all new buildings, the life-cycle carbon footprint will have to be calculated and shown in the energy performance certificate. The exact quantity of material is basic data for this – we provide it element by element from the model prepared for the fabrication drawings.