*2.3. Study of Mexican Energy Standard*

For the second aim of the methodology, we conducted an extensive search and review of the existing Mexican energy standard. Notably, there is no mandatory energy standard in Mexico. This situation is common for North America. In the United States, for example, some states have regional regulations that include an energy performance test. Others states only incorporate energy strategies in their own construction codes. The American energy codes rely on ASHRAE guidelines and reference values. In Mexico, there are some energy guidelines and a group of non-mandatory standards [25]. For this project, we used NOM-020-ENER-2011, published by the National Committee for the Efficient Use of Energy (CONUEE) [3]. With voluntary application, this norm uses the comparison of energy savings for a normal case and a saving-based case as its principal verification method (designed by the optimization of the thermal envelope of buildings and the heat gains analysis). The verification calculation begins with the definition of a reference model for the building using reference values (provided by attached tables and an application manual contained in the standard's documentation). The next step is the calculation of the same variables, but from the projected model for the building. This information is obtained by analyzing the building geometry and consulting the available data on the plans and material catalogue for the project [7].

With this previous information, the calculations provided by the standard provide the heat gains by radiation and conduction for the reference and projected models. If the heat gains obtained by the projected model are equal to or less than the heat gains of the reference model, then the building complies with the standards. Therefore, the standard focuses on the verification results only in terms of the performance of the thermal envelope, neglecting other results obtainable from a simulation. A verification standard is not an energy performance study. The use of elements, like internal gains, occupation calendars, and energy expenses of electricity and sanitary hot water, is difficult to implement in a standard because they are dependent ob the user. The Spanish Building Technical Code (CTE) does not consider them, either.

In this proposed model, the HVAC performance is important. The inclusion of HVAC design and performance in building energy performance for an energy certification is complex. HVAC design requires a specific study of demands and an input of actual user variables, like occupancy, space use, and schedules. The HVAC equipment design is also dependent the unique characteristics of the project, for which it is difficult to determine an exact result for a final energy certification for standard verification. Due to the functionality of the Mexican verification (applicable for preliminary architectural projects), the project can be simulated with ideal loads for HVAC equipment, because the standard work with heat gains, not energy demands. The simulation tools available on the market, like Energy Plus, can simulate and describe energy demands by defining certain HVAC equipment characteristics, but the customization options result in a large number of possible results. The simulator requires the collaboration with an expert in HVAC installations and sizing.The simulation parameters increase in complexity as the HVAC design evolves and requires more precise information. This is complicated because it implies a level of updating, which will increase the cost. It is recommended to first verify if a project complies with the standard and then perform a personalized energy simulation of the particular building project.

In summary, the principal focus of the NOM-020-ENER-2011 is the heat transfer control of the thermal envelope for improving energy demands. Instead of optimizing or pursuing the goal of a Nearly Zero Energy Building (like recent energy codes and certification models), the Mexican standard focuses on improving energy demands for cooling systems due to the prevalence of arid, warm, and humid climatic zones in the country. The principal energy consumption rates for thermal conditioning is due to cooling (40%) [11], The standard requires low improvement levels for maintaining low construction costs and the competitive housing affordability for most of the Mexican population. Nine of ten Mexican citizens are planning to acquire a house, but 45% of them cannot afford it [26]. The standard establishes several reference coefficients for calculating the energy efficiency: U-values, temperatures, glazing coefficient, etc. (Figure 5). However, as the scope of the standard is thermal envelope performance for heat transfer, it ignores thermal comfort considerations. This means that the the standard does not provide a comfort model for simulation, following only its own calculation method.


**Figure 5.** Example table of the reference values quoted in NOM-020-ENER-2011. It contains interior and exterior temperatures, U-values, and other coefficients.
