For major renovations and extensions, there is a requirement to meet a ‘cost optimal’ energy performance standard, a minimum of BER B2 and deemed to be technically, functionally and economically feasible.


This is where more than 25% of the surface area of the existing building’s thermal envelope undergoes renovation, calculating the entire surface area of the building where it can lose heat to the external environment or ground, this includes heat loss of walls, windows, floors and roof. If as part of your project your intention is to upgrade your existing doors, windows and install external wall insulation this could easily add up to 25% of the surface area of the thermal envelope.  Equally, if you are planning on adding an extension to your home, for example, extending the gable and rear wall, 25% of the heat loss surface of the existing dwelling is renovated.


McCarthy Construction advises to have your builder and/or architect to arrange for a BER assessor to determine whether you fall into this category, because if your project extension is more than 25% of the building thermal envelopment then the entire upgraded dwelling must either achieve:

  • BER B2 rating (whole-house energy performance of 125 kWh/sqm/yr as calculated by the BER assessor using the Dwelling Energy Assessment Procedure (DEAP) software
  • Implement pre-defined energy performance improvements, which could include 100mm of external wall insulation, 300mm of attic insulation and upgrading existing boilers to 91% efficiency gas boiler with full time, temperature and zone controls.


The final word from John McCarthy, “even though your energy performance is assessed at the start of your project, more air tightness tests need to be carried out during the project to ensure you are on target to meeting compliance and this should be facilitated by your builder as part of his site management obligations”.  He advises “this is important as the responsibility for compliance is with the architect, builder and owner of the home or dwelling”.


If you are planning a new build or renovation project, click here to contact us today to ensure you get the best advice and builder for your project