What must a number be able to do?

In building, we use numbers to compare very different solutions. Life-cycle assessments are meant to help us choose the option with the lower climate impact.

But what must such a number actually show if our starting point is: cut carbon now, as much as possible?

I want to find out with a simple case: one building, two functionally equivalent variants. Same geometry, same thermal performance, same building services. The main difference is the construction: timber and straw in one case, mineral construction in the other.

We start with the result used for assessment today – and then open the number up, step by step.

FULL COMPARISON · The complete comparative LCA of both building variants, including methodology, data and results, will be linked here shortly as a separate article and PDF.

Under QNG, the results are:

The timber–straw variant reaches QNG PREMIUM; the mineral variant QNG PLUS. Both therefore remain within the QNG assessment logic.

Converted back to our model building over the 50-year assessment period, those scores represent:

Difference: 44.91 tonnes CO₂e.

Tolerance

Tolerance is usually a good thing. Here, it is out of place.

Almost 45 tonnes of difference between two variants of the same building – and both remain within the QNG assessment logic.

This is not a calculation error. The assessment allows this difference.

So instead of looking only at the 50-year single score, look at what is already decided at the beginning when the construction is chosen.

Data basis: EFH-LCA master, NRF 167.19 m², 50-year assessment period. QNG thresholds and the full comparative LCA will be documented in the announced comparison article.

Manufacture of materials and building products · A1–A3

Difference: 83.24 tonnes CO₂e.

This difference arises before the building is used.

83.24 tonnes at the beginning.
44.91 tonnes in the overall assessment.

Where did the other 38.33 tonnes go?

Nowhere.

They have not disappeared. Product manufacture, operation, replacement and end of life are aggregated into one number over 50 years.

As a result, part of the difference that we lock in today through the choice of construction becomes less visible in the overall result.

Blur

The assessment can be correct – and still be blurred for a particular decision.

It answers the question of the modelled climate impact over the full assessment period.

But it does not show with the same clarity which emissions our decision today causes or avoids.