Assessment and Numerical Modeling of the Thermophysical Efficiency of Newly Developed Adaptive Building Envelopes Under Variable Climatic Impacts

Abstract

The relevance of this study is driven by the increasing requirements for the energy efficiency and indoor comfort of residential and public buildings, particularly in regions with extreme climatic conditions characterized by substantial daily and seasonal temperature fluctuations. Effective management of heat transfer through building envelopes has become a key factor in reducing energy consumption and improving indoor comfort. This paper presents the results of an experimental–numerical investigation of the thermal behavior of an adaptive exterior wall system with a controllable air cavity. Steady-state and transient simulations were performed for three envelope configurations: a baseline design, a design with vertical air channels, and an adaptive configuration equipped with adjustable openings. Quantitative analysis showed that during the winter period, the adaptive configuration increases the interior surface temperature by 1.5–2.3 °C compared to the baseline design, resulting in a 12–18% reduction in the specific heat flux through the wall. In the summer period, the temperature of the exterior cladding decreases by 3–5 °C relative to the baseline, which reduces heat gains by 8–14% and lowers the cooling load. Additional analysis of temperature fields demonstrated that the presence of vertical air channels has a limited effect during winter: temperature differences at the surfaces do not exceed 1 °C. A similar pattern is observed in warm periods; however, due to controlled air circulation, the adaptive configuration provides an improved thermal regime. The results confirm the effectiveness of the adaptive wall system under the climatic conditions of southern Kazakhstan, characterized by high solar radiation and large diurnal temperature variations. The practical significance of the study lies in the potential application of adaptive façades to enhance the energy efficiency of buildings during both winter and summer seasons.

1. Introduction

In the context of global warming and the steadily increasing cost of energy resources, improving the energy efficiency of buildings has become a particularly pressing issue. This is especially relevant for regions with a sharply continental climate, such as most of Kazakhstan, where hot and arid summers, cold winters, significant diurnal and seasonal temperature fluctuations, and low air humidity are typical. Under such conditions, there is a growing need for adaptive building envelopes capable of effectively reducing heat losses in winter while preventing indoor overheating in summer. This is of particular importance for the southern regions of the country, where summer temperatures reach extreme values, creating an additional load on cooling systems and increasing the overall energy consumption of buildings [1]. The development of energy-efficient architectural and structural solutions that account for regional climatic characteristics has therefore become a key direction in both construction and the renovation of the existing housing stock. Optimization of exterior building envelopes is especially crucial, as they determine the magnitude of heat losses, indoor thermal comfort, and the energy demand for heating and cooling. In Kazakhstan, with its pronounced temperature contrasts, the task of creating adaptive envelope systems is a priority [2]. Traditional approaches to improving building energy efficiency include multilayer walls with thermal insulation, reflective coatings, phase-change materials, and ventilated facades. Each of these methods addresses an individual challenge but does not provide comprehensive adaptation to the seasonal variability of the climate.

In recent decades, ventilated façades have attracted the most attention. They represent a wall system in which the cladding layer is separated from the thermal insulation by an air cavity [3]. In summer, this cavity enables natural ventilation and convective heat removal from the envelope, thereby reducing the thermal load on the building. In winter, with a properly designed configuration, the air layer can contribute to reducing heat losses due to its additional insulating effect. Moreover, the ventilation gap facilitates moisture removal from the structural layers, preventing degradation of the thermal performance of the insulation. However, under sharply continental climatic conditions, this system demonstrates several limitations: during winter, intensive ventilation leads to additional cooling of the wall, and in arid regions the cavity may become partially obstructed by dust, which decreases the overall efficiency of the façade system.

One of the promising directions for improving efficiency is the integration of transformable air channels into exterior wall assemblies. This approach allows the operating mode of the air cavity to change according to the season: a closed, insulating mode in winter and a ventilated, cooling mode in summer. Such a concept makes it possible to combine the advantages of traditional systems while eliminating their drawbacks through adaptation to varying climatic conditions [4,5,6]. Issues related to enhancing the energy performance of building envelopes have been extensively investigated in numerous studies, with particular attention to climatic factors and seasonal variations. Classical works describe the behavior of multilayer walls and the influence of enclosed air layers on heat transfer [7]. Studies [8,9] evaluate the impact of various insulation strategies on the thermal balance of buildings and the surrounding environment. The publication [10] examines energy-saving methods for residential buildings, including the use of foam insulation, mineral wool, and air cavities as effective thermal barriers. The study [11] provides a comparative analysis of foam insulation and fiberglass under hot and cold climatic conditions, demonstrating how insulation materials affect the overall energy performance of buildings. These investigations confirm that well-sealed air layers significantly reduce the heat transfer coefficient. Research works [12,13,14] focus on ventilated façades, which exhibit high efficiency in hot climates. Through natural or forced convection, heated air is removed from the cavity, thereby reducing heat gains indoors. The authors emphasize the critical role of cavity geometry, opening configuration, air-flow velocity, and external environmental parameters such as solar radiation and wind direction [12,13]. It has been shown that a properly designed façade can reduce exterior wall temperature by 5–10 °C [13]. However, in winter, continuous air circulation increases heat losses [14], highlighting the need for adaptive solutions capable of regulating ventilation intensity. Contemporary research confirms the importance of adaptive facades under changing climatic conditions. The study [15] presents an overview of adaptive building-envelope technologies, ranging from smart materials to experimental performance-evaluation methods, emphasizing the need to move from laboratory prototypes to practical, scalable systems. In ref. [16], a design methodology for adaptive facades based on dynamic modeling and parametric optimization is proposed. The reduction of heat loss through optimization of insulation and air-cavity configuration is discussed in detail in ref. [17]. The publication [18] provides an overview of modern energy-efficient materials used in Kazakhstan, with a focus on their thermal performance. Studies [19,20,21,22], which analyze the temperature-humidity behavior of walls with air cavities under the climatic conditions of Shymkent, identify zones prone to condensation and underscore the need to adapt wall assemblies to the regional climate [23,24].

In recent years, increasing attention has been paid to the influence of architectural elements on aerodynamics and heat transfer. The study [25] investigates heat and mass transfer in the windward façade zone equipped with vertical shading louvers. It has been shown that such elements significantly affect airflow patterns and can increase heat dissipation by nearly 40%, which must be taken into account when designing adaptive building envelopes. Numerical investigations [26] further demonstrate the need for a comprehensive assessment of façade aerodynamics. Vertical greening is of particular interest, and its effectiveness has been demonstrated in studies [27,28,29]. Experimental results show that green facades reduce heat flux during the summer period and decrease heat losses in winter, thereby providing a seasonally adaptive effect. The study [30] demonstrates that the transition from static structures to adaptive systems with integrated heat transfer mechanisms makes it possible not only to passively isolate a building, but also to actively manage its thermal balance depending on external conditions. This is confirmed by the results of numerical modeling, indicating a significant advantage of systems capable of changing their parameters in real time. The study [31] emphasizes that for cold and extreme climates, the use of dynamic strategies for controlling the ventilated layer is a key factor in realizing the energy-saving potential of the facade, which remains untapped in traditional “passive” schemes. The study [32] presents an annual performance analysis of adaptive dynamic shells with integrated solar components, confirming their high efficiency in operation in various climatic zones. An important aspect of the development of such systems is the integration of ventilation and heat recovery functions. The publication [33] systematizes achievements in the field of breathable enclosing structures, emphasizing their potential to improve the microclimate and reduce energy consumption.

Thus, contemporary research covers a wide range of issues related to thermal inertia, moisture protection, energy saving, and the adaptation of façade systems to climatic conditions. However, the seasonal transformation of air channels and their impact on thermophysical processes remain insufficiently studied. This highlights the relevance of the present research, which is aimed at developing adaptive structural solutions suitable for sharply continental climates. Accordingly, the aim of this study is to analyze the thermal and moisture regimes of multilayer exterior wall assemblies of residential buildings incorporating transformable air channels and to identify the optimal configuration that ensures enhanced energy efficiency under the climatic conditions of the city of Shymkent, located in the southern region of Kazakhstan.

2. Materials and Methods

2.1. Configuration and Material Characteristics of the Building Envelope

In the arid climatic conditions of the southern regions of Kazakhstan, particularly in the city of Shymkent, there is a growing need to improve the energy efficiency of building envelopes capable of adapting to seasonal and diurnal temperature fluctuations, as well as changes in relative humidity.

The object of this study is three fragments of multilayer exterior wall assemblies, as shown in Figure 1.

Figure 1. Multilayer wall assemblies (layer designations a, b, c, c*, d, e are provided in Table 1). (A) Scheme 1—a traditional multilayer exterior wall assembly consisting of homogeneous load-bearing and insulation layers, as well as a ventilated air cavity. (B) Scheme 2—a newly developed adaptive multilayer exterior wall assembly with a homogeneous load-bearing layer and a slab-type insulation layer incorporating vertical air channels. This configuration differs from the previous one by the presence of vertical air channels within the insulation layer. (C) Scheme 3—a newly developed adaptive multilayer exterior wall assembly with a homogeneous load-bearing layer and a slab-type insulation layer containing vertical air channels equipped with a radiant heat-reflective screen.

The main physical–mechanical and thermophysical properties of the materials, as well as the geometric parameters of the components comprising the investigated building envelope assemblies, are presented in Table 1 and Table 2 below. For the purposes of calculation and visualization of the results for Scheme 3, the insulation layer made of basalt-fiber boards of the “DiRock” brand (hereinafter referred to as BF which manufactured by Teplorock, Moscow, Russia), in which rectangular air chambers with various geometric parameters are incorporated (Table 3), is subdivided into two layers in the mathematical models: a solid BF layer without air cavities, and a layer in which air chambers alternating with BF segments occupy the full thickness of the insulation. Aluminum foil is considered the radiant heat-reflective screen. The building heights under consideration are 18 m and 48 m.

Table 1. Characteristics of the layers of the building envelope for Schemes 1–3.

Layer	Description		Thickness, mm	Width, mm	Thermal Conductivity, W/(m·°C)	Thermal Admittance (24 h Period), S, W/(m·°C)	Vapor Permeability μ, mg/(m·h·Pa)	Emissivity		Specific Heat Capacity, J/(kg·°C)
								Without Reflective Coating	With Reflective Coating
a	Cement-sand plaster		10	–	0.76	9.6	0.09	–	–	926
b	Ceramic brick masonry		380	–	0.7	9.2	0.11	–	–	924
c*	Insulation	“DiRock” insulation	Scheme	100	0.035	0.3	0.005	0.9	0.039	321
c		Alternating vertical channels (Figure 1—3 c*, c)	Scheme	Scheme	–	–		–	–	–
d	Air cavity (Figure 1—3 d)		Table 2 –		–	–	–	–	–	–
e	Porcelain stoneware cladding		10	–	3.49	25.04	0.008	–	–	–

Table 2. Geometric parameters of the exterior wall layers.

Wall Assembly Configuration	Thickness of the Ventilated Air Cavity mm (Figure 1—3 d)	Thickness of the Basalt-Fiber Insulation Layer “DiRock” mm (Figure 1)	Thickness of the Enclosed Air Cavity (Air Channel), mm (Figure 1)	Designation
Scheme 1	50	100	–	Scheme 1/50/100
	100	100	–	Scheme 1/100/100
Scheme 2	50	95	10	Scheme 2/50/95/10
	50	90	20	Scheme 2/50/90/20
	50	85	30	Scheme 2/50/85/30
	50	80	40	Scheme 2/50/80/40
	50	75	50	Scheme 2/50/75/50
	100	95	10	Scheme 2/100/95/10
	100	90	20	Scheme 2/100/90/20
	100	85	30	Scheme 2/100/85/30
	100	80	40	Scheme 2/100/80/40
	100	75	50	Scheme 2/100/75/50
	150	75	50	Scheme 2/150/75/50
Scheme 3	50	95	10	Scheme 3/50/95/10
	50	90	20	Scheme 3/50/90/20
	50	85	30	Scheme 3/50/85/30
	50	80	40	Scheme 3/50/80/40
	50	75	50	Scheme 3/50/75/50
	100	95	10	Scheme 3/100/95/10
	100	90	20	Scheme 3/100/90/20
	100	85	30	Scheme 3/100/85/30
	100	80	40	Scheme 3/100/80/40
	100	75	50	Scheme 3/100/75/50

Table 3. Climatic parameters of the region.

№	Parameter	Value
1	Absolute maximum temperature	44.2 °C
2	Absolute minimum temperature	−30.3 °C
3	Average temperature of the coldest five-day period with a probability of 0.92	−14.3 °C
4	Average monthly outdoor temperature in July	26.4 °C
5	Average relative humidity of the coldest month (January)	73%
6	Annual average humidity	57%
7	Maximum and average total solar radiation under clear-sky conditions in July for vertical surfaces with western orientation:	—maximum 749.3 MJ/m2; —average 175.8 MJ/m2.
8	Minimum of the mean wind speeds by compass points in July, occurring with a frequency of 16% or more	1.3 m/s
9	Maximum of the mean wind speeds by compass points in January	6 m/s
10	Duration of the heating season	137 days
11	Indoor temperature	winter 20–22 °C, summer 24–28 °C.
12	Indoor humidity at a temperature of:	up to 24 °C—50–60%, above 24 °C—40–50%.

Both cold and warm operational periods are considered, corresponding to different thermo-moisture exposure regimes. Typical indoor conditions are assumed: a temperature of 20–22 °C in winter and 24–28 °C in summer, with a relative humidity of 40–60%. The modeling is performed taking into account the geometric and physical characteristics of the air cavities, the insulation materials, as well as the parameters of the ventilated façade system of the “Granitogres” type. Aerodynamic characteristics are also considered, including the width of the inlet slit, local resistance coefficients, the building height (18 and 48 m), and the radiative properties of the exterior surface. Figure 2 presents the structural configurations of the newly developed adaptive exterior wall assemblies for the cold and warm periods, corresponding to the layouts shown in Figure 1—b, c.

Figure 2. Schemes of the newly developed adaptive exterior walls for the cold and warm periods according to Figure 1 (Schemes 2 and 3): (A) for the cold period; (B) for the warm period.

The adaptive behavior of the wall is numerically implemented by switching the boundary conditions of the ventilation openings. In the winter mode, these openings are modeled as solid boundaries (Wall), creating a stagnant air layer that maximizes thermal resistance. In the summer mode, the openings are set as pressure-based inlets and outlets, allowing for a fully coupled simulation of the stack effect, where air movement is driven by the temperature-induced density gradient.

2.2. Climatic and Indoor Boundary Conditions

The study is carried out under the climatic conditions of the city of Shymkent, taking into account high summer temperatures, significant diurnal temperature fluctuations, a prolonged heating season in the cold period, and a dry microclimate (Table 3) [34,35,36].

2.3. Finite Element Model. Convergence Study of the Numerical Solution

The investigations are carried out using the finite element method in the ANSYS 19.2/Workbench software environment, which provides the capability for comprehensive analysis of the proposed design solutions, taking into account the interaction of thermal and moisture processes in multilayer building envelope constructions.

Figure 3A shows the finite element model for Structural Scheme 1, while Figure 3B,C present the finite element models for Structural Scheme 2 and Structural Scheme 3 of the exterior walls for the cold and warm operating periods, considering different insulation-layer thicknesses and enclosed air-cavity dimensions. It should be noted that the finite element models of the exterior walls for Structural Scheme 2 and Structural Scheme 3 are constructed without the ventilated façade. Its influence is introduced by specifying the appropriate temperature boundary condition on the exterior side of the envelope. Thus, Schemes 2 and 3 are computed using identical finite element models. The effect of the heat-reflective screen inside the air channel is accounted for at the calculation stage by assigning the corresponding properties to the channels. Therefore, each finite element model can be used for the cases with ventilated air-cavity thicknesses of 50 mm and 100 mm.

Figure 3. Finite element model of the structure: (A) Scheme 1; (B) Scheme 2; (C) Scheme 3 of the exterior walls for the cold and warm operating periods.

It should also be noted that all generated finite element meshes are uniform and maintain stable element quality throughout the entire computational domain, which increases the accuracy of the numerical solution.

To assess the quality of the finite element models for various configurations of the wall envelope, a convergence study of the numerical solution was carried out for each enclosure variant. Two finite element mesh configurations were considered: a base mesh and a coarser mesh. The base finite element mesh was characterized by a representative element size of approximately 16 mm. The coarser finite element mesh was constructed using elements with twice the characteristic size, i.e., 32 mm. A homogeneous mesh structure ensures numerical stability of the solution and facilitates convergence control. In addition, a uniform mesh reduces the likelihood of local numerical errors and enables the use of more efficient solution algorithms due to improved structuring of the stiffness matrix.

A comparison of the temperature fields obtained using the base and coarser meshes revealed no significant differences (with deviations below 1%), which confirms the convergence of the developed models. For subsequent calculations, the coarser finite element mesh was adopted to ensure computational efficiency.

Table 4 and Table 5 present the characteristics of the finite element meshes for each computational scheme, including the total number of nodes and finite elements, as well as the averaged element quality evaluated based on geometric criteria.

Table 4. Parameters of the base finite element mesh for the computational schemes.

Scheme	Number of Nodes	Number of Finite Elements	Average Element Quality
Scheme 1	401,610	87,480	0.93023
Scheme 2–3/XX/95/10, cold period	415,420	98,766	0.90448
Scheme 2–3/XX/95/10, warm period	415,420	95,202	0.90448
Scheme 2–3/XX/90/20, cold period	401,768	95,480	0.94872
Scheme 2–3/XX/90/20, warm period	403,697	94,608	0.94837
Scheme 2–3/XX/85/30, cold period	421,726	100,320	0.95318
Scheme 2–3/XX/85/30, warm period	414,527	97,130	0.95356
Scheme 2–3/XX/80/40, cold period	427,078	101,640	0.9394
Scheme 2–3/XX/80/40, warm period	415,640	97,075	0.94708
Scheme 2–3/XX/75/50, cold period	401,514	95,418	0.93292
Scheme 2–3/XX/75/50, warm period	396,072	92,290	0.93953

Table 5. Parameters of the coarser finite element mesh for the computational schemes.

Scheme	Number of Nodes	Number of Finite Elements	Average Element Quality
Scheme 1	64,438	14,112	0.92011
Scheme 2–3/XX/95/10, cold period	121,270	27,460	0.87595
Scheme 2–3/XX/95/10, warm period	121,270	26,563	0.87595
Scheme 2–3/XX/90/20, cold period	123,175	28,354	0.86615
Scheme 2–3/XX/90/20, warm period	114,865	25,120	0.89854
Scheme 2–3/XX/85/30, cold period	102,109	23,362	0.86735
Scheme 2–3/XX/85/30, warm period	106,877	23,365	0.89153
Scheme 2–3/XX/80/40, cold period	102,109	23,362	0.87931
Scheme 2–3/XX/80/40, warm period	101,511	22,078	0.88712
Scheme 2–3/XX/75/50, cold period	91,377	20,788	0.8767
Scheme 2–3/XX/75/50, warm period	92,886	20,050	0.85173

In all computational schemes, the boundary conditions are defined as thermal contact with the surrounding environment in the form of convection. For the interior surface of the wall, a heat transfer coefficient (“film coefficient”) of 10 W/(m²·K) is used, while for the ventilated channels, air cavities, and the exterior side of the envelope, an intentionally increased value of 1000 W/(m²·K) is assigned, corresponding to a maximally conservative approximation.

The heat-reflective screen in the ANSYS models is represented as a coating with low surface emissivity. Assigning the “Surface Emissivity” parameter to the corresponding surfaces makes it possible to simulate the reflective properties of the materials and their effect on the heat flux within the air gaps. It is precisely the assignment of this parameter that distinguishes Structural Scheme 3 from Structural Scheme 2. At the same time, the surfaces with convection-type boundary conditions are identical for both Structural Scheme 2 and Structural Scheme 3.

2.4. Methodology for Determining the Moisture and Air Regimes of Building Envelopes

When analyzing the moisture regime of a building envelope, several tasks are addressed [34]:

1.

Determination of the vapor diffusion resistance of the building envelope.

2.

Identification of the water vapor condensation zone within the envelope.

3.

Determination of the amount of moisture accumulated in the condensation zone during the heating period and evaluation of the drying time of the envelope.

4.

Verification of the absence of condensate formation within the ventilated air cavity.

Tasks 1–3 are interrelated and are performed sequentially based on the temperature field within the envelope obtained using the finite element method, as well as the thermophysical properties of the construction materials.

The air permeability resistance of the external walls of a residential building shall not be less than the required air permeability resistance, which is determined by Equation (1) [34]:

$$R_{inf}^{req}={\displaystyle \frac{∆p}{G^{\mathsf{т}p}}},$$

(1)

where:

• $∆p$—the difference between the design air pressures on the external and internal surfaces of the building envelope;
• $G^{\mathsf{т}p}$—the prescribed air permeability of the external wall.

The pressure difference $∆p$ is determined by the following expression:

$$∆p=0.55H\left({\gamma }_{ext}-{\gamma }_{int}\right)+0.03{\gamma }_{ext}{v}^2,$$

where:

• $H$—the building height;
• ${\gamma }_{ext}={\displaystyle \frac{3463}{273+t_{ext}}}$—the specific weight of outdoor air;
• ${\gamma }_{int}={\displaystyle \frac{3463}{273+t_{int}}}$—the specific weight of indoor air;
• $t_{ext}$—the average temperature of the coldest five-day period with a reliability of 0.92;
• $t_{int}$—the indoor air temperature during the winter period;
• $v$—the maximum of the average wind speeds by direction for January, with a frequency of occurrence of 16% or higher.

Without considering the ventilated façade, the required air permeability of the external wall of a residential building shall not exceed $G^{\mathsf{т}p}=0.5\mathrm{k}\mathrm{g}/\left({\mathrm{m}}^2\mathrm{h}\right)$.

The air permeability resistance of the envelope $R_u,{\mathrm{m}}^2\mathrm{h}·\mathrm{P}\mathrm{a}/\mathrm{k}\mathrm{g}$ is determined as the sum of the air permeability resistances of the individual layers:

$$R_u={\sum }_{i=1}^n{R}_{u,i},$$

(2)

where $R_{u,i}$—the air permeability resistance of the i-th layer, ${\mathrm{m}}^2\mathrm{h}·\mathrm{P}\mathrm{a}/\mathrm{k}\mathrm{g}$. The air permeability resistance of air cavities is assumed to be equal to zero.

2.5. Input Data for Transient-State Analysis

To perform the transient-state analysis, the specific heat capacity of the materials is determined according to ref. [36], where the material properties used for the transient calculation are presented in Table 1.

In the cold period, the thermal conductivity of the enclosed air channel depends only weakly on temperature; therefore, its value is taken as 0.097 W/(m·°C). A transient thermal analysis is performed. The following boundary conditions are applied:

–

on the interior surface—Convection, 20 °C in winter; in summer: 21 °C until 06:00, 24 °C from 06:00 to 12:00, 26 °C from 12:00 to 20:00, and 24 °C from 20:00 to 24:00;

–

on the exterior surface of the envelope without considering the ventilated façade—outdoor air temperature in winter and outdoor air temperature including the effect of solar radiation in summer;

–

on the surface of the ventilated channel—outdoor air temperature in summer.

The initial conditions for the transient calculation are taken from the results of the steady-state temperature analysis at 00:00.

The hourly temperature for 15 January 2024 is shown in Figure 4.

Figure 4. Temperature values on 15 January 2024.

Solar radiation is determined for each hour according to refs. [34,35,36]. The western side of the building is considered. The temperature parameters and solar radiation values for 15 July 2024 are presented in Table 6.

Table 6. Temperature parameters and solar radiation on 15 July 2024.

Time	Temperature, °C	Wind Speed, m/s	Direct Radiation, W/m2	Reflected Radiation, W/m2	Temperature Considering Solar Radiation, °C
00:00	23	1.1	0	0	23
01:00	23	1.6	0	0	23
02:00	23	1.9	0	0	23
03:00	23	1.8	0	0	23
04:00	23	1.7	0	0	23
05:00	22	1.8	0	0	22
06:00	22	1.8	0	69.8	24
07:00	24	1.7	0	116.9	26
08:00	26	1.6	0	141.3	29
09:00	28	2.5	0	156.5	31
10:00	28	3.3	0	166.4	31
11:00	29	4.1	0	172.5	32
12:00	29	4.1	0	175.4	32
13:00	29	3.7	113.6	175.3	34
14:00	30	3.9	312.2	172.2	38
15:00	31	3.5	475.2	165.8	43
16:00	29	3.2	579.9	155.5	43
17:00	30	3.2	602.7	139.8	44
18:00	29	2.5	513.8	114.2	42
19:00	27	1.9	265.1	63.9	35
20:00	26	1.2	0	0	26
21:00	24	1.0	0	0	24
22:00	23	0.8	0	0	23
23:00	22	0.6	0	0	22

3. Results and Discussion

3.1. Calculation of Temperature Fields in Building Envelopes During the Cold Period

In accordance with the climatic parameters presented in Section 2.2, the analysis considers cases corresponding to the absolute minimum temperature, the average temperature of the coldest five-day period with a probability of 0.92, the absolute maximum temperature, and the average monthly outdoor air temperature in July. The calculation determines the temperature fields, the reduced thermal resistance, and the heat-flux density for various geometric parameters of the structure in accordance with Table 2.

Analysis of the data presented in Appendix A and Appendix B shows that the schemes incorporating enclosed vertical channels and a heat-reflective coating demonstrate significantly higher thermal resistance compared to a simple homogeneous wall assembly. This advantage is maintained both under extreme loading conditions during the cold period and under more moderate temperature gradients.

Accordingly, Figure 5 and Figure 6 present the temperature values between the layers of the building envelopes at the absolute minimum temperature of −30.3 °C and at the average temperature of the coldest five-day period with a probability of 0.92 equal to −14.3 °C for the developed adaptive design with heat-reflective screens and the traditional design with a ventilated façade.

Figure 5. Temperature fields in the building envelope for the case of the absolute minimum temperature: (A) Scheme 3/50/75/50; (B) Scheme 1/50/100.

Figure 6. Temperature fields in the building envelope for the case of the coldest five-day period: (A) Scheme 3/50/75/50; (B) Scheme 1/50/100.

The comparative analysis of the temperature fields showed that the presence of vertical air channels in the exterior wall assembly has almost no effect on the thermal performance of the wall during operation in the cold period. The temperature distributions in the configurations with and without channels exhibit similar values, and the temperature difference on both the interior and exterior surfaces of the envelope does not exceed 1 °C. This result indicates that under low outdoor temperatures, natural convection within the vertical channels is weakened, and the air gap functions predominantly as a layer of stagnant air, without causing any significant change in heat transfer through the building envelope. This behavior is attributed to the formation of a regime of restricted (confined) convection within the vertical channels when the inlet and outlet openings are sealed. Under these conditions, the velocity vector field is characterized by the presence of weakly pronounced local vortex (recirculation) zones, in which the air velocity approaches zero. The absence of a through airflow prevents convective heat removal from the structure, effectively transforming the air layer into a “thermal cushion” with high thermal resistance. This physical mechanism provides a clear explanation for the increase in the temperature observed on the interior surface of the wall in the present study.

3.2. Analysis of the Moisture Regime of the Building Envelope

Within the scope of the study, the partial vapor pressure values on the interior and exterior surfaces of the building envelope, as well as at the interfaces between layers, were analyzed. The calculation results are presented in Appendix E [33]. In all structural schemes, a small amount of moisture condensation is observed in the outermost layer of the insulation. At the same time, no condensation occurs either in the enclosed air channels or in the ventilated cavity. Therefore, when evaluating the parameters of the ventilated air cavity, only the variants with thicknesses of 50 mm and 100 mm are considered; the 150 mm case is excluded from the analysis as irrelevant.

Based on the obtained data, it can be concluded that all schemes exhibit a negligible amount of condensate, which forms only in the outermost insulation layer and does not significantly influence the overall performance of the building envelope. No condensation occurs in the enclosed air channels or in the ventilated cavity. Although the schemes with a heat-reflective layer show a slightly larger amount of moisture and a longer drying period, these values remain insignificant. Due to the absence of condensation and the low air-flow velocity in the ventilated cavity, it is advisable to use a cavity thickness of 50 mm for further calculations in the warm period.

The difference between the partial vapor pressure and the saturated vapor pressure ($E-e$) in the condensation zone is shown in the graphs presented in Figure 7 for the traditional configuration with the geometric parameters Scheme 1/50/100 and the developed configuration Scheme 3/50/75/50. The remaining structural schemes with different geometric parameters and the indication of possible condensation zones are presented in Appendix E.

Figure 7. Difference between the partial vapor pressure and the saturated vapor pressure (E − e): (A) traditional configuration with Scheme 1/50/100; (B) developed configuration with Scheme 3/50/75/50 in the insulation section; (C) developed configuration with Scheme 3/50/75/50 in the channel section.

For a quantitative assessment of moisture accumulation in the various structural schemes, the total amount of condensate formed within the building envelope and the average drying time were calculated. These indicators provide an additional evaluation of the moisture resistance of the different structural design solutions of the envelope elements. The calculation results are presented in Table 7.

Table 7. Amount of condensate formed in the building envelope and its average drying time.

Envelope Configuration	Condensate Mass, g/m2	Drying Rate, g/(Day·m2)	Drying Time, Days
Scheme 1/50/100	1.477	2.843	0.5
Scheme 1/100/100	1.659	2.816	0.6
Scheme 1/150/100	1.745	2.789	0.6
Scheme 2/50/95/10	2.174	2.644	1.0
Scheme 2/50/90/20	2.515	2.573	1.1
Scheme 2/50/85/30	2.759	3.018	1.2
Scheme 2/50/80/40	2.962	4.667	0.5
Scheme 2/50/75/50	3.186	4.609	0.6
Scheme 2/100/95/10	2.368	2.619	1.1
Scheme 2/100/90/20	2.7125	2.551	1.2
Scheme 2/100/85/30	2.96	2.994	1.2
Scheme 2/100/80/40	3.186	4.761	0.5
Scheme 2/100/75/50	3.399	4.702	0.6
Scheme 3/50/95/10	3.153	2.727	0.7
Scheme 3/50/90/20	4.030	2.803	2.3
Scheme 3/50/85/30	4.518	2.480	2.5
Scheme 3/50/80/40	5.014	3.14	2.6
Scheme 3/50/75/50	5.577	3.38	2.7
Scheme 3/100/95/10	3.275	2.689	0.7
Scheme 3/100/90/20	4.334	2.789	2.5
Scheme 3/100/85/30	4.518	2.939	2.5
Scheme 3/100/80/40	5.318	2.343	2.7
Scheme 3/100/75/50	5.757	4.548	0.9

Since no moisture condensation is observed within the ventilated cavity, it is reasonable to adopt the configuration with a 50 mm cavity thickness for subsequent calculations. Although Scheme 3 incorporating reflective layers exhibits an increase in the moisture drying period of up to 2.7 days, this does not have a significant impact on the operational reliability of the structure. The hydrophobized structure of the mineral wool boards DiRock prevents deep penetration of condensate into the material, while the presence of a protective corrosion-resistant coating on the reflective foil eliminates the risk of its oxidation during short-term contact with moisture. In addition, this moisture retention is cyclic in nature and is fully compensated during periods of intensified ventilation of the channels, as confirmed by compliance with the regulatory requirements for the moisture condition of building envelopes [34]. Taking into account the hydrophobic properties of the material and the fact that the calculated moisture content does not exceed the sorption limit, variations in the overall heat transfer coefficient remain within the margin of computational uncertainty (less than 1–2%). The short-term moisture retention period (up to 2.7 days) under extreme frost conditions is subsequently offset by the activation of airflow within the adaptive channels, thereby eliminating the risk of frost-induced degradation and ensuring long-term thermophysical stability of the envelope system.

3.3. Calculation of the Air Regime of Exterior Building Envelopes

The calculation of the air regime was performed in accordance with [33]. The minimum required values of air-permeability resistance ($R_{inf}^{req}$) were obtained (Table 8) and compared with the actual air-permeability resistance values ($R_u$).

Table 8. Minimum air-permeability resistance of the wall.

Envelope Configuration	${\mathit{R}}_{\mathit{i}\mathit{n}\mathit{f}}^{\mathit{r}\mathit{e}\mathit{q}},{\mathbf{m}}^2·\mathbf{h}·\mathbf{P}\mathbf{a}/\mathbf{k}\mathbf{g}$ 18 m	${\mathit{R}}_{\mathit{i}\mathit{n}\mathit{f}}^{\mathit{r}\mathit{e}\mathit{q}},{\mathbf{m}}^2·\mathbf{h}·\mathbf{P}\mathbf{a}/\mathbf{k}\mathbf{g}$ 48 m	${\mathit{R}}_{\mathit{u}},{\mathbf{m}}^2·\mathbf{h}·\mathbf{P}\mathbf{a}/\mathbf{k}\mathbf{g}$ 18 m	${\mathit{R}}_{\mathit{u}},{\mathbf{m}}^2·\mathbf{h}·\mathbf{P}\mathbf{a}/\mathbf{k}\mathbf{g}$ 48 m
Scheme 1/50/100	373	976	269	269
Scheme 1/100/100	374	976	269	269
Scheme 1/150/100	374	976	269	269
Scheme 2/50/95/10	390	1018	269	269
Scheme 2/50/90/20	405	1059	269	269
Scheme 2/50/85/30	421	1101	269	269
Scheme 2/50/80/40	437	1142	269	269
Scheme 2/50/75/50	453	1183	269	269
Scheme 2/100/95/10	390	1018	269	269
Scheme 2/100/90/20	405	1059	269	269
Scheme 2/100/85/30	421	1101	269	269
Scheme 2/100/80/40	437	1141	269	269
Scheme 2/100/75/50	453	1183	269	269
Scheme 3/50/95/10	390	1018	269	269
Scheme 3/50/90/20	405	1059	269	269
Scheme 3/50/85/30	421	1100	269	269
Scheme 3/50/80/40	437	1142	269	269
Scheme 3/50/75/50	453	1183	269	269
Scheme 3/100/95/10	390	1018	269	269
Scheme 3/100/90/20	405	1059	269	269
Scheme 3/100/85/30	421	1101	269	269
Scheme 3/100/80/40	437	1142	269	269
Scheme 3/100/75/50	453	1183	269	269

All the investigated building envelope configurations exhibit high air permeability, which is particularly noticeable for medium- and high-rise residential buildings. This is associated with the increased pressure difference on the façades as the building height increases, as well as with specific features of the wall-assembly design. These results highlight the need for implementing measures to reduce air permeability in order to ensure energy efficiency and maintain a comfortable indoor microclimate.

To increase the air permeability resistance and ensure the required airtightness of the structure, an additional plaster layer may be applied between the masonry and the thermal insulation layer or the air channel. At the same time, to achieve maximum efficiency and to protect the thermal insulation from air exfiltration and infiltration, it is recommended to incorporate a dedicated air barrier membrane into the system. Installing such a membrane on the outer side of the insulation layer or adjacent to the air channel minimizes uncontrolled airflows through joints and material pores, which is critically important for maintaining stable thermal resistance of the adaptive layer, particularly under winter operating conditions.

3.4. Calculation of Temperature Fields in Building Envelopes During the Hot Period

Analysis of the hot-period data presented in Appendix C and Appendix D shows that the building envelope is capable of withstanding thermal loads and maintaining an acceptable indoor temperature regime during the warm season, even under extreme weather conditions. This indicates that the envelope effectively limits heat gains and ensures indoor comfort. The results of the thermophysical analysis for the absolute maximum outdoor temperature of 44.2 °C (extreme case) and the average monthly outdoor temperature for July of 26.4 °C are presented in Figure 8 and Figure 9.

Figure 8. Temperature fields in the building envelope for the extreme case: (A) Scheme 3/50/75/50, 18 m; (B) Scheme 3/50/75/50, 48 m; (C) Scheme 1/50/100.

Figure 9. Temperature fields in the building envelope for the average case: (A) Scheme 3/50/75/50, 18 m; (B) Scheme 3/50/75/50, 48 m; (C) Scheme 1/50/100.

A similar trend is also observed during the warm period: the temperature difference between the configurations with vertical air channels and those without them does not exceed 1 °C. Despite such a small discrepancy, even a slight reduction in the interior surface temperature can have a positive effect on the energy efficiency of the building envelope during prolonged operation. This is particularly important in the summer season, when reducing heat gains through exterior walls decreases the thermal load on indoor spaces, lowers the demand on air-conditioning systems, and consequently reduces the building’s energy consumption. Thus, the presence of vertical air channels, even with minimal temperature differences, contributes to improved thermal comfort and enhanced energy efficiency of residential spaces during hot weather. This behavior is explained by the development of pronounced natural convection within the channels when the openings are open, driven by the stack effect. Under these conditions, the velocity vector field exhibits a directed laminar–turbulent upward airflow. The flow velocities increase significantly, ensuring efficient removal of the accumulated solar heat from the surfaces of the thermal insulation and the cladding into the surrounding environment. It is precisely this intensified air exchange that accounts for the observed reduction in heat gains to the indoor space.

3.5. Analysis of the Transient Regime

Since the temperature load on 15 July 2024 varies over a wide range (22–44 °C), maintaining a constant indoor temperature is not practical. Therefore, the case without active air conditioning in the indoor environment is considered. The time-dependent temperature profile is shown in Figure 10.

Figure 10. Average heat flux through the envelope under the temperature load of 15 January 2024 with a constant indoor temperature of 20 °C.

The results of the calculated temperature of the interior wall surface under the temperature load of 15 July 2024 are presented in Figure 11.

Figure 11. Temperature of the interior wall surface under the temperature load of 15 July 2024.

Along the abscissa axis, the time of day (0–24 h) is shown, and along the ordinate axis, the temperature of the interior wall surface is presented. Throughout the day, the temperature varies within a narrow range of 23.5–24.2 °C. At the beginning of the day, the temperature is approximately 23.8 °C, gradually decreases to 23.5 °C by 14:00, and then increases again, reaching 24.2 °C by the end of the day. This curve shape reflects the thermal inertia of the structure: cooling lags behind the nighttime decrease in outdoor temperature, and the minimum temperature is reached only at midday. The diurnal variation of less than 0.7 °C confirms the high thermal stability of the envelope and the steadiness of the temperature regime. Indoor air temperature exhibits minimal variation (Figure 11), which also indicates reliable protection against overheating. Overall, the structure ensures a stable indoor microclimate even during short-term interruptions of air conditioning in the summer period.

The results of the numerical analysis confirmed the effectiveness of the adaptive exterior wall design in both winter and summer operating periods. In winter, the adaptive scheme provides an increase of 1.5–2.3 °C in the interior surface temperature compared to the baseline configuration, which leads to a reduction of 12–18% in the heat flux through the envelope. This demonstrates a clear relationship between regulating the air cavity and decreasing heat transfer, ensuring enhanced stability of the indoor thermal regime. In summer, a 3–5 °C reduction in the exterior cladding temperature decreases heat gains into the indoor space by 8–14%, thereby reducing the load on air-conditioning systems and improving the indoor microclimate.

A comparison of the simulation results for the configurations with vertical air channels showed that, during the winter period, the temperature difference between the schemes does not exceed 1 °C, indicating weak natural convection at low outdoor temperatures. This confirms that adaptive control of the air cavity is a key factor in improving the energy efficiency of the envelope. In the warm period, a similar temperature difference also does not exceed 1 °C; however, even such a small effect contributes to an overall improvement of the thermal regime by reducing overheating of the interior surfaces and lowering the load on cooling systems.

The physical advantage of the developed adaptive structure is most pronounced during the summer period, when activation of natural ventilation within the channels results in a reduction of the cladding surface temperature by 3–5 °C and a decrease in heat gains by 8–14%. This effect is achieved due to the stack effect, driven by the density difference between cooler air and air heated by solar radiation, which ensures efficient removal of excess heat and prevents overheating of the inner wall layers, in accordance with the operating principles of ventilated façade systems [15,23]. The obtained results correlate well with the findings of refs. [19,20], which demonstrated that dynamic control of airflow enables higher energy efficiency compared to static systems. In this context, the simplified mechanical regulation proposed in the present study exhibits efficiency comparable to that of more complex systems [21], while requiring significantly lower implementation costs.

The results of the study are further supported by findings reported by international researchers. According to the review presented in ref. [37], adaptive façade systems are capable of reducing the annual energy consumption of buildings by approximately 15%, which is consistent with the conclusions drawn in this work. The review in refs. [38,39] indicates that variations in the configuration and thickness of the air cavity have a substantial influence on the thermal regime, which is corroborated by the observed effect of reducing the cavity thickness from 100 to 50 mm in the present study. Similar conclusions are reported in the experimental investigation [40], which emphasizes that adaptive wall systems are particularly effective under conditions of high solar radiation, characteristic of the climate of southern regions of Kazakhstan. This makes the proposed solution highly relevant for improving building energy efficiency under extreme climatic conditions.

The scientific novelty of the study lies in the fact that, for the first time, a transient numerical simulation of an adaptive wall assembly has been performed specifically for the climatic conditions of southern Kazakhstan, which are characterized by high solar radiation and significant diurnal temperature fluctuations. Quantitative relationships describing the variation of temperature fields under different air-cavity configurations, including cases of limited and active convection, have been established. In addition, a new adaptive envelope scheme with controllable openings has been proposed, providing seasonal regulation of heat transfer [41,42].

The study has certain limitations. The modeling did not account for wind gusts exceeding 6 m/s, which may significantly alter the airflow structure within the air cavity. Solar radiation was assigned as an averaged value without considering temporal cloudiness, reflected radiation, or local shading effects. No full-scale validation on an actual building was performed, which limits the accuracy of evaluating the behavior of the structure under real operating conditions. The current calculations were performed under the assumption of a constant indoor microclimate maintained by HVAC systems, which made it possible to identify the energy-saving potential specifically attributable to the adaptive properties of the building envelope. In addition, aerodynamic effects in the upper sections of high-rise buildings, where vortex flows may form, were not considered.

Despite these limitations, the obtained results have high practical significance. The adaptive scheme reduces heat losses in winter by 12–18% and heat gains in summer by 8–14%, which leads to a substantial reduction in operating costs. The design can be effectively applied in both new and retrofitted buildings with heights ranging from 5 to 25 floors. The simplicity of integration and the absence of the need for complex engineering solutions make the adaptive system a promising component for façade modernization in regions with hot climates.

Thus, the conducted study demonstrates that the adaptive wall assembly has significant potential for improving the energy efficiency of buildings, particularly under the climatic conditions of southern Kazakhstan. The modeling confirms the feasibility of effective seasonal control of heat transfer through the envelope by regulating the air cavity, which aligns with global trends in the development of adaptive façade systems.

4. Conclusions

• The comparative analysis showed that the schemes with enclosed vertical channels and a heat-reflective layer exhibit higher thermal resistance than the simple homogeneous scheme under both extreme and moderate thermal loads. Schemes without heat-reflective screens are less effective under the given conditions.
• According to the calculation results, the most effective configuration is Scheme 3/50/75/50, which provides the optimal combination of thermal inertia, heat-transfer resistance, and resistance to overheating in the summer period. The thickness of the air cavity significantly affects convection: at 50 mm the air velocity increases, whereas at 100 mm it decreases considerably. Overall, the natural convection velocity remains very low due to the small height difference between the inlet and outlet.
• In the winter period, the adaptive scheme (Scheme 3) increases the interior surface temperature by 1.5–2.3 °C compared with the baseline configuration, resulting in a 12–18% reduction in heat flux through the envelope. In the summer period, the exterior cladding temperature in the adaptive scheme is 3–5 °C lower, reducing indoor heat gains by 8–14% and decreasing the cooling load.
• The moisture-regime analysis showed that all investigated schemes exhibit condensation in the outermost insulation layer. The amount of condensate is minimal and does not affect the thermal performance of the envelope. No condensation was detected in either the enclosed vertical channels or the ventilated cavity. Schemes with heat-reflective layers generate slightly more condensate and exhibit longer drying times; however, the values are sufficiently small and do not influence the selection of the optimal scheme.
• In the summer period, the use of schemes with ventilated vertical channels reduces heat flux through the envelope by up to 36% under average July temperature conditions. Under extreme temperatures, the performance changes: the configuration with homogeneous insulation becomes more stable due to the sharp reduction in natural convection caused by strong heating of the exterior surface. The heat-reflective layer has virtually no effect on the envelope performance under warm conditions.
• All investigated schemes demonstrate significantly excessive air permeability, particularly in the design of residential buildings approximately 48 m high. To increase air-permeability resistance, the use of an additional plaster layer between the brick masonry and the insulation, or the application of denser vapor-barrier materials, is recommended.
• Transient thermal modeling for Scheme 3/50/75/50 under the conditions of 15 January and 15 July 2024 showed high thermal stability of the envelope. In winter, the heat flux through the envelope did not exceed 6.2 W/m² while maintaining an indoor air temperature of 20 °C. In summer conditions without active air conditioning, the amplitude of temperature fluctuations at the interior surface did not exceed 0.7 °C, despite an outdoor temperature amplitude of about 22 °C, confirming the high thermal inertia of the adaptive structure.
• A comprehensive comparison of all schemes confirms that Scheme 3/50/75/50 is the most balanced solution for both winter and summer conditions of the southern Kazakhstan climate. This scheme ensures a stable thermal regime, minimization of heat losses, reduction of heat gains in the hot period, low sensitivity to outdoor-temperature fluctuations, high thermal stability, and reliable performance under intense solar radiation.

The results of numerical modeling confirm the high efficiency of the adaptive system under extreme temperature conditions. Accordingly, a promising direction for future research is the optimization of the thickness of the DiRock thermal insulation layer depending on the construction region. For the northern regions of Kazakhstan, it is advisable to consider increasing the insulation thickness while preserving the geometry of the adaptive channels, which would minimize the load on heating systems while maintaining the benefits of natural façade ventilation during the summer period.

5. Patents

As a result of the work, we obtained a patent for utility model of the Republic of Kazakhstan «Adaptive energy-efficient exterior wall construction», 2025, No. 11495 [41] and patent for invention of the Republic of Kazakhstan «Energy-saving wall enclosing structure with air channels», 2024, No. 36701 [42].