Review of NZEB Criteria: Design of Life Containers in Operations Area

Abstract

The Spanish Ministry of Defense is currently attempting to reduce the amount of energy that is consumed by its military bases and has therefore raised concerns about how to make their facilities more energy efficient. To fulfill this objective, the Spanish army has developed various studies and projects, as well as a technical prescription sheet that defines the thermal transmittance values of the materials that are to be used to construct the different elements of the containers that make up the temporary housing units at Spanish military camps. Both governments and private entities have developed initiatives that are aimed at improving the energy efficiency of buildings, which are classified into two groups: those aimed at the development of mandatory building codes and those that are based on voluntary certification programs. The use of passive strategies is one of the key actions that is being implemented to achieve the NZEB category, as its first requirement is to be a “very low energy consumption building”. This paper compares the energy efficiency requirements of the tents and containers that are used in military camps and the energy-efficient design requirements that are demanded by the energy efficiency standards for buildings in the civil sector. Through this comparison, we determine how energy efficient the current living spaces in military camps are in order to define strategies that can be implemented to improve the design requirements of these living spaces so to reduce the consumption and operation logistics and to improve both operability and safety in military camp facilities.

1. Introduction: Evolution of Energy Efficiency in the Military Bases

The development of the Paris Agreement at the United Nations Framework Convention on Climate Change has resulted in the increased energy efficiency of buildings and the reduction of the GHG emissions of buildings, being one of the most important issues related to energy policy. Despite significant improvements in recent years, the global share of the final energy demand in buildings and the CO₂ emissions stood at 36% and 37%, respectively, in 2020 [1,2].

NZEBs (net-zero energy buildings) are regarded as an integrated solution that can be implemented to address problems that are related to energy-saving, environmental protection, and CO₂ emission reduction in buildings and in the construction sector. NZEBs mainly involve three kinds of energy efficient measures: passive design, service system, and power generation from RES, as shown in Figure 1.

The use of passive strategies is one of the key actions that can be taken to achieve the NZEB category, since being a “very low energy consumption building” is the first requisite to achieve this status. Through these methods, a building’s energy consumption can be reduced by evaluating different passive strategies during the design stage and by implementing the most appropriate solutions according to the location, climate, cost, and available materials [3].

Presently, militaries are concerned about the requirements of the decarbonization of the building and construction sector by 2050, a requirement that has been implemented because army facilities consume a large quantity of energy from nonrenewable sources [4]. In the last two decades, the armed forces of different countries, as well as international military alliances, have made significant progress in making their facilities more energy efficient by developing studies that are focused on reducing their energy consumption.

The North Atlantic Treaty Organization (NATO) is assessing the effect of different energy efficiency measures and is developing studies to determine how to reduce energy consumption during humanitarian and military operations together with the use of sustainable alternatives to fossil fuel. In 2014, NATO published the NATO Green Defense Framework, which is a multifaceted project that includes a wide range of activities, such as operational effectiveness, environmental protection, and energy efficiency activities [5]. NATO operations have a significant environmental impact; therefore, research is focused on reducing energy consumption in NATO facilities, which results in the collateral impact of reducing threats to soldiers due to the reduced logistical footprint and movement of military units in conflict territories. Furthermore, in the context of this framework, the so-called expert group Smart Energy Team (SENT) has been working with universities and research institutes that are integrated in the Science for Peace and Security Programme [6] in order to promote greater environmental awareness.

The Spanish Ministry of Defense is also concerned about reducing the energy consumption and improving energy efficiency of its facilities. These concerns are highlighted in the Geothermal and Renewable Energy in Modular Architecture System (GREENMAR) [7], which discusses the development and construction of a hangar in the military airport of Seville, “La Maestranza”, through the use of a rapid building system and that will have a modular architecture and high energy efficiency through the dominant use of renewable, mostly geothermal, sources of energy. For the air conditioning and electricity supply, La Marañosa Technological Institute (ITM) developed a prototype of a more efficient electric generator.

In order to promote research projects focusing on energy efficiency measures, the Spanish Army is developing the “2020 Military Camps in Energy Efficiency” initiative [8]. The aim of this initiative is to analyze measures that would enable the energy consumption in military camps to be reduced, stressing the use of different types of lighting as well as the use of highly efficient generators and tents. This initiative has received a high level of commitment, with the following different military departments pledging their participation: Operational Logistics Force, Engineers Command, High Availability Ground Headquarters, and the Canary Islands and Force Support Command (Army Logistics Support Command, Directorate of Education, Instruction, Training and Evaluation, coordinated by the Training and Doctrine Command (Directorate of Research, Doctrine, Organization and Materials)).

Furthermore, the Centro Mixto Universidad de Granada (CEMIX) and Mando de Adiestramiento y Doctrina are developing a methodology that will be able to register the amount of energy that is consumed at military camps, as well as each camp´s energy efficiency, in order to design an energy management plan that has been specifically adapted to military camps, the so-called PGEMi (Energy Management Plans for Military Camps).

Nowadays, the Spanish Army has a Technical Specifications Document [9] which defines the conditions for the purchase of the containers used as temporary living spaces at military camps. This document defines the data for the procurement of ablution containers (C17) and includes the following:

• The values of the thermal transmission coefficients for the different construction elements of the containers must be at least the following: 0.35 w/m²K for floors, 0.25 w/m²K for ceilings, 0.33 w/m²K for walls, 0.30 w/m²K for doors and 2.15 w/m²K for windows;
• Lighting requirements indicate the use of three double fluorescent light fixtures with a voltage of 2 × 36 W and a light intensity of 200 lux;
• Pressure groups must have a minimum power of 1.5 C. V., with a minimum pressure of 150 kPa for showers and 100 kPa for taps;
• The hot water storage tanks must have a power of 2500 W;
• The ventilation system must have a minimum flow rate of 500 m³/h;
• There must be an electric radiator with a minimum power of 1500 W for heating.

Other containers, such as the ISO 20′ 1CC (designed according STANAG 2828:15 standard) with inner fittings that have been adapted to the Military Emergency Unit (UME) [10] do not have any type of thermal insulation according to their technical specifications. On the other hand, the company CONTAINEX [11], who specializes in containers, as well as Enhanced Forward Presence (EFP) (Latvia), one of the most prominent suppliers of military containers, have developed their own solutions, and their parameters for the heat transmission coefficients are shown in

Table 1. Thermal transmittance (W/m² K) of different living spaces in military camps.

Infrastructure	U Walls	U Roofs	U Doors	U Floors	U Windows
CONTAINEX	0.59	0.37	0.59	0.54	2.50
EFP	0.27	0.22	-	0.22	-
Containers C17	0.33	0.25	0.30	0.35	2.15
Tents TC	0.7	0.7	0.7	-	-

.

In the military world, there are no technical instructions regulating the parameters that are used to design the living spaces in military camps in order to reduce their energy consumption. Since envelopes are one of the most important parts of the studies on energy consumption among the different elements of a building [12], the main objective of this study was to determine the appropriate heat transmission coefficient for the design of military tents and containers. It is important to consider that these containers and tents are for temporary use only, and one has to consider where and for how long these structures will be used during military operations. It is almost as important to consider the cost of these structures; however, in cases where the use of the selected heat transfer coefficient is not economically profitable, then other aspects such as operability and facility safety should be considered.

2. Nearly Zero-Energy Buildings (NZEB) Standards: Heat Transmission Coefficients

When designing a NZEB, the following should be considered in order to reduce the energy demands of the building: the building envelope, optimal orientation, climatic information, comfort conditions, solar protection, solar and controlled ventilation, and the energy efficiency of the building’s facilities should be analyzed, and a study should be conducted into whether these energy needs can be met through the use of renewable energy sources [2]. The Energy Performance of Buildings Directive EPBD 2010/31/EU on the energy performance of buildings promotes policies that help to ensure that numerous buildings will be highly energy efficient and decarbonized by 2050. It also mandates that the member states must draw up national plans to increase the number of nearly zero-energy buildings, and to define the values and measures necessary to improve energy efficiency and to support the development of renewable energies in order for each member state to achieve the EU requirements by 2050 [13].

Attia [14] presents a summary of which European countries have transposed Directive EPBD 2010/31/EU in its national legislation. The measures that have been implemented are focused on the establishment of the maximum primary energy consumption required for residential buildings as well as on the use of minimum share of renewable energies. It should be noted that there is a great disparity in the requirements that have been proposed by the different countries, i.e., Denmark has set a number of high-level requirements, whereas the requirements that have been set by Romania are less demanding. Because of such discrepancies, this article shows these data and includes those from similar areas, such as Portugal and Spain.

Certain European projects such as Zebra2020 [15] are focused on tracking the market transition to nearly zero-energy buildings and was launched in 2014 with the aim of generating data and evidence for the evaluation and optimization of energy policies. Its main objective is the creation of an NZEB building observatory that is based on market studies and various data tools in order to help the 17 member states that are participating in the project to draft appropriate legislation for its implementation, with the aim of achieving the 20/20/20 objectives. In addition, other projects that have already ended focused on delivering the EU energy targets, such as the Intelligent Energy Europe Programme (IEE), supported concrete projects to provide training on new construction techniques and promote renewable energy sources.

2.1. Spanish Building Standard

In Spain, there are two building standards that have been implanted to achieve these goals: the Royal Decree 314/2006, 17 March, which approved the Spanish Technical Building Code (CTE), and the order FOM/1635/2013, 10 September, which updates the Spanish Building Standard on energy efficiency in buildings. This regulation is the result of implementing both the EPBD and Directive 2009/28/EC, which was developed by the European Parliament and of the Council of 23 April 2009, on the promotion of the use of energy from renewable sources [16]. This standard establishes a first step towards meeting the objective of NZEBs in Spain by defining different transmittance values for all of the elements comprising the envelope of a building according to each of the five climate zones that have been defined in Spain. Concerning the revision of the thermal transmission coefficient, Table 2 shows the evolution of the values of this parameter and the expected future value for only one of these five climate zones that have been defined in Spain, which correspond to the largest coastal area of Spain, zone C2.

Table 2. Thermal transmittance in Spain’s C2 zone [17].

U-Value (W/m2k)	CTE 2006	CTE 2013 1	CTE 2013 2	NZEB	Technical Limits
Façade walls	0.95	0.75	0.29	0.2	0.1
Roof	0.59	0.5	0.23	0.2	0.1
Facings in contact with the ground	0.65	0.5	0.36	0.25	0.15
Openings	4.4–3.5 3	3.1	1.6–2.0	<1.6	0.8

¹ Table 2 and

Table 3. Passive House parameters.

	Parameters			Alternative Parameters
Heating
Heating demand (KWh/(m2a))	≤15			-
Heating load (W/m2)		-		≤10
Cooling
Cooling demand + dehumidification (KWh/(m2a))	≤15 + dehum. contribution			variable
Cooling load (W/m2)		-		≤10
Tightness
Pressure tightness test n50 (l/h)		≤0.6
Renewable primary energy (PER)	Classic	Plus	Premium
Energy demand (KWh/(m2a))	≤60	≤45	≤30	±15
Generation of renewable energy (KWh/(m2a))		≥60	≥120

. CTE DB HE1 2013, limit values for zone C2 [16]. ² CTE 2013- Appendix E of CTE 2013. Indicative values of the characteristic parameters of thermal transmittance for residential use. ³ According to orientation.

Table 2. Thermal transmittance in Spain’s C2 zone [17].

U-Value (W/m2k)	CTE 2006	CTE 2013 1	CTE 2013 2	NZEB	Technical Limits
Façade walls	0.95	0.75	0.29	0.2	0.1
Roof	0.59	0.5	0.23	0.2	0.1
Facings in contact with the ground	0.65	0.5	0.36	0.25	0.15
Openings	4.4–3.5 3	3.1	1.6–2.0	<1.6	0.8

¹ Table 2 and Table 3. CTE DB HE1 2013, limit values for zone C2 [16]. ² CTE 2013- Appendix E of CTE 2013. Indicative values of the characteristic parameters of thermal transmittance for residential use. ³ According to orientation.

2.2. International NZEB Standards

There are various certification systems that have been internationally accepted that have defined the criteria that are relevant to NZEBs and through which minimum insulation values have been established. Among the most prominent are the following: Klimaaktiv (Equilibrium (Canada)), LEED (America), CSH (UK), Minergie (Switzerland), Building Class (Denmark), Effinergie (France), Passive house (Germany), ASHRAE (USA), etc. Three of the most relevant certificates are presented below.

2.2.1. Passive House Standard

The Passive House Standard establishes the criteria that are currently the benchmark in many countries. This standard defines the different and an increased number of concrete parameters that can be used for building certification. This standard divides the map into different zones, as shown in Figure 2.

Figure 2. Passive House Institute climate zone map [18].

Figure 2. Passive House Institute climate zone map [18].

The Passive House Institute has also established a set of design parameters [19], shown in Table 3, for the three categories of passive houses that have been defined by the standard: classic, plus, and premium certificates.

There is an alternative definition of the heating or cooling demand value depending on the climatic conditions of the building environment: for warm climate, 15 KWh/(m²a); for warm-temperate, 20 KWh/(m²a); for cold temperate, 25 KWh/(m²a); for cold, 30 KWh/(m²a); for polar, 35 KWh/(m²a), as shown in Figure 3.

The Passive House certification standard defines the thermal transmittance coefficients required for each building component depending on the climate conditions, as shown in

Table 4. Thermal transmittance (W/m²K) according to Passive House Institute [19].

	Opaque Envelope with Respect to the…
	…Ground	…Outside Air
Climate Zone	Insulation	Outdoor Insulation	Indoor Insulation
Artic	Specifically determined in the PHPP for each project.	0.09	0.25
Cold		0.12	0.3
Cool–temperate		0.15	0.35
Warm–temperate		0.3	0.5
Warm		0.5	0.75
Hot		0.5	0.75
Very hot		0.25	0.45

.

2.2.2. ASHRAE

ASHRAE, a United States organization founded in 1894, promotes human well-being through sustainable technology. According to the Standard 169 Climate Data for Building Design Standards, ASHRAE follows building design criteria that are based on the Passive House criteria. This standard defines different climate zones according to following temperature values: cooling degree day (CDD) and heating degree days (HDD), values that are calculated from their base temperature according to the Energy Smart strategy [20].

According to PHIUS [21], there are different climate zones in the United States, with each climate zone requiring different thermal transmittances values for walls, roofs, and windows. The values for these parameters are shown in Table 5.

Table 5. Thermal transmittance (W/m² K) according to ASHRAE [22].

Climate Zone	U Walls	U Roofs	U Windows
0 (Extremely Hot)	0.151	0.032	0.30
1 (Very Hot)	0.151	0.039	0.48
2 (Hot)	0.123	0.039	0.35
3 (Warm)	0.104	0.039	0.31
4 (Mixed)	0.086	0.030	0.29
5 (Cool)	0.076	0.030	0.29
6 (Cold)	0.067	0.030	0.29
7 (Very Cold)	0.067	0.027	0.27
8 (Subarctic)	0.046	0.027	0.24

Table 5. Thermal transmittance (W/m² K) according to ASHRAE [22].

Climate Zone	U Walls	U Roofs	U Windows
0 (Extremely Hot)	0.151	0.032	0.30
1 (Very Hot)	0.151	0.039	0.48
2 (Hot)	0.123	0.039	0.35
3 (Warm)	0.104	0.039	0.31
4 (Mixed)	0.086	0.030	0.29
5 (Cool)	0.076	0.030	0.29
6 (Cold)	0.067	0.030	0.29
7 (Very Cold)	0.067	0.027	0.27
8 (Subarctic)	0.046	0.027	0.24

2.2.3. MINERGIE

This is a standard that has been registered in Switzerland and that aims to achieve high-quality and energy-efficient products.

There are different certificates within this standard, as shown in Table 6.

Table 6. Summary table on the MINERGIE standard.

Certificate 1	Main Requirement (kWh/(m²a))	PV Must Cover Energy Requirements	Heating Demand Compared to MuKEN 2	Energy Demand without PV (kWh/(m²a))
Standard	55	NO	=	New 35% No New 60%
Minergie-P	50	NO	New 70% No New 90%	New 35% No New 60%
Minergie-A	35	YES	=	New 35% No New 60%

¹ All require controlled air renewal, heat protection in summer, and no fuel consumption in new buildings [23]. ² MuKEN: model rules for cantons in the energy sector.

Table 6. Summary table on the MINERGIE standard.

Certificate 1	Main Requirement (kWh/(m²a))	PV Must Cover Energy Requirements	Heating Demand Compared to MuKEN 2	Energy Demand without PV (kWh/(m²a))
Standard	55	NO	=	New 35% No New 60%
Minergie-P	50	NO	New 70% No New 90%	New 35% No New 60%
Minergie-A	35	YES	=	New 35% No New 60%

¹ All require controlled air renewal, heat protection in summer, and no fuel consumption in new buildings [23]. ² MuKEN: model rules for cantons in the energy sector.

The MINERGIE-ECO Standard [24] is based on the Passive House certification, shown in Table 3, but introduces six new aspects, such as daylight, sound insulation, indoor climate, sustainable building concepts, grey energy and materialization, and processes. As result, this standard analyzes 79 criteria, 12 of which are exclusive.

2.3. National Standard of UE Member States

The European Commission has a project called ENTRANZE which aims to support the definition of the NZEB criteria for the EU member states and to support renewable energies. For these purposes, an online tool has been published that includes statistical data on the thermal performance of the 27 countries that comprise the European Union and include data on thermal quality, size, age, use, structure, heating, cooling, energy demand, etc. Among these data, the thermal transmission coefficient of the building elements that are recommended in different countries are defined. The included data are from the years leading up to 2008 because the economic crisis may have affected these criteria [25].

There is a variety of analyses on thermal transmittance coefficients, such as the one shown in

Table 7. Thermal transmittance (W/m² K) according to EURIMA project.

City	U Walls	City	U Walls	City	U Walls
Brussels	0.18	Helsinki	0.17	Lisbon	0.35
Sofia	0.20	Paris	0.19	Bucharest	0.21
Latvia	0.18	Berlin	0.18	Athens	0.22
Prague	0.19	Vilnius	0.17	Budapest	0.21
Amsterdam	0.18	Copenhagen	0.16	Dublin	0.18
Tallinn	0.17	Warsaw	0.19	Rome	0.26

[26], which shows the maximum thermal transmittance value for vertical walls in different European sites of the EURIMA project.

2.3.1. Danish Requirements

Denmark is one of the European countries that has demonstrated a commitment to the definition of NZEB values and the setting of a roadmap towards the European target.

Since its development in 2014, the BedreBolig program has aimed to improve the energy consumption of buildings. The requirements that are sought included nonresidential requirements of 25 kWh/m²a and residential requirements of 20 kWh/m²a.

2.3.2. Romanian Requirements

Romania divides its territory into five zones and defines its requirements for each of the building elements as shown in Table 8.

Table 8. Requirements of thermal transmittance (W/m²K) in Romania for NZEB [27].

Type	U Walls	U Roof	U Floor	U Window
Residential	0.56	0.20	0.35 with basement 0.22 without basement	1.30

Table 8. Requirements of thermal transmittance (W/m²K) in Romania for NZEB [27].

Type	U Walls	U Roof	U Floor	U Window
Residential	0.56	0.20	0.35 with basement 0.22 without basement	1.30

2.3.3. Portuguese Requirements

The Portuguese territory is divided in three different climate zones that are determined according to the winter and summer seasons [28]. For each zone, different thermal transmittance coefficient values are required for both opaque elements and glazed enclosures, as shown in

Table 9. Thermal transmittance (W/m²K) and solar factor of openings [28].

Winter climate zone	I1	I2	I3
External and internal vertical opaque elements	0.7	0.6	0.5
Exterior and interior horizontal opaque elements	0.5	0.45	0.4
External glazing	4.3	3.3	3.3
Summer climate zone	V1	V2	V3
Solar factor of openings (without shading devices)	0.25	0.2	0.15

.

3. Comparison of Transmittance Values

The most important international certificates or national standards were presented in the previous section. In this section, how well the 2017 Spanish army ablution containers (C17) and tents (TC) fulfilled these requirements will be analyzed.

Table 10. Number of comparative zones.

UE Member State	Zones
	National Standards	Passive House	ASHRAE
Spain	5	2	2
Romania	5	3	3
Denmark	4	1	1
Portugal	6	2	2

,

Table 11. Primary consumption coefficient (kWh/m²a) comparative.

National Standards				Passive House	Minergie-A
Spain	Romania	Denmark	Portugal
Developing	93–217	20	Developing	15	35

,

Table 12. Thermal transmittance in windows (W/m²K) comparative.

		Spain	Romania	Denmark	Portugal
National Standards		1.6–2	1.3	0.8	4.3–3.3
	C17	NO	NO	NO	YES
Passive House		1.05–1.25		0.85	1.05–1.25
	C17	NO	NO	NO	NO
Entranze		3.1		1.71	3.15
	C17	YES	NO	NO	YES

and

Table 13. Thermal transmittance in walls coefficient (W/m² K) comparative.

		Spain	Romania	Denmark	Portugal
National Standards		0.29	0.56	0.2	0.3–0.5
	C17	NO	YES	NO	YES
	TC	NO	NO	NO	NO
Passive House		0.3–0.5	0.3–0.15	0,15	0.3–0.5
	C17	O	O	NO	O
	TC	NO	NO	NO	NO
Entranze		0.8	0.9	0.25	0.77
	C17	YES	YES	NO	YES
	TC	YES	YES	NO	YES

O: Depends on the subzone deployed within the same zone.

show a comparison of the parameters according to various standards. In Table 13, the rows C17 and TC depict a comparison between the parameters of the 2017 ablution containers and the tents with the corresponding certificate or national standard, with compliance or noncompliance being indicated according to the climate zone.

Although the Passive House and ASHRAE criteria consider the same number of zones, they are defined differently. The fact that the zones that are defined by both sets of parameters are not available to the public.

Selecting the climate zone that corresponds to the area in which a building will be constructed is critical due to the large amount of varaibility that can be observed among the parameter values that have been defined depending on the certificate or the state regulation that has been chosen [29,30]. According to Walsh [31], it is essential to anlayze the climate of each zone, as it affects the performance of the building by up to 37%.

In terms of the required primary consumption (Table 11), major differences can be highlighted between the Passive House criteria and the state standards that have been implemented by Romania; however, this difference is also similar to the one seen between the Romanian and Danish standards. This demonstrates the need to improve the requirements in some countries. It must be noted in this context that that the Passive House system not only requires reducing demands, but also requires information about how dwellings are used so that they can be in accordance with the design and planning.

The disparity in the value thresholds in each country is highlighted, such as those for cooling, which vary from 5 to 100 kWh/m² or more due to the existing climate change crisis and the continuous extreme heat waves, making it difficult to be able to specify these unpredictable data.

The analysis of the thermal transmittance coefficient in the window areas, shown in Table 12, is a critical issue for building energy efficiency. It can be seen that the new ENTRANZE parameters are quite close to the state standards, but even so, they are not quite adjusted to the state standards in some areas. For other areas, they are quite far from these parameters, and because of this, they were updated in 2008. As for the Passive House parameters, they are once again the most restrictive. On the other hand, it can be seen that Denmark’s parameters have become more developed in later revisions of their regulations.

Finally, the transmittances of the vertical walls, shown in Table 13, have been analyzed. It should be noted that the current U-value of the tents that are offered to the Ministry of Defense for any area is 0.7 W/m² K. This would no longer comply with Spanish state regulations, and in other more severe climates, these tents would have major inefficiency problems. For containers, a U-value of 0.33 W/m² K for container walls (Spain) [10] and 0.27 W/m² K (Latvia) [32] in a cold climate stand out. According to the Passive House requirements, the latter values should be 0.12 W/m² K, so that the improvement margin is large, both in military operations and in Spain as a country.

Sierra-Pérez [33] conducted a comparative analysis on the energy saving efficiency of one of the classroom buildings at the General Military Academy by taking into account the real conventional built parameters and the Passive House parameters. The results were significant because the implementation of the Passive House requirements for this building allowed energy savings of between 60% and 80% with respect to the current building characteristics. The ENERPHIT solution allows the highest possible energy savings to be achieved, although the alternative solution avoids impacts on energy consumption that are associated with the construction process, making this solution 5% better than the conventional one, with the ENERPHIT solution being 30% better.

The implementation of the Passive House Standard in the building design phase can significantly reduce energy consumption. According to Schnieders [34], energy consumption can be reduced by 75–95% when Passive House parameters are considered versus traditional building construction parameters. When the Passive House parameters are properly applied during the building design and construction phase, it can be guaranteed that the building energy consumption can be reduced to less than 10 W/m², regardless of where the building is in the world.

4. Results and Discussion

There is a wide range of thermal transmittance coefficients that need to be applied depending on the country or standard certifications. The decision to use one value or another depends, in many cases, on the economic aspects resulting from the temporary use of the military tents and containers as living spaces. The economic analysis must take into account both the investment costs and the savings in energy costs over the lifetime period of the structure.

The annual energy cost of a living space is calculated based on unit area as

$$Ca,i=\frac{\mathrm{86,400}\ast HDD\ast C_f}{ \left(R_{wt}+R_{ins}\right)\ast H_u\ast \phi }$$

(1)

where HDD represents the annual cooling degree hours, R_wt represents the thermal resistance of the initial wall, R_ins represents the total thermal of the additional insulation material, C_f in USD/kg represents the fuel cost, H_u in J/kg represents the fuel heating value, and η represents the system efficiency.

On the other hand, the cost of insulation in USD/m² is given by

$$C_{ins}=C_i\ast x$$

(2)

where x is the insulation thickness and C_i is the cost of insulation in USD/m³.

Finally, the net present value over the lifetime period is given by

$$VNA=-C_I+{\displaystyle \sum }_{i=0}^{i}CSa,i\ast Rd,i$$

(3)

where i represents the calculation period, C_I represents the initial investment costs of additional isolation material, CSa,i represents the annual energy cost saving during the year i, and Rd,i represents the discount factor for year i based on discount rate r, which can be calculated as

$$Rd,i={\left(\frac{1}{1+r/100}\right)}^i$$

(4)

where p represents the number of years from the starting period and r represents the real discount rate.

The case study that was presented in this paper focused on determining the extra insulation thickness that is necessary to add to the insulation material that is used to construct the containers that are used in the Spanish base in Lebanon in other to satisfy the different standards. The parameters that were used for the simulations are shown in

Table 14. The container parameters used for base Miguel de Cervantes in Marjayun (Lebanon).

Parameters	Data
Cooling degree hours (CDH)	1203 °C-h [35]
Lifetime (i)	10 years
Insulate material	Polyurethane
Heat transfer coefficient of the initial wall (Uwt)	0.35 W/m2k
Thermal conductivity of polyurethane (R)	0.024 W/mk
Polyurethane costs	260 ($/m3)
Coefficiente of performance	0.70
Hu	31.35 × 106 J/kg
Fuel cost	0.105 $/kg
Discount rate	3.1%
Inside temperature	19 °C

.

Figure 3 shows the results of the calculations that were performed. The cost per m² to add the thicker polyurethane insulation to the container to reduce its transmittance is shown. The simulations were conducted by considering that the initial thickness is 0.08m.

On the other hand, the energy savings per m² over a ten-year period that would result from reducing the transmittance of the walls are presented. Finally, the VNA of the project to increase the insulation thickness with respect to the initial situation is presented.

Table 15. Results of the study case.

	Passive House	ENTRANZE	National Standards
			Spain	Romania	Denmark	Portugal
Thermal transmittance coefficient	0.3	0.8	0.29	0.56	0.2	0.3
Extra insulation thickness	0.015	0	0.0105	0	0	0.015

shows the extra thickness that is necessary in order to comply with the standards that were previously presented. The outcomes that were achieved allow us to assert that the transmittance values that are required for the different standards that were studied and imply an economic profitability that is very close to the optimal value.

5. Conclusions

In the military world, there are no technical instructions that regulate the parameters that are used to design living spaces in military camps to reduce their energy consumption. Therefore, we analyzed the insulation needs of containers that are intended for temporary use in military camps by considering the national standards or those that are internationally recognized to be applicable to permanent housing.

Armed forces must be equipped with living containers that can fulfil the characteristics that are required based on the deployment location, and should also have lower energy consumption, allowing them to contribute to a reduction in the logistical footprint.

The optimal design is a container that is adaptable to different situations, that is resistant and durable over time, and that would be able to facilitate the initial comfort of army troops in the operation area. As these containers are used for short periods of time in different locations, we propose that increasing the thickness of the insulation is required to achieve the adequate thermal transmittance coefficient that corresponds to the location of the deployment of the troops.

This study provides a guide for people who are working in the field to design better quality insulation. When proper insulation materials at an appropriate thickness are used, they provide economic and environmental advantages while decreasing the heat transmission from the walls.