Methodology to achieve cost optimal NZEB renovation

Nearly Zero Energy Building (NZEB) standard becomes mandatory for buildings in Europe after 2020. Furthermore, according to the EPBD, NZEB renovations should be cost-optimal. Achieving those requirements is challenging and highly dependent on the individual case, therefore a method allowing for consistent evaluation and selection of renovation solutions is needed. According to the impact assessment of the EU Commission a renovation rate of 3% per year is necessary to reach the desired goals set by the EU. Currently the achieved renovation rate varies from approximately 0.1 to 1.5 % (depending on the country and year).

The ReCO2ST methodology can help increase the renovation rate by reducing cost via establishing cost-optimal scenarios accounting for energy savings and renewable energy production. Additionally, the methodology will allow for saving time by providing a structured outline for determination of appropriate renovation interventions to be implemented in a building. The structure and provided consequent actions will also allow reducing complexity of the renovation process. The user of the method shall be able to select a solution without exploring too many unnecessary options.


The aim of the method is to assist engineers, designers and professional building owners in selecting renovation scenarios composed of the most relevant and cost optimal actions. At the same time, the methodology should provide possibility for prioritization of renovation actions in the scenarios, which promote healthy indoor climate and provide added value. The general idea behind the ReCO2ST methodology is to reduce the cost of NZEB renovation by identifying the cost optimal balance point between building energy efficiency and on-site renewable energy generation. The method should serve as decision support through the early stages in the renovation process.

The method begins with various activities related to determination of the existing state and energy demand of the building under consideration. Site visits combined with theoretical calculations are suggested in order to quantify the magnitude of energy savings, obtained by applying different interventions. Thereafter a cost efficiency value is calculated for each energy-related renovation action. It indicates the cost of saved kWh and is used as a guiding value when compiling renovation scenarios. A similar approach is followed for renewable energy producing technologies. Each possible renewable energy technology is investigated in terms of investment cost and production capacity. Initially a combination of the cheapest options in terms of energy savings and energy production are combined and evaluated using the EU imposed global cost method. Following, scenarios are optimized for specific targets, synergies and finances.

The proposed activities are structured in a way, which allows for obtaining the parameters required by both, the EU cost optimal methodology, as well as regional requirements for envelope, systems and/or total building efficiency and energy demand. An important part is the heat balance calculations, which determines the energy demand of the project under consideration. The calculations are used for documentation to authorities and estimation of achieved energy saving by the applied renovation interventions.

Life cycle cost (LCC) calculations are another important part of the EU cost optimal methodology; thereby cost estimates for the renovation investment, operation and maintenance, and in some cases, end of life are necessary. Furthermore, assumptions for energy price development are needed for determination of the global costs. In some countries, there are also minimum requirements for indoor environmental quality levels. Therefore, it is a goal that the methodology is capable of linking the evaluated renovation actions and their improvement to the existing indoor environmental quality.

Currently the necessary parameters to be integrated in the methodology have been determined. An outline presenting consequential steps to be followed is established and aligned to the EU cost optimal methodology. The main remaining challenges are: 1) To set meaningful constrains of how the investment balance is to be decided; 2) How to link and prioritize renovation actions and indoor environmental quality improvements.
Ongoing considerations about how the balance should be established point towards linking the balance with the project targets, possible synergies and available finances. For example, the type of NZEB would have a big influence on which energy-producing technologies are considered and how an economic balance between building efficiency and energy production would be achieved. Future work will also focus on precise definition, description and examples of how the balance can be obtained. Demonstrations will be provided using one or more of the ReCO2ST demonstration buildings.