Energy Consumption in Mexican Homes: Using a Reference Building as a Launchpad for Achieving Nearly Zero Energy

Abstract

The residential sector accounts for over a third of the world’s energy use. Even though this ratio is lower in Mexico, there is a pressing housing deficit, especially regarding low-cost homes. This research aimed to create a reference building (RB) to understand the current energy consumption of multi-family buildings across different climatic zones. The aim was to assess their energy performance and promote reduced energy requirements as a guideline for designing and constructing affordable, low-energy, or zero-energy buildings. The present work conducts a diagnosis of the current energy consumption of multi-family buildings in eight cities in Mexico. First, a reference building was developed to represent typical Mexican building geometry and construction practices, and then the building’s fixed and variable energy requirements were simulated. Finally, a comparison was made between the energy requirement and the data reported by the national energy survey. Therefore, it was possible to generate a reference building from national data sources complying with national regulations, where materials, occupant behavior, and equipment were chosen to help represent the building’s thermal behavior. Domestic water heating was identified as a driver of variable energy requirements in all cities. In contrast, the simulated heating and cooling requirements were directly linked to the city’s climate. Electricity bills tended to mostly correspond with the results that excluded the use of heating systems. Lastly, while comparing LPG (Liquified Petroleum Gas) consumption was challenging due to the unavailability of national data, LPG requirements were closely estimated for temperate cities. The definition of a reference building is an important step towards developing nZEB in Mexico, as these buildings are valuable tools that can contribute to the energy evaluation of specific types of buildings. This characteristic makes them convenient for revising a building code or setting new national energy policy goals.

1. Introduction

The effects of climate change are more evident than ever, and people worldwide are experiencing their consequences. Countries have signed agreements focusing on limiting global warming, such as the Paris Agreement, whose main objective is to prevent the earth’s temperature from increasing by over 2 °C [1]. Due to the relationship between energy use and the emission of greenhouse gases (GHG), energy efficiency projects and policies are very important in order to achieve these goals, especially in the sectors with the highest consumption. One of the critical sectors in which energy efficiency strategies can be applied is the building sector, which represents 34% of energy demand and 37% of CO₂ emissions [2]. Achieving substantial and sustained GHG reductions can limit climate change, even though some effects may take decades to stabilize [1].

According to estimates produced by the International Agency for Energy [3], to achieve the goal of zero carbon emissions by the year 2050, direct CO₂ emissions from buildings must be reduced by half by 2030. Even though, more than ever before, buildings worldwide are built following energy efficiency codes, it is necessary to strengthen and expand actions to achieve decarbonization goals. Another important commitment is the 17 Sustainable Development Goals (SDGs) within the 2030 Agenda of the United Nations (UN). Some of these objectives are related to energy use in buildings due to their high percentage of global energy consumption. Goal 7 (Affordable and Clean Energy), in its Target 7.3, establishes the need to double the global energy efficiency improvement rate. Goal 11 (Sustainable Cities and Communities) emphasizes reducing the environmental impact of cities and improving the use of natural resources [4]. Also, Target 11.3 of Goal 11 (Sustainable cities and Communities) highlights the importance of developing sustainable infrastructure [5]. Within SDG 11, there is an indicator in which the ratio of used land to population growth must be taken into account, indicating benefits in the development of compact urbanizations.

According to the 2020 National Housing Survey [6], 21.1% of Mexican households reported that some inhabitants needed to rent, buy, or build homes. This document indicates that around 8.2 million homes are required in the country to cover this need. In contrast, the Housing Registry (RUV) platform reports that only 3,009,435 homes were built and registered between 2013 and 2024 [7], classifying homes into eight categories (Economy, Popular128, Popular158, Popular200, Traditional, Medium, Residential, and Residential-Plus). In the report, it is observed that the three most built homes are the Traditional (29.66%), Popular200 (23.86%), and the Popular158 (19.60%), while the most common constructed area size is between 45–60 m² (44.91%), >80 m² (20.66%), and 60–80 m² (19.84%). On the other hand, horizontal housing represents 72.21%, while the vertical housing accounts for 27.79%.

Nationwide, programs and policies have been put into practice to develop the country’s energy efficiency targets. The document “Perspective of energy efficiency programs and policies in residential buildings in Mexico” [8] groups sixteen representative policies according to the policy categories proposed by Tori et al. [9]. The analysis found that in Mexico, there are three regulatory-type policies: five economic policies, three related to building performance, three regarding information and motivation, and one policy in the categories of market transformation and target-group specific approaches. Other policies have also been implemented in Mexico, such as the sustainability policy within the National Housing Program 2019–2024, which aimed to ensure the right to adequate and affordable housing. The first target of the program sought to provide financial, technical, and social solutions to families; another target addressed the importance of promoting the use of energy-saving technologies [10]. Similarly, the Science Secretary SECIHTI (former Science Council CONAHCYT) includes actions on housing among its National Strategic Programs (PRONACES) due to its interest in promoting the development of sustainable housing [11].

In addition to national policies, urban planning plays a critical role in energy efficiency. Unplanned city expansion leads to increased energy consumption, GHG emissions, climate change, and environmental degradation. Because of this, and in line with SDG 11, the UN Urban Agenda favors compact, high-density, and well-planned urban development. Such developments can generate positive environmental impacts by relying less on motorized transport, reducing the urban footprint’s expansion rate, making better use of terrestrial ecosystems, and providing more efficient services [12,13].

Nevertheless, most vertical housing in Mexico has been allocated to the residential and higher-income segments. Most of the population requires low-cost housing, and it is observed that there is a limited supply of this type of construction [14]. This also represents a barrier to the attainment of affordable housing objectives. Although vertical housing currently represents a smaller percentage of housing in Mexico, it is essential to explore the benefits of vertical housing in the development of cities. The importance of the compact development of cities has been recognized in SDG 11. Among the national goals referring to Objective 11 of the SDGs, it has been proposed that by the year 2030, at least 66% of new homes built in the country’s cities follow vertical development criteria [15].

However, in Mexico, few documents focus on energy efficiency in apartments or multi-family buildings. For instance, Córdova and Alpuche Cruz [16] conducted a study in Hermosillo, Sonora, which evaluated how urban densification affects energy consumption in homes in a hot-dry climate. Through simulations with EnergyPlus and measurements with a thermal imaging camera, it was observed that vertical densification from four levels onwards significantly reduces (up to 57%) energy use intensity. Indicators such as the land use coefficient and the volume-to-land ratio showed a strong correlation (R² = 0.71) with the energy use intensity, highlighting their relevance for designing more sustainable cities. In another study in Hermosillo by Lopez-Ordoñez et al. [17], they analyzed how vertical densification reduces residential cooling demand. The authors carried out energy simulations of eight case studies of single and multi-family houses, supported by a comparison against historical electricity bills. The results showed that multi-family buildings consume up to 72% less energy for cooling than single-family houses due to a lower surface-to-volume ratio and a consequent decrease in heat exchange through the building envelope. These results highlight the potential of vertical housing to save energy and reduce energy poverty in cities with extreme climates.

Multiple studies were found for other parts of the world. For example, Sarkar and Jana [18] carried out an analysis in India to determine differences between marginalized horizontal dwellings and vertical retrofitted buildings. The authors determined that the change to vertical dwellings can have a positive effect on the satisfaction of the occupants. Greater thermal comfort was achieved, in addition to modifying their behavior, in the preference for indoor activities, the selection of the type of appliances and their time of use, among others. Also, Del Hierro López et al. [19] carried out a study on the potential application of self-sufficient photovoltaic systems in Madrid, Spain. To do so, the authors classified the buildings by construction year and by number of floors. They found that single-family buildings can be self-sufficient in producing more than 70% of the electricity they consume. In comparison, this percentage decreases by up to 10% in multi-family buildings with an increasing number of floors.

Similarly, Li et al. [20] analyzed data on energy consumption for heating and cooling of 36 multi-family buildings with up to 34 floors located in cold areas of China, with minimum and maximum temperatures of up to −17 °C and 37 °C, respectively. The findings showed that the annual heating consumption in Beijing is 45 to 80 kWh/m², while in Qinhaungdao, this consumption rises to 80 to 95 kWh/m². As for the cooling consumption during the summer, the requirement is between 2 and 28 kWh/m². On the other hand, Alonso et al. [21] analyzed the energy performance of two five-story multi-family buildings in Madrid, Spain, as part of a methodological proposal for the characterization and improvement of social buildings. They determined the gas consumption required to heat domestic water and radiators for air heating (21.22 kWh/m² and 29.98 kWh/m², respectively). The electrical consumption for lighting and household appliances was 31.34 kWh/m² and 44 kWh/m², whereas the total consumption was lower than the reference values stipulated in the legislation. Such a finding was associated with the unmet minimum levels of comfort and air quality.

Vallati et al. [22] simulated different energy consumption scenarios for heating and water heating in a three-story residential building in Rome, Italy. The study intended to improve energy efficiency by using various heat generation systems as substitutes for a natural gas boiler. The authors determined that reducing the primary energy demand by up to 28% is possible by using a hybrid system without modifying the building envelope. Subsequently, Vallati et al. [23] analyzed the same building to determine the potential for using heat pumps and thermal–photovoltaic hybrid collectors. Finally, Hernandez-Cruz et al. [24] measured the energy consumption of centralized systems for heating and domestic hot water systems in six buildings in the Basque Country. The authors determined that the average annual consumption for heating was 15.9 kWh/m² and 14.5 kWh/m² for water heating. Through the investigation, they could relate each home’s consumption to factors such as the home orientation, number of inhabitants, and their behavior.

In Mexico, there is a clear interest in improving the efficiency of buildings and developing sustainable housing. This is demonstrated by the implementation of national policies such as energy efficiency standards and similar programs implemented over the last decades. Nevertheless, although the general policies already exist, it was found that few studies explore the current state of energy usage on vertical residential buildings in Mexico. The current low availability of such studies represents a technical barrier that limits the development of the sector and delays the sustainable transition. The main objective of this research was to create a reference building to diagnose the current energy consumption of a multi-family building in eight cities in Mexico located in five different Köppen–Geiger climate zones. The analysis is presented as a first step in understanding their energy performance and influencing the reduction in energy consumption to promote the design and construction of affordable low- or zero-energy-use buildings. The addressed research questions include the following: (1) What is the magnitude of the current energy requirement of a proposed Mexican multi-family residential building? (2) What are the energy consumption differences between cities with different climates? (3) Which energy service drives the energy requirements of multi-family buildings in Mexico?

2. Methods

The methodology of this work consists of three main sections. The first focuses on selecting the study cities and their climatological parameters. Subsequently, the considerations taken for establishing a reference building, which is the basis for this study, are detailed. The Energy Performance of Buildings Directive (EPBD) (EU/2010/31 [25]), which established the criteria for developing nearly zero energy buildings (nZEB) in the European Union, was used as a basis for this work. Considering the directive’s requirements, statistical information from existing governmental sources [6,26,27,28,29] was collected to create a reference building representative of a multi-family vertical dwelling in Mexico. Later, the thermal and energy performance of the building was evaluated in seven Mexican cities.

Lastly, the parameters considered for the calculation of the building’s energy requirement are presented. The requirements are divided into two types: (1) fixed energy consumption (lighting, appliances, cooking) and (2) variable energy consumption (DHW (Domestic hot water), heating and cooling).

2.1. Studied Cities and Climate

The ratio of required to inhabited housing was used as the main criterion to select the cities for this study [6]. This value was used as it is foreseen that the states with the greatest need for housing will shortly have to build more housing. Consequently, they will also need to develop sustainable buildings. Secondary criteria included geographical and climatic type representation.

The selected cities can be observed in Figure 1. In this investigation, eight cities were chosen for this study. We used their characteristic climatic conditions in the energy building simulation to understand how the building performs under these conditions. The selected cities were chosen considering two factors: (a) The ratio of required to inhabited housing, as cities in states with a higher value were preferred. It is foreseen that the greatest need for housing will shortly increase the demand for housing construction, consequently increasing the need to develop sustainable buildings. (b) The geographic and climatic distribution, distribution of cities in the territory was privileged. Although this was not a restrictive parameter, diverse climatic representation was favored.

The climate data of the selected cities were obtained from the Meteonorm v8 program. The Meteonorm v8 program was used to obtain a Typical Meteorological Year (TMY2) for each selected city. These files were later incorporated into the TRNSYS 18 software through Type-15, one of the elements that allows the user to manage weather variables in TRNSYS. TMY is a data file with the local climate of the Typical Meteorological Year. These data consider a representative year that summarizes the climatic information characterizing the place from data measured over a long period, ideally the lifetime of the subject of interest, in this case, a building.

Table 1. Climate parameters of the studied cities.

City	Köppen Climate Classification	Ambient Temperature (°C)			Relative Humidity 1 (%)	Global Horizontal Irradiation 1 (kWh/m2)	Precipitation 1 (mm)
		Annual Average	Minimum	Maximum
Chilpancingo	Aw	23.6	14.0	34.1	48.4	2244	991
Durango	BSk	18.6	3.6	32.7	40.8	2051	651
Hermosillo	BWh	25.6	9.1	39.7	37.3	2036	661
La Paz	BWh	25.7	13.5	37.2	52.8	2153	210
Mexico City	Cwb	18.0	8.4	26.2	46.7	1826	887
Tepic	Csa	22.6	9.9	32.8	65.8	2038	2277
Tlaxcala	Cwb	16.8	6.1	27.2	52.4	2033	1061
Tuxtla	Aw	24.3	15.4	33.3	66.4	1963	2281

¹ Annual average.

shows the annual averaged ambient temperatures, relative humidity, global horizontal radiation, precipitation, and minimum and maximum ambient temperatures. The annual average ambient temperature in the selected cities ranges from 16.8 °C (Tlaxcala) to 25.7 °C (La Paz). The city with the highest ambient temperature is Hermosillo (39.7 °C), while the one with the lowest is Durango (3.6 °C).

2.2. Reference Building (RB)

Using reference buildings as a baseline has proven useful when assessing buildings from a national policy perspective. The concept is also used in the EU/2010/31 Directive, where it is established that the reference building has characteristics (e.g., construction, functional, geometric, geographic) that make it representative of the national stock of its building type (residential, non-residential, among others). One of the methods described in the literature as helpful in creating a reference building is the Synthetical Average Building. Such a method uses the statistical information available of certain buildings to generate a reference building [30]. Therefore, in this investigation, a reference building was designed to represent the geometry and construction practices used in the country. This aims to serve as a starting point for understanding the energy requirements of a multi-family building built in different cities in Mexico.

According to the Housing Building Code [28], single-family homes and homes grouped in horizontal buildings must have a maximum of 3 levels. In contrast, multi-family homes grouped in vertical buildings must have a maximum of 5 levels, with or without an elevator. For the reference building, it was decided to outline a building with a large surface area to promote the greatest possible coverage by renewable energy systems and favor the compact urban development recommended for sustainable cities [12,13,31]. Different examples of multi-family housing were analyzed for the distribution of interior spaces. A configuration resulting in a symmetrical building was chosen to limit the effect of the building orientation on the energy simulations. The distribution of the rooms in the homes, their dimensions, and their height (2.7 m) were selected considering the criteria defined in the Housing Building Code [28].

The building was modeled in TRNSYS 18, where five thermal zones were defined for each floor in the building model.

Table 2. Thermal comfort temperatures [32].

Ciudad	Minimum Temperature (°C)	Maximum Temperature (°C)
Chilpancingo	21.4	28.2
Durango	21.7	28.3
Hermosillo	21.7	28.4
La Paz	21.3	28.1
Mexico City	21.5	28.1
Tepic	21.2	27.5
Tlaxcala	21.3	28.0
Tuxtla	20.9	27.8

shows the comfort temperatures, which were calculated by evaluating typical climatic conditions during winter and summer in each of the seven cities. With these characteristics, the ASHRAE-55 standard was used to calculate the thermal comfort range for each city [32]. Here, thermal comfort can be defined as a state of mind in which a person feels satisfied with their thermal environment and does not feel cold or hot. A clothing level (clo) of 0.5 was used, and an indoor wind speed of 0.2 m/s was implemented for the summer. In contrast, a clo of 1.0 and an indoor wind speed of 0.05 m/s were considered for winter.

The thermal comfort range and the home’s occupancy profile determine when the air conditioning systems are active. Table 2 lists each city’s temperatures used to activate the air conditioning systems. The heating system activates at the minimum temperature, while the cooling system turns on at the maximum. Heating systems are devices that use energy to produce heat to maintain the air temperature at a desired level. The gap between these two temperatures has an average value of 6.67 °C among the eight cities.

2.3. Energy

Electricity and gas consumption of homes were calculated for each studied city. Fixed energy consumption was calculated based on lighting, appliances, and cooking. In contrast, variable energy consumption includes heating, cooling, and water heating. Lastly, it is essential to highlight that in estimating electrical energy consumption, the energy consumption by the air conditioning system was calculated through TRNSYS simulations. Likewise, the gas required for water heating was calculated for each city’s climatological conditions.

2.3.1. Fixed Energy Consumption

Lighting

According to the results from the National Survey on Energy Consumption in Homes (ENCEVI) [26], fluorescent lamps are the most common type, as 72% of respondents said they use them in their homes. Consequently, fluorescent lamps were used to quantify the lighting energy consumption.

A study was carried out with the DIALux^® evo 11 software to quantify the natural lighting and the power needed for the lighting systems within the proposed building geometry. This free-use program simulates the lighting conditions of buildings using photometric files of real luminaires [33]. For the calculation, a work area was established in each space at a height of 0.8 m from the ground. Luminaires that used a compact fluorescent type lamp and whose application was residential type were selected from the LUMsearch^® library. The lighting system was designed to meet recommendations for power density and illuminance levels, since no official regulations exist nationwide for residential buildings [34,35]. Lastly, each home’s annual energy requirement for lighting was obtained using the software LENICALC V3 from the Italian National Agency for New Technologies, Energy, and Sustainable Economic Development (ENEA) [36].

Appliances

Data on appliance usage in Mexican homes and their standard energy efficiency ratings were gathered. Such information was collected mainly from the ENCEVI Survey [26] and the NOM-ENER standards [37,38] (

Table 3. Appliance usage [26,29,37,38,39,40].

Appliance	Frequency of Use (Days/Month)	Average Daily Use (min)	Electrical Power (W)
Blender	18	10	550
Coffee maker	19	29	900
Hairdryer	13	17	1875
Iron	4	65	1302
Laptop/PC	16	153	32
Microwave	16	15	1570
Mixer	3	16	200
Modem	30	1384	5
Refrigerator	30	1440	401 *
Television	28	224	50
Toaster	9	12	800
Washing machine	4	127	203 **

* Annual average energy consumption (kWh), ** average energy consumption per wash cycle (kWh).

). Table 3 lists appliances commonly found in a typical Mexican home. Also, the average number of days they are used per month and for how much time (in minutes per day) were added, according to ENCEVI data.

In addition, information from the quality reports of the Federal Consumer Protection Bureau (PROFECO), which analyzes products in the Mexican market, was used to select a typical power for each device when possible [29,39,40], as shown in Table 3. For most appliances, consumption was calculated by multiplying its electrical power and average use time. The two exceptions were the refrigerator and the washing machine, for which energy consumption was reported as an annual average value and as an average per wash cycle, respectively. The frequency of use varies significantly among appliances; however, even a device with a high frequency will have low consumption if the electrical power is small.

Cooking

According to information from ENCEVI, most Mexican homes have a gas stove for cooking. Likewise, it is reported that most people use two burners simultaneously for an average of 1 h and 54 min. Also, the quality study data from PROFECO was used to select a commonly bought stove [41].

2.3.2. Variable Energy Consumption

Domestic Hot Water (DHW)

In Mexico, 43.5% of homes have equipment intended for water heating, with 14.3 million units in use [26]. Gas is the most used fuel in these units, with 11.2 million gas water heaters; in contrast, there are only 1.9 million solar heaters. Additional data show that the most common water heater technologies are gas heaters with storage tanks (47%), instantaneous gas heaters (17%), and solar heaters (12%).

The methodology described by García-Valladares and Ituna-Yudonago was adapted to determine the amount of gas used in the building to heat water [42] by not considering solar contribution. In this methodology, the amount of heat necessary to raise the water temperature from the network to a desired temperature is quantified. Subsequently, the amount of gas required is calculated using such heat requirement. To obtain the temperature of the water in the network in different cities, it was decided to consider that the water in the building is stored in an underground space before being distributed to the homes. Therefore, it is estimated that the temperature of the water in the network is similar to the ground temperature. Each person is assumed to require 40 L of hot water per day at 40 °C. Thus, the daily hot water demand is calculated by multiplying the number of inhabitants by the per capita water requirement. The calculation of the energy necessary to heat the water was carried out with Equation (1):

$$Q_{req}=m_{water}{Cp}_{water}\left(T_{water,user}-T_{water,network}\right),$$

(1)

where $Q_{req}$ is the energy required daily for water heating in the house, in MJ; $m_{water}$ is the mass of water used daily, in kg; ${Cp}_{water}$ is the specific heat of the water at constant pressure, in MJ/(kg °C); $T_{water,user}$ is the user’s required hot water temperature, in °C; and $T_{water,network}$ is the temperature of the water from the house network, in °C. The calculation considered monthly temperature averages in the six cities of interest. Subsequently, the amount of gas needed ($m_{gas}$) was obtained applying Equation (2):

$$m_{gas}={\displaystyle \frac{Q_{req}}{{\eta }_{heater}{CV}_{gas}}},$$

(2)

using a heating efficiency (${\eta }_{heater}$) of 0.76 obtained from a system in the market that complies with the minimum efficiency allowed [43], and the calorific value (${CV}_{gas}$) of liquefied petroleum gas (LP gas or LPG) (48.36 MJ/kg).

Heating and Cooling

For this study, mini-split equipment was selected to maintain a comfortable temperature indoors. According to the ENCEVI survey [26], the most commonly used cooling system in Mexico is the on–off mini-split (40%). On the other hand, ENCEVI shows that only 6.3% of homes in the country have a heating system. As a consequence, an electric heating system was implemented to estimate the heating requirements of the buildings, as it has an estimated usage of 48.3% of the total. The device selected has a SEER of 11.5 W/W for cooling and a COP of 3 W/W for heating, meeting the standard for this type of air conditioning system [44].

The energy needed for maintaining the air temperature within the comfort ranges was calculated through dynamic simulation using the TRNSYS 18 software. During the simulation, the air conditioning system activates when the air room temperature of each house is out of the comfort range and when the occupancy profile indicates that there are people inside the home.

A significant difference between the calculated data and the survey results is the number of homes that use air conditioning. ENCEVI groups the states in the northern, center, and southern regions. For further reference, Supplementary File S1 shows this information and lists the percentages of use of cooling and heating systems stock by state. Appendix A summarizes the characteristics and parameters considered for the reference building.

2.4. Comparison with Available Data

The estimated electricity consumption was compared with data from the ENCEVI survey [26]. Although the survey does not directly measure household electricity use, it provides self-reported electricity bills from respondents. Consequently, the average bimonthly payment was calculated by considering the applicable electricity tariff for each studied city. Then, the bill payment was computed considering the applicable rate, and finally, a comparison was carried out.

A similar comparison was made regarding gas consumption to assess the calculation made with the RB’s assumptions. It is essential to mention that the most common gas purchase in Mexico is made in the form of gas tanks delivered to homes. ENCEVI states that the most frequent gas purchases are for 30 kg (large) and 20 kg tanks (small). Large tanks are the most bought form in La Paz, Durango, Hermosillo, Tepic, and Tuxtla. Meanwhile, small tanks are most widely sold in Mexico City, Chilpancingo, and Tlaxcala. Later, an estimation of the duration of the most common recharge (either small or large tank) in each average home was made. Next, such estimated duration was compared to the ENCEVI answers. As a result of being limited to this range and its percentage distribution, the comparison consists of finding the number of cities which estimated gas duration falls into the most common duration for each city.

3. Results

This section presents the results in three parts: (1) parameters and characteristics of the reference building, (2) energy use analysis concerning its fixed, variable, and total energy, and (3) a comparison of calculated energy use with the ENCEVI survey.

3.1. Configuration of the Reference Building

The RB was designed to reflect typical Mexican construction practices and building geometry, aiming to provide an understanding of the energy requirements of a multi-family building. Below, all the characteristics of the reference building are described, which were based on statistical information of buildings constructed in the country and the energy use in an average home.

3.1.1. Building

The RB floor is built on a 368.64 m² surface; the total built area is 1843.20 m², whereas the habitable area is 1293.12 m². Figure 2 shows the building design and the residential floor’s configuration. The building has five floors; the ground floor is occupied by commercial space, and the four subsequent floors correspond to residential use. The residential floors have the same configuration: four homes were placed on each floor; two are two-bedroom homes, and the other two are three-bedroom homes.

The distribution of the two housing configurations is shown in Figure 3. Here, it is shown that both houses have external dimensions of 13.25 m wide by 6.10 m long, with a total surface area of 80.82 m². According to the Housing Building Code, it is placed in the Traditional housing category (average constructed area of more than 70 m²). Also, the building has five thermal zones on each floor, which results in 25 thermal zones for the entire model (Figure 4).

3.1.2. Materials

According to the 2020 National Housing Survey [6], 92.4% of the country’s homes have walls consisting of materials such as bricks and concrete blocks [45,46]. When considering the development of multi-family housing, walls built of concrete blocks facilitate their reinforcement with steel [47]. Concerning the roofs, the National Housing Survey (ENVI) indicates that 78.4% of homes have a concrete slab roof or a joist system with a vault. Of the 28,953 homes interviewed in the Energy Consumption Survey (ENCEVI), 4.7% stated that they had ceiling insulation, 1.0% indicated that they had some insulation in the walls, and only 0.54% had specialized windows [26].

Therefore, for this study, the external walls of the building were considered to be uninsulated concrete blocks with exterior mortar coating and interior plaster. Interior walls were made of concrete blocks with plaster on both sides, and the roof was a reinforced concrete slab with a joist and vault system.

Table 4. Constructive systems configuration and properties [48,49,50,51].

Building Component	Exterior Walls	Interior Walls	Roof	Windows
Materials	Mortar, concrete block, gypsum plaster	Gypsum plaster, concrete block, gypsum plaster	Cast concrete, lightened slab, stucco	Single-glazed, aluminum frame
Thickness (m)	0.200	0.190	0.170	0.003
U-value (W/m2·K)	2.312	2.279	0.530	0.855 *

* SHGC: Solar heat gain coefficient.

shows the elements of the building configuration and its properties.

3.1.3. Occupancy

With data from 2018, the National Housing Commission (CONAVI) indicates that most homes are inhabited by three and five people [52]. Similarly, in 2020, the average number of occupants per home was 3.6 [53]. For this study, it has been decided to consider an average occupancy of 3.5, with 14 persons per floor, totaling 56 persons in the building. The family configuration is 2: two adults, 3: two adults, one minor, 4: two adults, two minors, 5: two adults, two minors, and a 65+ adult. The distribution of inhabitants on each floor is described in

Table 5. Floor occupancy.

	W (2 Bedroom)	N (3 Bedroom)	E (2 Bedroom)	S (3 Bedroom)
P1 (first floor)	2	4	3	5
P2 (second floor)	3	5	2	4
P3 (third floor)	2	4	3	5
P4 (fourth floor)	3	5	2	4

.

Statistical information from the 2019 National Time Use Survey (ENUT) [27] on how Mexicans use their time was used to create an occupancy profile for all home inhabitants. From this survey, the average time that Mexicans spend in outdoor activities (cultural and sports events, sports and exercise, and shopping) was calculated. It was found that, on average, each spends 11 h of non-domestic activities per week and sleeps an average of 7.57 h daily. Considering this, an overall occupancy profile was defined where each occupant carries out 59 h of domestic activities and sleeps 53 h per week. In total, each house has 112 weekly hours where a person is inside the home and needs potential air conditioning to achieve a comfortable environment. The simulation considered different occupations from Monday to Friday and weekends, but no other seasonal variations like holidays or similar. It is worth mentioning that the house with five inhabitants has a particular characteristic, as the 65+ family members stay at home 24 h. Figure 5 shows the occupation profile, where the hours that the person spends at home and uses air conditioning are marked with an “x”.

3.2. Energy Use

3.2.1. Fixed Energy Consumption

Fixed energy consumption includes lighting, appliances, and cooking. The energy consumption of lighting and appliances was added to estimate the fixed energy consumption, as shown in

Table 6. Summary of monthly fixed energy consumption.

Home	Appliances (kWh/Month)	Lighting (kWh/Month)	Fixed Electricity (kWh/Month)	Cooking (kg/Month)	Fixed LPG (kg/Month)
Two-bedroom	76.52	30.27	106.79	11.12	11.12
Three-bedroom	82.95	32.37	115.32	11.12	11.12

. For the two-bedroom house, the estimated annual energy consumption for lighting was 363.3 kWh/year (30.27 kWh/month), and for the three-bedroom home, it was 388.5 kWh/year (32.37 kWh/month).

Furthermore, as described in the Appliances Section, each appliance’s consumption was calculated by multiplying its average electrical power and average use time (Table 3). The refrigerator was an exception, for which its annual energy consumption is reported in its technical data due to the intermittent behavior of its components. Therefore, the fixed electricity consumption is 76.52 kWh/month for the two-bedroom home and 82.95 kWh/month for the three-bedroom home. Lastly, a stove whose standard burners have a measured thermal capacity of 5126 kJ/h and an efficiency of 59% was chosen. Considering that LPG has a calorific value of 48.36 MJ/kg [54] and a density of 0.54 kg/L [55], the calculated daily consumption of gas for cooking food was 0.40 kg (11.12 kg per month).

3.2.2. Variable Energy Consumption

The variable energy requirement, which corresponds to DHW and the use of heating and cooling systems, is shown in Figure 6a and Figure 6b, respectively. Regarding DHW (Figure 6a), it was expected that cities with temperate climates had high energy requirements for this energy service. Tlaxcala and Mexico City need up to 32.8% more energy than the city that requires the least (La Paz). In contrast, Figure 6b shows the cities in descending order regarding energy requirements to condition the spaces. Tlaxcala, Durango, and Hermosillo are the cities that need the most energy. Tlaxcala, Durango, and Mexico City would have a dominant energy requirement for heating, while Hermosillo, La Paz, Tuxtla, and Chilpancingo have dominant cooling requirements.

DHW

It was found that domestic hot water (DHW) is a driver of the variable energy requirement for all cities. For the calculation, it is assumed that each person requires 40 L of water per day (at 40 °C); therefore, 2240 L per day needs to be heated for the building.

Table 7. Building’s annual DHW requirement.

City	Calorific Energy (MJ/Year)	LPG (kg/Year)
Chilpancingo	54,681	1488
Durango	65,959	1795
Hermosillo	47,846	1302
La Paz	45,795	1246
Mexico City	76,554	2083
Tepic	52,972	1441
Tlaxcala	76,554	2083
Tuxtla	56,390	1534

shows the heating energy requirements and the quantity of LPG in kilograms for the building in each city. The buildings in cities requiring the most energy would need 2083 kg of LPG annually (Tlaxcala and Mexico City). In contrast, La Paz and Hermosillo need 1246 kg and 1302 kg, respectively.

Heating

The cities in Figure 7 and Figure 8 (heating and cooling energy requirement) are arranged in descending order concerning energy requirements. In addition, the dwellings can be identified by their floor number (1, 2, 3, 4) and their orientation (N, S, W, E), i.e., 3W refers to the third-floor dwelling facing west.

Figure 7 shows that the reference buildings in Durango and Tlaxcala have the highest heating requirements. Tuxtla and Chilpancingo have a negligible heating requirement. In La Paz, it is considered low; this could result in the families opting not to install a heating system. The top-floor dwellings require more heating energy than those on lower floors. Indicating that in vertical housing, middle dwellings benefit from losing less heat through the roof compared to upper dwellings. Considering the above and keeping in mind balancing the energy consumption of the building, it may be advisable to add thermal insulation to the roof of the upper dwellings. But it is not advisable to add it to the mezzanine floors of the building.

The dwelling that consumes the least amount of heating is the 2S (with only four occupants), which is to be expected due to the more significant amount of solar radiation received during the year. Likewise, the dwellings facing west (except those on the 4th floor), together with those facing south (except those with five occupants), are those that consume the least amount of heating.

Cooling

Regarding cooling energy (Figure 8), Tlaxcala and Mexico City did not show cooling energy requirements for this purpose. Hermosillo and La Paz have substantial cooling needs; this is natural due to their warm climate. The home that requires the most energy for cooling is 4N in all cases, while the one that requires the least energy is 2S. On the other hand, when looking at the national average per floor, it can be noted that the first floor requires more cooling than the following floors. The dwellings that consume the least energy for cooling are 2E and 4E.

3.2.3. Total Energy Use

Table 8. Summary of energy requirements of the building (kWh/year).

	Chilpancingo	Durango	Hermosillo	La Paz	Mexico City	Tepic	Tlaxcala	Tuxtla
Lighting 1	6014
Appliances 1	15,309
Cooking 2	28,669
Water heating 2	19,981	24,102	17,483	16,743	27,973	19,356	27,973	20,605
Heating 1	241	17,072	5824	835	11,901	1812	17,460	287
Cooling 1	1527	343	7559	6256	5	2084	1	2968
Fixed energy	49,992
Variable energy	21,749	41,517	30,866	23,834	39,879	23,252	45,434	23,860
Total	71,741	91,509	80,858	73,826	89,871	73,244	95,426	73,852
Electricity	23,091	38,738	34,706	28,414	33,229	25,219	38,784	24,578
LPG	48,650	52,771	46,152	45,412	56,642	48,025	56,642	49,274

¹ Electricity, ² LPG.

summarizes the building’s annual fixed and variable energy requirements. The reference building in Tlaxcala has the highest energy requirement (95,426 kWh/year). In contrast, the reference building with the lowest requirement is in Chilpancingo (71,741 kWh/year); this represents a difference of almost 25% between the cities with the highest and lowest consumption. Also, it is observed that the elevated energy requirement in Tlaxcala is mainly driven by water heating systems and the need for space heating.

The reference building has an annual fixed energy requirement of 49,992 kWh, of which 6014 kWh is for lighting, 15,309 kWh is for the operation of household appliances, and 28,669 kWh is for cooking; such requirements are the same for all the cities. On the other hand, the variable energy requirements of the reference building, which are different for each city, show that the building in Tlaxcala (45,434 kWh/year) would require more variable energy; contrastingly, Chilpancingo requires the least (21,749 kWh/year).

It is worthwhile to point out that DHW energy requirements are substantial for all cities. DHW accounts for 57% to 92% of variable energy requirements and 22% to 31% of total building energy requirements. Mexico City and Tlaxcala have the highest DHW energy requirements, and La Paz has the lowest. There is a 40% difference between La Paz and the cities with the highest energy requirements.

On the other hand, when analyzing heating requirements between cities, the reference buildings that require the least energy are those in Chilpancingo (241 kWh/year), Tuxtla (287 kWh/year), and La Paz (835 kWh/year). The buildings that need the most energy are those in temperate climates, such as Tlaxcala (17,460 kWh/year) and Durango (17,072 kWh/year). It is observed that there are two cities in which the building would not need cooling systems: Tlaxcala and Mexico City. On the contrary, cities such as Hermosillo and La Paz would need up to 7559 kWh/year and 6256 kWh/year, respectively.

Also, when examining energy consumption concerning fuel (electricity or LPG), it is observed that the buildings that would need more electricity are Tlaxcala and Durango, which have temperate climates. The cities that follow such high electricity consumption are Hermosillo and Mexico City. On the other hand, the buildings that would use more LPG are those in Mexico City and Tlaxcala, remembering that these cities also have the highest requirement for DHW fueled by gas.

3.3. Comparison

3.3.1. Electricity

Table 9. Comparison of average electricity billing against ENCEVI ($) [26].

City	Average Bill According to ENCEVI	Estimated Energy Bill for the Average Simulated House
		Fixed Energy		Fixed Energy and Cooling and Heating		Fixed Energy and Cooling
	($MXN)	Average ($MXN)	Differences (%)	Average ($MXN)	Differences (%)	Average ($MXN)	Differences (%)
Chilpancingo	344	230	33	251	27	248	28
Durango	253		9	721	185	234	8
Hermosillo	392		41	392	0	321	18
La Paz	375		39	315	16	305	19
Mexico	277		17	522	88	229	17
Tepic	289		21	277	4	254	12
Tlaxcala	208		10	721	247	229	10
Tuxtla	249		8	269	8	265	6
Average	298		22	434	72	261	15

shows a comparison of the average electricity bill reported by the participants of the ENCEVI survey, sorted by state and for houses of similar size to the one in this study, against the simulated results from the homes in the reference building. Three different sets of results are presented to explore variations in household energy use: (a) fixed energy (only lighting and appliances), (b) fixed energy plus cooling and heating usage, and (c) fixed energy and cooling.

First, Table 9 presents the comparison against a home that consumes electricity only for basic needs (lighting and appliances) and does not use heating and cooling equipment. It is observed that such a bill is the same for all cities; this is because, within all electricity tariffs, the winter billing in the first 75 kWh (150 kWh bimonthly billing), known as the basic range, is the same for all the country. The energy billing differentiation starts from the intermediate range onwards, but such a range is not achieved for the analyzed homes. In this comparison, the differences between the energy bills in the cities range from 8% to 39%.

Table 9 also shows that when the cost of electricity bills accounts for the use of both cooling and heating systems, many differences occur in cities with temperate climates (Durango, 185%; Mexico City, 88%; Tlaxcala, 247%). This phenomenon seems consistent with the information reported in ENCEVI regarding the low rate of heating used in homes (≅20%). Thus, an estimation was made considering electricity bills excluding heating, where the differences are lower, ranging from 6% to 28%. The city with the greatest difference is Chilpancingo (28%), while a minor difference occurred in Tuxtla Gutiérrez (6%). Supplementary File S2 shows a detailed comparison between the billing of all dwellings and ENCEVI.

Although the simulation of the homes guarantees perfect thermal comfort, in practice, not all homes have the resources to acquire the equipment or the disposition to condition the indoor air whenever required. For example, according to ENCEVI data, the northern states use air conditioning systems in the highest percentage. However, even in these states, only 48% of homes have a cooling system, while only 20% use some system to heat the indoor air (see more details in Supplementary File S1).

3.3.2. Gas

The estimated gas consumption for the average home was calculated and presented in

Table 10. Comparison of average gas consumption against ENCEVI [26].

City	Reference Building			Gas Duration (Months) in Local Surveyed Homes; Percentage Distribution According to ENCEVI					Accordance Between RB and the Most Frequent Answer in ENCEVI
	Average Home Gas Consumption	Most Common Gas Recharge	Estimated Recharge Duration
	(kg/Month)		(Months)	<1	1–2	2–4	4–6	>6
Chilpancingo	18.9	Small	1.1	21%	37%	32%	8%	2%	✓
Durango	20.5	Large	1.5	18%	33%	32%	13%	4%	✓
Hermosillo	17.9	Large	1.7	9%	30%	32%	19%	9%
La Paz	17.6	Large	1.7	5%	20%	37%	21%	16%
Mexico City	22.0	Small	0.9	41%	38%	13%	6%	1%	✓
Tepic	18.6	Large	1.6	2%	18%	43%	24%	13%
Tlaxcala	22.0	Small	0.9	46%	36%	16%	2%	0%	✓
Tuxtla	19.1	Large	1.6	6%	20%	43%	16%	14%

. After making this comparison, an agreement was found between the survey data and the simulation results from Chilpancingo, Durango, Mexico City, and Tlaxcala. In Chilpancingo and Durango, the survey reports that most homes have a one to two-month duration of their LPG tanks, while this investigation results estimated a 1.1 and 1.5 month recharge duration, respectively. Similarly, this work reports a duration of 0.9 months in Mexico City and Tlaxcala, matching the survey reports. In contrast, the gas tank duration was underestimated for the rest of the cities, meaning that the gas tank would last for less time than in reality. For example, in Hermosillo, the ENCEVI discloses that homes have a gas duration of two to four months (32%), and in this calculation, it was less than two months (1.7). However, it is observed that the second most common tank duration in this city (30%) was between one and two months, so this work’s estimated value is not as far from what was reported.

On the contrary, if we look at the results of cities with less extreme climates—more homogeneous temperatures throughout the year (i.e., Tepic)—where the most frequent tank duration is between two and four months (43%). The second most frequent duration is four to six (24%), and the third frequency is one to two months (18%), which matches this work’s estimate.

4. Discussion

This study develops a reference building (RB) and an energy assessment of its operation in eight Mexican cities as an initial step toward nZEB implementation in Mexico. The RB was created using statistical data available from the national residential stock.

The proposed RB requires land of less than 370 m² and has a living area of 1293 m², distributed in four dwellings of 80.82 m² per floor. According to the Housing Building Code, this dwelling size classifies it as “traditional” [28]. The building consists of concrete block walls, the roof has a lightweight concrete slab, and the glazing comprises single-glazed windows with aluminum frames.

The energy requirements of the RB were calculated using simulations in TRNSYS, differentiating between fixed energy (appliances, lighting, and cooking) and variable energy (DHW, heating, and cooling). The energy requirements were calculated for the entire building, but the information was also disaggregated by dwelling, floor, and orientation. Similarly, the energy consumption was classified according to the energy source used: electricity or LPG.

Monthly fixed energy requirements ascend to 107 kWh and 115 kWh of electricity in the two-bedroom and three-bedroom houses, respectively, and 11.12 kg of LPG. Such requirements range on average from 53% (in Tlaxcala) to 70% (in Chilpancingo) of the total energy requirement of the house.

On the other hand, the need to heat sanitary water (DHW) is a key driver of the variable energy requirements in all cities. This is particularly true in the cities of Tlaxcala and Mexico City, which do not have a particularly warm climate. This contrasts Hermosillo and La Paz, which are cities with a predominance of hot weather throughout the year.

Tlaxcala and Durango, which have temperate climates, have the highest electricity consumption for RB, mainly used to operate heating systems, representing more than 97% of the electricity associated with the thermal comfort of the occupants. In contrast, the cities that require the least energy are Chilpancingo, Tuxtla Gutierrez, and Tepic, and they have a dominant requirement for cooling.

Dwellings on the upper floor require more heating energy. This results in advantages for the dwellings on the other floors, as they benefit from losing less heat through the roof. Similarly, homes facing south—except for the homes with five persons and homes facing west—except those on the top floor, consume the least heating energy. This is because homes with five occupants (1S, 2N, 3S, and 4N) have higher heating and cooling consumption; such a phenomenon occurs since these homes have 24 h usage of heating and cooling systems. This consideration was made assuming that one of the family members remains home all day; however, this scenario also represents homes with poor energy habits regarding using these systems.

Comparisons with ENCEVI data show that the reference building model allows consumption to be calculated within the range of values that occur in practice within the Mexican housing sector. The electricity comparison is a good match, especially considering that most Mexican households do not own heating systems or prefer not to use them, probably due to economic and cultural practices.

When analyzing electricity bills that include fixed energy, heating, and cooling, significant discrepancies were found between ENCEVI survey data and simulation results. In contrast, when the use of heating systems was excluded, the differences were reduced to an average of 15% (ranging from 6% to 28% in the cities studied). Regarding gas use, the results indicated that the implemented methodology estimates gas consumption more closely in cities with temperate climates but may underestimate consumption in other climates.

Therefore, the gas comparison was more challenging due to ENCEVI survey data format limitations. Such data shows that RB tends to adequately reproduce gas consumption in temperate climates but overestimates it in hot climates. Nevertheless, identifying the actual overestimation with the available data format was found to be unfeasible.

In this investigation, the characteristics of the building were chosen based on the available statistical data. These decisions aim to represent the average conditions of the dwellers. However, in practice, several cultural customs or habits that are not considered in this study due to their complexity and variability are known factors that influence energy consumption in buildings. These actions may include construction decisions, regional holidays, individual energy habits, families suffering from energy poverty, or personal preferences. For example, some families prefer a distinct water temperature for showering, and others do not own heating or cooling systems.

Also, it is crucial to consider that it is complicated to reproduce the conditions and customs of each home. Lastly, it is vital to remark that it would be desirable to have access to more quality and recent data sources to evaluate this investigation’s findings properly.

5. Conclusions

Although a reference building is designed to represent a specific building type, it cannot fully replicate the diversity of the national building stock. Nevertheless, RBs can be considered a valuable tool for evaluating energy performance studies, particularly those carried out during the development of national policies, where it is important to have a baseline from a wide group of buildings of the same kind. In Mexico, there are still not enough tools and databases focused specifically on multi-family buildings, despite the need to densify some regions of the country.

It is considered that through the development of this study, a positive impact can be attained for different actors. Mainly due to the importance of the building sector and the role that housing plays in people’s daily lives. Considering the above, the development of energy-efficient housing can help reduce energy poverty in homes. In addition to the economic aspects, the development of low or zero-energy housing can favor people’s health. Likewise, homes designed to maintain a comfortable temperature range contribute to reducing the effects of fluctuating temperatures inside homes. Lastly, developing energy-efficient housing can help reduce air pollution levels through the diminished use of electricity and LPG, reducing cardiorespiratory diseases derived from air pollution.

Vertical and multi-family buildings are a key option for developing sustainable housing. This study lays groundwork for the transition towards nearly zero-energy buildings (nZEB) in Mexico by providing an initial energy model of a typical multi-family building. However, this transition faces challenges nationwide, which still need to be addressed, alongside technical aspects in future studies. For example, only a few studies address the actual energy use in vertical buildings, limiting the availability of primary data for designing effective strategies and policies for the transition to nZEB. It is also necessary to take into consideration economic and financing constraints. The need to build low-cost housing to meet current demand may compete with the initial investment in nZEB technologies if there are no appropriate financing mechanisms. Finally, due to electric tariffs being subsidized for residential users in Mexico, particularly in warm climates, nZEB may be perceived as less favorable from the end user perspective.

When designing and implementing nZEB buildings, specific implications must be considered, especially in regions where these concepts have not yet been incorporated into current regulations. For instance, nZEBs are designed to minimize energy consumption through various techniques that can significantly reduce dependence on fossil energy sources. Also, many nZEBs generate energy from renewable sources, contributing to sustainability and reducing carbon emissions. However, some places have not fully developed the infrastructure for integrating renewable energy or affordable, efficient technologies. The nZEBs help mitigate climate change by reducing greenhouse gas emissions associated with buildings, among the largest energy consumers globally.

The following policy recommendations and building code revisions can be considered to facilitate the transition to nearly zero-energy buildings (nZEB):

• Update building codes by incorporating energy efficiency requirements and minimums on renewable energy use into national building codes, such as establishing limits on primary energy consumption and promoting using materials that enhance thermal performance.
• Develop professional training programs for architects, engineers, and builders in techniques and technologies. Create skills certification systems to ensure compliance with standards.
• Launch educational campaigns to inform the public about the benefits of better-performing buildings in terms of energy efficiency, economic savings, and environmental impact.
• Conscious implementation of subsidies or low-interest financing for projects that adopt nZEB standards, motivating developers and owners to invest in sustainable technologies.
• Intensify actions to promote research and development by funding research projects that seek innovations in design, materials, and technologies for nZEB buildings adapted to local cultural and climatic conditions.

On the other hand, in Mexico and other countries, nZEB standards are nonexistent, inadequate, or ongoing. When generated, they must be tailored to specific climatic, economic, and social conditions, including developing regulatory frameworks promoting implementation. The lack of clear and consistent regulations and the absence of incentives can hinder the implementation of effective policies. The construction and energy industries tend to be cautious, which creates resistance to adopting new practices and technologies. In addition, financial and technological challenges exist, as implementing nZEBs can be initially costly due to adopting business-as-usual practices and incorporating new technologies. However, energy savings and environmental benefits are known to offset the costs in the long run.