1. The ten per cent rule
According to information published by the Ministry of Climate the division is threshold-based, which keeps the calculation manageable. The threshold is set in the regulation.
In a building with several uses, each use whose heated area exceeds 10 per cent of the building's total heated area is assigned the energy performance value corresponding to that use. A use below ten per cent is not accounted for separately.
The practical point concerns the boundary: a use close to ten per cent must be calculated precisely, because crossing the threshold adds one component to the calculation. Near the boundary the calculation must be rechecked.
2. How the weighted average is calculated
Two separate calculations that must not be conflated, and forgetting one is a common error. They concern the result and the requirement.
| What is calculated | How |
|---|---|
| The building's energy performance value, i.e. the result | the weighted average of the parts' values by heated area |
| The maximum permitted energy performance value, i.e. the requirement | the weighted average of the parts' limit values by heated area |
Compliance means that the first does not exceed the second. Weighting is by heated area in both cases, which makes the definition of area particularly important here.
3. What it means in practice
Three consequences, and the third works in the client's favour. They concern how the parts are weighted.
A good result in one part of the building can compensate for a weaker one in another, because the average is weighted, but the compensation happens within that same weighted logic: a larger part weighs more. The weighting makes that possible.
The effect of a small but energy-intensive part may be greater than its area suggests, because the limit value for that use is usually higher and therefore raises the weighted permitted level. Professionally this means the distribution of uses affects the requirement, not only the result.
4. Why the definition of uses is strategic
Three reasons make describing the uses the basis of the calculation rather than a formality. The description affects the result directly.
| What the use determines | Consequence |
|---|---|
| Which limit value applies to that part | direct effect on the requirement |
| The combination of uses | determines the weighted permitted level |
| The 10 per cent threshold | determines whether a use is counted separately at all |
The use must nonetheless correspond to reality, and this is checked during the procedure. Describing uses inaccurately is not optimisation but a risk.
5. What to check
Four things, and the fourth is the commonest source of error. It concerns the split of heated area.
Check whether any use exceeds ten per cent of the heated area, whether the heated area has been calculated within the meaning of the regulation, and whether each use's limit value has been taken from the annex in force. These three set the structure of the calculation.
Check finally whether the weighted average has been calculated on both sides, for the result and for the limit value. This is where most errors occur: the result is averaged and averaging the requirement is forgotten.
Summary and four practical rules
In a building with several uses, each use whose heated area exceeds ten per cent of the building's total heated area is assigned its own energy performance value, and uses below the threshold are not counted separately. Both the building's energy performance value and its maximum permitted value are weighted averages by heated area, and compliance means the first does not exceed the second. The distribution of uses therefore affects the requirement and not only the result, but the uses must correspond to reality, since this is checked during the procedure.
Four rules: calculate each use's heated area separately to test the threshold. Average both the result and the limit value. Use the regulation's concept of area, not the register's. Describe the uses accurately, since the procedure checks them.
This article offers professional orientation as at the date of verification. It replaces neither the regulations in force nor the assessment of a competent specialist.