Thermal Performance of the Building Envelope: Original Methods and Advanced Solutions

1. Introduction

In the European Union, buildings are responsible for 40% of the final energy demand and approximately 36% of greenhouse gas emissions [1]. The existing building stock’s substantial energy demand is mostly due to the poor thermal performance of their envelope components (e.g., roofs, walls, and windows), which is especially relevant in buildings that were built when the current EU regulations addressing energy efficiency in buildings were not in force; at present, it is possible to state that roughly 75% of the EU stock is energy inefficient [2].

In this framework, a correct design of the building envelope, including an informed choice of the materials and their thermal properties, is a key requisite to conjugate energy efficiency, the durability of the envelope, and indoor microclimate. This involves a strategic selection of building materials based on their thermal properties and performance. Designers must adopt a multi-scale approach, evaluating the thermal performance of individual envelope components and optimizing the overall energy efficiency of the building. In situ and laboratory measurements can improve the accuracy of the analysis. Indeed, in situ measurements allow for the assessment of actual building performance under real-life conditions, identifying areas of thermal weakness and potential improvements [3]. Laboratory measurements enable precise control over the testing conditions and the evaluation of material properties and their thermal performance in a controlled environment. These measurements are also useful for dynamic simulation techniques, which account for transient thermal behavior and occupant interaction with the building’s performance [4]. This activity should be carried out using reliable tools able to simulate advanced envelope solutions and must be complemented by effective and original evaluation methods to identify the best performing solution.

2. Contributions

This Special Issue titled “The Thermal Performance of the Building Envelope—Original Methods and Advanced Solutions” presents advanced research about the above-mentioned topics. It includes 10 articles, namely 9 original research articles and 1 perspective paper. The 10 articles were authored by 37 scholars from Italy, Serbia, Portugal, Turkey, Romania, Hungary, Canada, Egypt, India, Vietnam, and Japan. The studies concern the following main topics:

• The thermal performance of the building envelope.
• The energy optimization of buildings.
• The integration of renewable energies.

Several different approaches to these topics were followed, as detailed below.

2.1. The Thermal Performance of the Building Envelope

The thermal performance of the building envelope is assessed based on the following:

• The use of different nondestructive techniques.
• The integration of experimental measurements and numerical analyses.
• The development of advanced simulation models.

The integration of different nondestructive techniques allows for a comprehensive analysis of the thermal performance of the building envelope without damaging the structure. Among these techniques, infrared thermography (IRT) detects infrared radiation (IR) emitted by objects to visualize temperature variations, highlighting areas of heat loss, thermal bridges, and insulation deficiencies. IRT provides real-time and accurate data that can be compared with temperature measurements from thermal sensors to understand the thermal properties of materials. Živanović et al. [5] investigated the relationship between measurements performed using IRT and direct temperature measurements using embedded sensors through the testing of an early-age hydration process of a cement-based paste. They prepared two types of cubic samples with different heights using a cement-based paste, with 20% of the cement (by mass) being replaced with fly ash. Infrared (IR) images were obtained using an FLIR E6 IR camera positioned at 0.60 m from the sample, through a tube, to stabilize the environment between the camera and the surface of the samples. The IR camera captures images every half hour during stable temperature periods and every 10 min during periods of intensive temperature fluctuation. The temperature fluctuations were modeled using the asymmetric Gaussian function. The measurements from both the IR camera and surface sensors show excellent agreement with the modeling coefficients, as the temperature differences between the thermo-sensors on the surface and the thermal camera are less than 2 °C. The values obtained from the sensor placed in the central part of the samples are higher than the temperatures measured either through sensors placed on the surface or with the thermal camera. The temperatures measured on the surface with the two different devices show very good accordance.

Integrating experimental measurements and numerical analyses allows for a detailed evaluation of the thermal performance of building envelopes, supporting the validation of theoretical models and the development of a more effective building design. Experimental measurements provide real-world data, ensuring accuracy and reliability. Numerical analyses, on the other hand, allow for the simulation of various conditions and scenarios that may not be feasible to test experimentally. In this direction, Evola and Gagliano [6] studied, both experimentally and numerically, the thermal distribution in a thermal bridge corresponding to a reinforced concrete corner pillar in a building dating back to the 1980s and located in Southern Italy. The authors measured the inner surface temperatures in several points near the corner pillar by using both Pt 1000 temperature probes and thermal imaging techniques. Moreover, 2D finite element simulations were performed based on the same indoor and outdoor conditions as those measured on site during the experimental campaign. The results show that the thermal imaging technique is very useful for obtaining visual information about the presence and extension of the thermal bridging effect, but it is less accurate when quantifying the information.

Similarly, Santos et al. [7] conducted an evaluation of bio-based (pine wood) and recycled (rubber–cork composite) materials to mitigate the thermal bridge effect caused by steel profiles in Lightweight Steel-Framed (LSF) walls. In fact, the high thermal conductivity of steel and the associated thermal bridges can significantly compromise the thermal performance of LSF walls. Their study involved both controlled laboratory measurements and numerical simulation models. The thermal resistance (R-value) of the LSF wall was measured using the Heat Flow Meter (HFM) method in a climatic chamber. These measurements were then compared with calculations from 2D (THERM models) and 3D numerical simulations (ANSYS models), showing a difference of less than ±2%. Both materials performed similarly, with pine wood having a slight advantage due to its greater thickness. Overall, bio-based and recycled materials present an environmentally friendly solution as thermal break strips.

Recent advancements in simulation models for building envelopes could support innovative approaches in environmental design to optimize energy efficiency and thermal performance. These include solar greenhouse technologies, greenery systems, solar chimneys, and ventilated façade systems. Indeed, Kaliakatsos et al. [8] simulated the thermal performance of a bioclimatic greenhouse to assess the influence of features like the type of glazing, thermal mass, size, shading, and ventilation systems, thus providing a general overview of the features that a south-facing solar greenhouse should have when attached to a building in the Mediterranean area. According to their results, the solar greenhouse contributes to greater energy savings in the winter if it has a reduced depth and low-emissivity double glazing. In the summer, the greatest energy savings are achieved by favoring the ventilation of the greenhouse and activating solar shadings to reduce the counterproductive effects of overheating.

Among the possible strategies, the implementation of resilient technologies for the building envelope, such as vertical greenery systems (VGSs), is gaining ground. However, existing models in the literature are not sufficiently detailed in describing all phenomena occurring in a VGS. Nesci et al. [9] tried to overcome this research gap by identifying and improving two mathematical models for green façades and living walls. To this end, a dedicated calculation code to estimate the effect of VGSs on a building’s energy performance and indoor thermal comfort was developed and implemented within the EnergyPlus calculation software. Through a BESTest case study selected from ASHRAE 140, it was shown that the shading effect of the vegetation layer and the evapotranspiration process significantly lower the exterior wall surface temperatures during the summer, thus improving the building’s energy performance and occupant comfort. Indeed, the shielding effect reduces the incident solar radiation on the back wall, and the evapotranspiration effect of both the vegetation and substrate, based on the technology applied, involves cooling in terms of the external surface temperature and immediate surroundings. Instead, the thermal energy needs for cooling in the summer decrease for green façades and for living walls by 15.2% and 8.5%, respectively. The proposed model can easily be used within a dynamic energy simulation tool.

Similarly, using passive strategies, Nguyen et al. [10] studied solar chimneys to reduce the solar heat gain on a building envelope and enhance natural ventilation. In this work, three configurations of two solar chimneys combined with a heated wall to naturally ventilate a room are proposed: (I) the chimneys are connected serially, (II) the chimneys are parallel and exhaust air at two separate outlets, and (III) the chimneys are parallel but the outlets are combined. The airflow rate achieved with each configuration was predicted with a Computational Fluid Dynamics (CFD) model. The results show the effects of the heat flux in each channel and the geometries of the channels. Configuration (II) shows the highest flow rate. Particularly, the proposed configurations significantly enhance the flow rate by up to 40% compared to the typical setup with a single-channel solar chimney. The findings offer a novel design option for building façades to reduce their solar heat gain and enhance natural ventilation. Lastly, Petresevics and Nagy [11] evaluated the point thermal transmittances created by brackets and anchors in ventilated façade claddings using the 3D Finite Element Method (FEM) to carry out thermal modeling. Ventilated façade systems are popular not only for their esthetic properties, but also because they provide mechanical and acoustic protection for the façade and reduce the building’s energy demand. However, point thermal bridges of the fastening system with brackets and anchors are often neglected during simplified energy performance calculations and practical design tasks. To address this, a comprehensive point thermal bridge catalog is created, considering multiple factors of ventilated façades. This research investigates the effects of these parameters on a broader scale than previous studies. Numerical simulations predict the effects of metal fasteners on the thermal performance of the building envelope. The FEM-based results indicate that thermal breaks/isolators can only reduce point thermal transmittances by 2% to 28% depending on the materials of the brackets and isolators. The material and geometric properties of the brackets can result in up to a 70% difference between the corrected and uncorrected thermal transmittance values. The results of the numerical simulations clearly show that considering only the anchors and doweling for mechanical fixings is insufficient. The effect of the brackets on the point thermal transmittance is also significant. Additionally, significant differences are observed when brackets are applied to different types of masonry or reinforced concrete walls.

2.2. Energy Optimization in Buildings

Several papers discuss models to optimize the energy of entire buildings. Particularly noteworthy is the contribution of Chidiac and Marjaba [12], who introduced a new metric known as the building envelope coefficient of performance (BECOP). This comprehensive metric evaluates the thermal performance of building envelopes by comparing them to an ideal system, ensuring applicability across building types and climate zones. The BECOP captures the combined influence of the thermal resistance, climate zone, and internal heat gains. As the heating degree days (HDDs) increase, the BECOP highlights the enhanced impact of an efficient building envelope. A noted weakness is the range of the BECOP given the low efficiency of the building envelope compared to the ideal system. Nonetheless, this weakness can become a catalyst for designing a more efficient building envelope. Furthermore, the BECOP values underscore the energy saving potential achievable with innovative building envelope systems.

Additionally, Kallioğlu et al. [13] illustrated the effects of different insulation materials and fuel types on the cooling and heating performances of buildings situated in hot and dry, warm and humid, composite, and cold climatic conditions in India. Ten different locations were chosen from diverse climatic regions, and various potential parameters for expanded polystyrene and extruded polystyrene insulation materials were evaluated. Their study demonstrates that applying insulation to buildings’ exterior walls results in significant annual savings with a payback period of less than five years, indicating economic feasibility. Additionally, increasing the insulation thickness reduces greenhouse gas (GHG) emissions from fuels. Future research should aim to determine optimal insulation thicknesses for various climate zones and materials.

2.3. The Integration of Renewable Energies

Finally, the perspective paper introduces a new theme: the integration of renewable energy sources (RESs) into the building envelope as a novel research outlook that combines energy efficiency with energy transition. Specifically, the study focuses on the built environment, concentrating on the greatest barriers to the application of these technologies. Particularly, Lucchi [14] explored RES integration within architectural heritage settings, addressing the need to reduce the energy demand and environmental impact without compromising heritage values and esthetic, historical, and material integrity. The perspective study reviews recent studies in the literature to identify key topics, challenges, and advanced solutions for applying solar, wind, geothermal, and bioenergy in heritage contexts. It also highlights acceptability, design criteria, and state-of-the-art technologies through illustrative case studies, offering an understanding of practical implementation strategies. The RES integration criteria in architectural heritage include ensuring conservation compatibility and minimal visual impact to preserve historical and esthetic integrity. Additionally, installations should be reversible, environmentally considerate, and compliant with regulatory standards to balance sustainability with heritage preservation.

3. Conclusions

Adopting a comprehensive design strategy that includes the informed selection of materials with optimal thermal properties and the detailed design of building envelopes is a key action in optimizing the thermal performance of the building envelope. Future research may refer to the following:

• The development of advanced materials, especially those based on bio-based solutions, nanotechnologies, Super Insulating Materials (SIMs), and smart materials.
• The interaction between real-time measurements and advanced simulation tools, thanks to the application of Digital Twins into the building sector.
• The study of hybrid simulation models that combine different tools and techniques (e.g., CFD, FEM, and building energy modeling) to achieve more accurate and comprehensive predictions of a building’s performance.