Cost-optimal passive versus active nZEB. How cost-optimal calculations for retrofit may change nZEB best practice in Ireland

Arising from Ireland's first measured nearly Zero Energy Building (nZEB) retrofit, this paper explores the potential impact of EU member state (MS) proposed cost-optimal nZEB retrofit implementation on Irish building codes, and in turn how that may impact current best practice in low-energy retrofit design practices. The paper compares cost-optimal methodologies to best practice and discusses how design retrofit strategies could change in the context of market drivers and value engineering. The implications for capital investment and return are discussed, highlighting variations from EU guidance, with specific regard to net present value (NPV), scenario analysis, building lifespan, residual value and calculation period in the Irish cost-optimal calculations. A case study of a 1974 non-residential building highlights nZEB retrofit best practice immediately prior to MS cost-optimal calculations, demonstrating the importance of fabric-first interventions in extending building lifespan and reducing energy demand. The reported demonstrator project is the first measured site nZEB retrofit in an Irish context, outperforming reported cost-optimal energy consumption, costing less than new-build and adding an additional 40-year building lifespan. The paper concludes that the proposed Irish cost-optimal nZEB retrofit packages may drive design strategies towards active solutions, and away from fabric-driven retrofits, whilst not complying with EU guidance.