Following the procedure in TGD L, the energy performance of the proposed dwelling is not assessed in absolute terms of energy consumption, but measured against a Reference Dwelling with the same geometry of our proposal and basic specs that comply with 2005 standards.
The required 60% reduction on energy performance must be measured against this Reference Dwelling. This means a more efficient geometry will make compliance more difficult! We show why:
A more compact house will have an improved Energy Value, but this will also make the Reference Dwelling better… so a still lower Energy Value will be required to comply with the regulations.
We carried out an analysis for 4 dwelling types, with shapes ranging from most efficient (mid-mid apartment with just one external wall) to least efficient (a detached house with a large perimeter/area ratio). We show the resulting graphs below: Red relates to space heating, orange to water heating, and yellow shows electricity consumption.
For each of the dwellings, the bar on the right shows the Energy Value requirement to comply: a 60% improvement from the Reference Dwelling. We’ve already assumed 100% low-energy lighting and solar water heating (to comply with minimum renewable contribution), so all the extra reduction must be made on space heating by reducing fabric loss (red part of the bar).
For the most efficient geometry, the required extra reduction is in practice impossible to achieve, so we get penalised by being forced to use expensive renewables to comply!
We suggest a single target figure for the Energy Value is applied for all dwellings, as this would be simpler (a Reference Dwelling would no longer be necessary) and fairer (choosing more compact geometries would be rewarded).