1. Introduction
The rapid development of the social economy has significantly accelerated urbanization in China, resulting in considerable energy consumption and carbon emissions [
1]. According to the statistics, China’s building energy consumption has risen from 21% to 33% of total energy consumption in the past decade [
2,
3], leading to negative impacts on the economy and environment. In light of this, the concept of a building’s energy savings has become a primary concern for the built environment, and has a significant potential for energy conservation and emission reductions.
Prompted by the policy of energy conservation and emissions reduction, various regions in China have launched building energy-saving design standards successively. Energy-saving design requirements have also evolved from the 30% in 1986 to the 75% raised in the latest standard (JGJ 26-2018) implemented on 1 August 2019 [
4,
5]. The thermal performance of a building envelope is the key factor influencing building energy consumption, improvements to which can reduce the amount of energy required to heat and cool interior spaces and may therefore play a significant role in reducing overall energy consumption. Building energy efficiency standards [
5,
6] require the finite value of the heat transfer coefficient of the building envelope.
Research on the improvement of the thermal performance of the building envelope primarily focuses on the optimization of the thermal insulation performance of building envelopes. Kaynakli [
7] optimized the thickness of wall insulation in different heat supply models. Dylewski [
8] analyzed the effect of different insulation materials on building energy consumption and specifically optimized the thickness of composite walls. Additionally, Daouas [
9] studied the effect of environmental factors on the heat transfer coefficient of the building envelope and modified it. Furthermore, Song et al. [
10] used the economic net present value (NPV) method to evaluate different envelope modification methods. Zhang et al. [
11] analyzed the effect of different material insulation and construction forms on building energy consumption using an energy simulation method and constructed an economic optimization method in Chengdu. Moreover, Dodoo et al. [
12] analyzed the relationship between the building envelope economy and building energy consumption from the perspective of the whole building life cycle, and optimized the selection of materials for the economy. Subsequently, Arnas Majumder [
13] studied the optimization of recyclable material insulation panels and proved the feasibility of the technological routes.
Unlike thermal insulation, which has been studied extensively, the thermal mass and its effects on a building’s energy and thermal performance are yet to be researched comprehensively [
14]. Zhu et al. [
15] studied the thermal mass of the envelope and found that the effective use of the thermal mass could significantly reduce a building’s energy consumption. Ghoreishi [
16] found that a building envelope with a high thermal mass could induce a significant thermal lag, delaying the effect of peak temperatures and reducing the amplitude of the heat gain, which results in reductions in seasonal heating and cooling loads. Additionally, Al-Sanea et al. [
17] studied thermal mass insulated wall buildings and showed that the buildings were relatively energy efficient in the spring and autumn seasons, but not in summer and winter. Dodoo et al. [
18], Wang et al. [
19], Reilly and Kinnane [
20], and Deng et al. [
21] studied the thermal mass of buildings in specific climatic zones, the their results showed that in hot summer and cold winter regions or hot regions, the increase of the thermal mass could effectively reduce a building’s cooling energy consumption, and the advantages were better highlighted in areas with a large temperature difference between day and night. Hoes, Trcka, Hensen, and Bonnema [
22,
23] studied the effects of a high thermal mass on the reduction of overheating in a mid-European climate, where heating may not be the governing design factor. Furthermore, some other scholars studied the effect of the thermal mass and insulation through construction layers of different thicknesses, but these studies were never able to respond positively to the effect of the thermal mass on the energy demand in cold climates [
16,
24,
25,
26].
Moreover, although coupling the thermal mass with code-required thermal insulation commonly and inherently takes place in everyday construction projects, only a few studies have comprehensively discussed the impact of such a combination on a building’s energy use in a generalizable manner, resulting in increased errors in predicting energy consumption at the beginning of the building’s design [
27,
28].
To resolve these limitations, this article presents a study of a comprehensive index evaluating the effect of a material’s thermal mass—in conjunction with thermal insulation—on the overall energy performance of residential buildings across a range of climate zones in China. It considers the characteristics of buildings’ exterior walls, as the interface between indoor and outdoor energy exchange, to assess the impact of these parameters on annual energy demands. By using the comprehensive evaluation index, coupled with the thermal mass with insulation, the thermal performance of conventional building materials in five main Chinese climate zones could be quantified.
In summary, this study is theoretical research focusing on varying building construction parameters, such as wall thickness, to investigate the effect of coupling thermal mass with insulation on a building’s energy performance, but not necessarily the building’s practical design. The purpose of this paper is to find a method combining the thermal mass and insulation to evaluate the performance of building materials and to discover the correlation between it and building energy demands. It also aims to give the appropriate wall thickness interval for different materials in five main climate zones, with the factors of the comprehensive index improvement and energy-saving rate of a building’s energy consumption, to provide a reference for the energy-saving design of a building at the early stage of its design.
2. Research Methodology
To establish a comprehensive evaluation index of the thermal mass and insulation, Four relevant metrics, namely the heat transfer coefficient, thermal inertia index, temperature wave attenuation, and phase detention time, were taken. For this paper, five common building materials, namely reinforced concrete, aerated concrete, rammed earth, clay brick, and hollow clay brick were selected, and, based on the above comprehensive evaluation index, the design interval of a building structure’s thickness suitable for the typical cities of different thermal zones are given in combination with the room ambient design temperature. In addition, the energy demands were simulated with WUFI plus to prove the comprehensive evaluation index.
2.1. Materials and Properties
Taking the “Code for thermal design of the civil building’’ as a reference [
29], the density, thermal conductivity, specific heat capacity, and thermal storage coefficient of the above five building materials are listed, as shown in
Table 1.
2.2. Typical Cities in the Thermal Zones
Thermal zones in China can be roughly divided into a hot summer and warm winter zone, a mild zone, a hot summer and cold winter zone, and cold and harsh regions, and five cities—namely Guangzhou, Kunming, Shanghai, Beijing, and Harbin—were selected in turn as typical representative cities in each thermal zone. The climatic parameters of the typical cities are shown in
Table 2 [
30].
2.3. Base Case Model
This paper uses a two-story house as a prototype for the energy simulation to study the effect of different thicknesses of walls on the energy demand of a building, which is similar in scale to the majority of residential houses in China. Its length, width, and height of the building are 12.9 m, 7.7 m, and 6.3 m, respectively; the height of the ground floor is 3.6 m and that of the first floor is 3.3 m. The appearance of the building is shown in
Figure 1, with each floor of the building consisting of three main thermal zones: a separate bedrooms on each side and an open living room in the middle.
To assess the impact of the thermal mass and insulation of different materials within various wall constructions on the energy demand, ten different external wall thicknesses were chosen, namely 100, 200, 300, 400, 500, 600, 700, 800, 900, and 1000 mm. The specifications of the base case model are seen in
Table 3.
2.4. Comprehensive Evaluation Index
It is well known that the thermal performance of wall materials is mainly measured by their thermal mass and insulation. The wall insulation performance depends on the heat transfer coefficient of the wall, and the heat storage coefficient of the material is the key factor affecting the thermal mass of the wall. With the same thermal storage coefficient, the greater the thermal inertia index and temperature wave attenuation, the longer the phase detention time, and the better the thermal performance of the wall are. Through the derivation of the formula, it can be found that the above factors are all related to the thickness of the wall. Therefore, with the thickness as the only variable, a comprehensive evaluation index was established with four parameters, namely the heat transfer coefficient, thermal inertia index, temperature wave attenuation, and phase detention time, seen in Equation (1):
where,
a,
b,
c, and
e represent the sensitivity coefficients of the above parameters with respect to the change in wall thickness, respectively.
β represents the linear relationship after the dimensionless treatment of each subfactor.
5. Conclusions
In this paper, it was reported how a comprehensive index for evaluating the thermal mass and insulation properties of walls was constructed by correlating the thermodynamic influences. Taking five building materials as examples, their respective weights were calculated in different climate zones, and the thickness intervals of different materials when constructing walls was optimized by combining them with energy-saving codes. Meanwhile, this study analyzed and demonstrated the feasibility of the comprehensive evaluation index of a building’s thermal performance from the perspective of energy consumption, and found that the thickness optimization interval of walls under the comprehensive evaluation index system was consistent with that under the influence of energy consumption. The purpose of this study was to examine the relationship between wall insulation and the thermal mass and seek the balance between the thermal performance and energy demand of walls that optimized the thickness interval of designed walls and provided a reference for the initial design of the building structures in various Chinese climate zones. Below are the main findings:
- (a)
Through research and demonstration, the numerical formula of the integrated evaluation index for measuring building insulation and the thermal mass was found as follows: M = aβ
1/U + bβ
D + cβ
V + eβ
ξ, and correlation coefficients are shown in
Section 3 of this paper.
- (b)
The correlation R2 between comprehensive index and energy consumption was 0.7736–0.8215, which was approximately equal to twice the correlation of heat transfer coefficient, and it was scientific to use M as the index for building energy consumption prediction.
- (c)
Using wall thickness as a general index, the correlation between the thermal performance of walls and building energy consumption was constructed and a balance between the two was sought; in addition, through numerical calculations and energy consumption simulations, the ideal wall thickness interval of five materials are shown in
Table 22.
- (d)
Considering the complex working conditions during practical construction, the conclusions of the paper provide a prospective application of the research: constructing the equivalence index between materials and transforming the single-material wall into a composite wall with a better performance, which is the direction the authors need to study subsequently.
Regardless of the foregoing, this study has the following limitations that need to be addressed in future research. First, the specific meaning of the balance point of the energy-saving rate and the thermal performance improvement rate must be explored and studied. Second, this paper uses five typical cities as research subjects in different climate zones; thus, the research object is not extensive, and later work needs to discuss more cities to verify whether the conclusions are universal.