## 2 Methodology
They can then be implemented in the cost-optimal study through building improvements requested by the HAPPEN project. * Clustering Method 2: A total of 37 reference buildings obtained through redundancy clustering of envelope geometries and qualities. * Clustering Method 3: Nine reference buildings obtained through the hierarchical clustering study with [PERSON]'s minimum variation method. [PERSON]'s Dendogram of the first level. * Clustering Method 4: Four reference buildings obtained through the hierarchical clustering study with [PERSON]'s minimum variation method. [PERSON]'s dendrogram in the second level  
The procedure was as follows:  
* Step 1: Study and definition of constructive properties. * Step 2: Generation of forms to be filled in by member countries. * Step 3: Data collection, analysis, and generation of reference buildings on Clustering Method 1. * Step 4: Definition of scales based on U-values differentiating two groups: first, for roofs, slabs on grade, and walls, and second, for windows. Definition of scales: definition of scales based on construction geometry. * Step 5: Classification of reference buildings on scales defined in step 4. Generation of reference buildings on the Clustering Method 2. * Step 6: Calculation of annual energy consumption of the 42 reference buildings using the approximate method. * Step 7: Implement [PERSON]'s minimum variation method and generate a dendrogram. Two levels of clustering, the second level with nine reference buildings and the first level (Clustering Method 3) with four reference buildings (Clustering Method 4). First, the study and definition of constructive properties are necessary for the characterization of the building stock of the member countries. Forms to be filled in by member countries were generated to request the necessary information. The forms will have three construction periods and two types of housing in each period, for a total of six buildings for each country. Through the collaboration of seven MED countries, the HAPPEN Project developed a harmonized structure for residential building typologies. A set of typical residential buildings was developed for each participant country, and data in terms of construction time and building type were collected. RBs are considered as examples and theoretical buildings according to the definition mentioned above. There are an endless number of permutations in the current building stock, and there is no \"correct average\". Therefore, the creation of a typical, in terms of size, number of floors, and use, RB for each building category (SFH-MFH) is recommended. The majority of these input data for present and former building regulations and standards may be gathered from the sources indicated above for the definition of a building's geometrical characteristics. More specifically, the definition of their thermal characteristics can be achieved in several different ways. The related U-value criteria are stated since the building stock is classified into groups depending on the year of construction, which might correspond to the periods for changes in building regulations (or other changes of relevance for the assessments to be performed). RBs might be defined with U-values for each of these periods of time for each building category. The collected data structure for each MED country is depicted in Figure 1.  
Figure 1: Data structure for RBs for each country per construction period. Each existing real building demonstrates a different constructive nature; therefore, it is necessary to develop a classification with the objective of agglomerating the types that are congruent with each other. This method is called clustering. Descriptive indicators considered include:  
* Geometrical characteristics, which define the dimensions of the envelope. * Thermal characteristics, which define the construction materials and methods. * Positional characteristics, which define the position of the building in relation to other buildings.