Today is not someday

In “What must a number be able to do?” we opened up a life-cycle number. It turned out to combine events that happen at very different times: manufacture and construction now, operation over decades, later replacements and, finally, a modelled end of life.

What happens if we open up not only the number, but time as well?

The manufacturing emissions of a building arise with its construction. What we choose today in terms of construction and materials takes effect immediately. Operation, by contrast, stretches over decades and depends, among other things, on how our energy system develops. Replacements and end of life lie further away again. So the life-cycle calculation can also be read as a landscape through time.

Data basis: EFH-LCA master, timber–straw and mineral construction, 50 years. The 147.89/192.80 t CO₂e end values come from the QNG calculation; the alternative electricity paths use the decarbonisation path in the working model. 2045 marks Germany’s target year for greenhouse-gas neutrality. The intermediate paths in this working graphic will be checked again against the master graphic data in the comparative-LCA article.

Today is not someday.

A tonne emitted today increases atmospheric loading from today onward. An emission that actually occurs in 2045 or 2076 only does so from that point onward. For a climate task in which the coming years matter in particular, that difference is not incidental.

At the same time, the further we move along the time axis, the character of our knowledge changes. We decide today on the construction installed today. For the future we calculate: if the electricity mix develops as assumed, if the heat pump, PV system or battery is replaced at the assumed time, if the building reaches its modelled end of life after 50 years.

The further into the future we calculate, the longer the chain of ifs becomes.

The negative bet

This becomes particularly clear with biogenic materials. Timber and straw bring carbon into the building that plants previously removed from the atmosphere. In the life-cycle calculation that carbon is booked back at end of life when the chosen scenario assumes thermal recovery.

For building straw, for example, today’s scenario assumes thermal recovery at end of life. Reuse is not included in the EPD because experience is currently lacking. We therefore write into today’s calculation that a biogenic material we are installing for decades will ultimately be burnt.

That is a negative bet.

The comparison with the plastic insulation we examined is revealing. There, a future recovery or circularity scenario may already be used at end of life. That, too, is an assumption about what will happen decades from now to a material installed today.

If that is how the game is played, we can just as well place the positive bet for biogenic materials.

What would that do to our number? As a simple stress test, we assume 50% reuse of the straw insulation in wall and roof and 50% reuse of the structural timber in our timber–straw building. For timber, we deliberately include only the substantial, untreated and readily demountable structural sections in walls, floors and roof – plates and rails, joists, columns, beams and rafters. Windows, doors and other timber products are excluded.

Fifty per cent is not a forecast. For these structural timbers the assumption is deliberately conservative. The point here is simply to see how sensitive the life-cycle number is to whether we assume combustion or continued use in 2076.

Stress test: 50% reuse of straw insulation in wall and roof and 50% of the defined structural timbers in cost groups 330/350/360. In the master, the modelled C3 return emissions are 12.29 t CO₂e for the two main straw positions and 12.46 t CO₂e for the selected structural-timber positions. In the simplified 50% scenario, 12.38 t are therefore not released within the assessment period. Result: 89.9 → 77.5 t CO₂e. QNG thresholds converted to this model building: PREMIUM 167.2 t, PLUS 200.6 t (NRF 167.19 m² · 50 years).