"AN ELABORATION ON WHETHER OUR ZERO CARBON BUILDINGS ARE ALSO ZERO ENERGY BUILDINGS" Anju Pradhan, Christina J. Hopfe and Rob S. McLeod ABSTRACT The UK government has set stringent targets to reduce carbon impacts from the built environment sector in order to combat climate change. To attain these targets, all the buildings built beyond 2016 have to be ‘zero carbon’ buildings. At the same time Energy Performance Building Directive (EPBD) proposes to produce ‘nearly zero energy’ buildings by 2020 driving the need to look at energy efficient means to improve energy performance of the dwellings. This paper investigates the underlying relation between zero carbon buildings and zero energy buildings. The analysis is based on two different approaches, ultra low energy efficient Passivhaus Standard and the Fabric Energy Efficiency Standard (FEES) proposed by Zero Carbon Hub (ZCH) and Community and local Government (CLG). In a conducted case study, using the Passive House Planning Package (PHPP) and the Government’s Standard Assessment Procedure (SAP2009), an attempt was made to understand the underlying assumptions and differences between these two tools and findings of the comparative study are presented in this study. The results from the analysis provide a probable estimation of real world carbon emission and energy consumption for the domestic dwellings and show that our ‘zero carbon’ buildings are not inherently ‘zero energy’ buildings. Keywords: zero carbon, zero energy, passive house 1.0 INTRODUCTION Climate change impacts are widely known and well documented [1], [44]. CO 2 is one of the major gases which cause green house effect and the 100 year equivalent impact of a CO 2 molecule (CO 2 100 eq. ) is typically used as a proxy indicator for all Green House Gas emissions [44]. The accumulation of CO 2 emissions in the atmosphere is one of the critical factors that lead to the global climate change. The ambitious target to cut down the CO 2 emissions from all sectors by the year 2050 to 80 % [1] enforces us to cut down emissions and maximise the efficient way of energy use. In context of UK, 27 % of CO 2 emissions is produced from domestic dwellings alone, from both regulated (space and water heating, lighting and building services etc) and unregulated energy use (cooking and household appliances etc.) [2].In the process to limit carbon emissions from built environment sector, the UK government’s proposed timelines are such that by the year 2010, a 25% reduction in regulated carbon emission is proposed compared to part L (Building Regulation 2006), by the year 2013, a 44% reduction and by the year 2016, a 100 % reduction from domestic dwellings. That means all the dwellings built beyond 2016 have to be zero carbon buildings [3] in relation to their SAP regulated CO 2 emissions. Under the current ‘zero carbon’, definition, CO 2 emissions from the operational energy use, that is regulated energy use from space heating cooling, hot water ventilation and lightings are permissible across the year, provided they are balanced by the negative or offset emissions across the period. That means that it allows fossil fuels or grid electricity as an energy source provided the emissions produced is balanced by negative emission from the use of low or zero carbon technologies across the year or by the offsetting of these emissions under the new concept of ‘allowable solutions’. Not included within the ‘zero carbon’ definition are the embodied energy, construction, transport, demolition and recycling of the materials. Furthermore it is based on emissions from the predicted energy use calculated at the design stage but it does not consider the emissions from the actual energy use post construction [3], [4]. According to the Zero Carbon Hub (ZCH), the unregulated energy uses arises from cooking, household appliances are no longer included in the ‘zero carbon’ definition [5]. Only regulated energy use from space and water heating, fixed lighting and building services need to be considered in the definition [6]. Alongside with the target timeline proposed by the UK government, the European Energy Performance Building Directive (EPBD) on its recast -2 proposed that by the year 2020 all the new dwellings have to be ‘nearly zero’ energy buildings. A ‘nearly zero’ energy building is a building that has very high energy performance. The low energy requirement is fulfilled by the use of renewable energy sources. Since the housing act 2004 implemented the requirement of the Energy Performance Certificate (EPC) of the EPBD in the