Regulatory Gap in Fenestration Thermal Performance: Integrating Linear Thermal Transmittance into Energy Codes

Abstract

Fenestration systems play a critical role in building thermal performance, particularly in cooling-dominated climates where envelope inefficiencies directly amplify electricity demand. In Saudi Arabia and other Gulf Cooperation Council (GCC) countries, cooling accounts for the majority of building energy consumption. Nevertheless, the facade and insulated glass industries are experiencing rapid market expansion. Despite this technological evolution, prevailing regulatory frameworks, including the Saudi Building Code Energy Conservation Requirements (SBC 601), ASHRAE 90.1, and the International Energy Conservation Code (IECC), primarily rely on area-weighted U-values and solar heat gain coefficients (SHGCs) without explicitly integrating multidimensional thermal bridge effects such as linear thermal transmittance (ψ). This paper examines the omission of ψ from current energy compliance systems, evaluates its implications in cooling-dominated climates, and proposes a phased regulatory integration pathway aligned with sustainability objectives under Vision 2030. Literature reports indicate that thermal bridges may increase cooling loads by up to 25% and total building energy use by 5–30%, depending on climate severity and façade configuration. The findings highlight the need to transition from simplified prescriptive compliance toward a physics-informed governance capable of addressing evolving facade complexity in hot-arid environments. The proposed framework offers a systematic pathway for integrating linear thermal transmittance requirements while supporting regional sustainability goals and advancing high-performance building technologies.

1. Introduction

1.1. Fenestration Systems in Building Energy Performance

Fenestration systems are among the most thermally vulnerable components of the building envelope, significantly influencing overall energy consumption and occupant comfort. In cooling-dominated climates, buildings experience persistent thermal stress driven by high ambient temperatures, intense solar radiation, and extended cooling seasons [1]. The building sector in hot-arid regions such as Saudi Arabia accounts for approximately 75% of total electrical energy consumption, with cooling demand accounting for the dominant share of electricity use [2]. Research indicates that significant temperature increases amplify cooling loads, underscoring the sensitivity of energy systems to envelope performance and the necessity of accurate thermal assessment methodologies.

Comparable evidence from Taiwan’s cooling-dominated climate reinforces the importance of envelope thermal optimisation. Studies using climatic data report benchmark U-values of approximately 0.794 W/m² K for exterior walls and 0.99 W/m² K for rooftops under standard practice; however, improved performance requires lower U-values and higher R-values, particularly under high solar radiation and humidity. Additional research confirms that optimised insulation thickness, high-performance glazing, and enhanced facade detailing can significantly reduce cooling loads and annual energy demand in subtropical environments, underscoring the need for physics-based envelope assessment in hot and humid climates [3,4,5].

While glazing technologies have evolved considerably toward insulated glass units (IGUs), low-emissivity (Low-E) coatings, thermally broken frames, and curtain wall assemblies, regulatory compliance frameworks have largely retained simplified evaluation approaches. Most energy codes assess window performance using area-weighted U-values and solar heat gain coefficients (SHGCs) without the explicit integration of multidimensional heat transfer effects occurring at glazing–frame interfaces. This structural simplification may lead to significant performance discrepancies between nominal compliance values and actual thermal behaviour in cooling-dominated climates [6].

1.2. Research Questions and Study Objectives

This study addresses three fundamental research questions that guide the literature review and analysis. First, it examines the extent to which current energy codes explicitly evaluate linear thermal transmittance (ψ) in fenestration systems. Second, it examines how this omission affects performance reliability in cooling-dominated climates where the facade market is rapidly expanding. Third, it explores how a phased regulatory framework can integrate thermal bridge evaluation while supporting sustainability objectives under Vision 2030 [7].

This study positions linear thermal transmittance not merely as a thermal parameter, but as a governance variable that mediates the relationship between complexity and national energy resilience.

1.3. Significance for Sustainability and Vision 2030

The rapid expansion of the Saudi facade and glazing market further amplifies the importance of accurate thermal bridge evaluation. Saudi Arabia’s Vision 2030 emphasises sustainable development through economic diversification and energy-efficiency initiatives. Research demonstrates that optimising building envelope design can support these sustainability goals by reducing reliance on energy-intensive cooling systems and offering significant environmental and economic benefits. Understanding and addressing regulatory omissions in thermal performance assessment is therefore essential for achieving national climate objectives [2,8].

The side-by-side visualisation contrasts the absolute scale and projected growth of the global glass curtain wall market with those of the regional Saudi Arabian construction glass sector. Global Market Trends: The left panel shows the global glass curtain wall market, valued at USD 62.4 billion in 2024 and projected to reach USD 107.91 billion by 2032 (refer to Figure 1). This consistent upward trajectory reflects a robust global demand for advanced, energy-efficient building envelopes. The right panel illustrates the construction glass market in Saudi Arabia, which is projected to grow from approximately USD 0.67 billion in 2024 to USD 1.1 billion by 2032, according to Vyansa Intelligence. The regional expansion is primarily driven by massive infrastructure developments and a transition toward high-performance glazing technologies, mandated by the Saudi Building Code (SBC) [9] and Vision 2030 sustainability targets. The rapid expansion of the Saudi facade and glazing industry further intensifies the importance of accurate performance assessment. While the Saudi market remains significantly smaller in absolute terms than the global curtain wall sector, its growth trajectory closely mirrors global expansion rates, underscoring the Kingdom’s accelerating infrastructure development, giga projects under Vision 2030, and increasing adoption of energy-efficient glazing technologies [10,11,12].

Unlike previous studies that focus primarily on technological mitigation strategies or simulation-based optimisation, this study reframes thermal bridge omission as a governance and regulatory design issue, particularly in cooling-dominated economies undergoing rapid expansion of the facade market. Unlike prior technical optimisation studies, this research develops a structured comparative regulatory evaluation, combined with analytical scenario modelling, to quantify the systemic implications of omitting ψ in cooling-dominated climates.

1.4. Theoretical Framework and Quantification Methods

Thermal bridges at window installations significantly affect the energy performance and indoor comfort of buildings, particularly in nearly zero-energy buildings (nZEB) [13]. Linear thermal transmittance (ψ-value) quantifies additional heat losses occurring at junctions between building components, representing a heat flow that simplified one-dimensional U-value calculations cannot capture [14].

Numerical calculations of heat flux through building structure joints have become increasingly relevant due to EU requirements for nearly zero-energy buildings. Window installation perimeters are particularly problematic, where frame fastenings to load-bearing structures create linear thermal bridges. Calculations performed using specialised software in accordance with ISO 10211 [15] standards demonstrate that ψ-values can range widely from 0.025 W/(m K) to −0.005 W/(m·K) for top and side installations, with optimal installation depths of 9–15 cm depending on the insulation layer thickness [16]. Negative ψ-values may occur when insulation overlap results in localised thermal improvement.

A critical comparison of thermal bridge calculation methods reveals significant differences in predicted heat losses. Research examining whole-building thermal bridges compared multiple techniques, including simplified regulatory methods, ISO 14683 [17,18], and conjugate heat-and-moisture simulation. Overall heat losses varied by 30% depending on the calculation method, with linear heat losses ranging between 12% and 32% of surface heat losses. The evaluation demonstrated that simplified methods produced the lowest heat losses, while ISO 14683 [17] produced the highest results, with numerically simulated results falling between the two [19,20].

Fenestration systems exhibit multidimensional heat transfer behaviour due to interactions among glazing layers, spacer bars, frames, and structural interfaces. Unlike opaque walls, window assemblies involve significant two-dimensional conductive effects, particularly at glazing edges. Fenestration systems exhibit multidimensional heat transfer behaviour due to interactions among glazing layers, spacer bars, frames, and structural interfaces. Unlike opaque walls, window assemblies involve significant two-dimensional conductive effects, particularly at glazing edges (see Figure 2).

The overall heat transfer coefficient of a window is typically expressed by Equation (1):

$$U_{window}={\displaystyle \frac{A_g{U}_g+A_f{U}_f+L\psi }{A_{total}}}$$

(1)

where

• $U_g$ = glazing transmittance;
• $U_f$ = frame transmittance;
• $A_g$, $A_f$ = glazing and frame areas;
• $L$ = length of glazing–frame interface;
• $\psi$ = linear thermal transmittance.

• where L_ψ represents the product of the glazing–frame interface length and the corresponding linear thermal transmittance, expressed in W/m² K after normalisation by total area. Linear thermal transmittance represents additional heat flow not captured by area-weighted terms alone. In high-performance glazing systems, ψ may contribute a non-negligible proportion of total thermal loss.

Hierarchical representation of building-scale energy demand, whole-window U-value assessment, and component-level parameters including frame transmittance (U_f), glazing transmittance (U_g), and linear thermal transmittance (ψ), illustrating multidimensional heat transfer effects.

As shown in Figure 3, thermal bridging can significantly influence building energy performance, with reported increases in overall energy consumption ranging from approximately 5% to 30% [21,22]. The effect is particularly pronounced in heating-dominated conditions, where energy demand may increase by up to 32%. In comparison, cooling loads may rise by as much as 25% depending on facade configuration and insulation continuity. These findings underscore the importance of mitigating linear and point thermal bridges in high-performance building envelopes to meet energy efficiency standards and sustainability targets [14].

Figure 4 compares the implementation of thermal bridge calculation methods across selected European countries. It examines whether countries use detailed ψ-value calculations, default values, mean U-values, or simplified basic rules in national energy performance assessments.

The results show considerable variation in methodological approaches, reflecting differences in regulatory frameworks and levels of calculation accuracy across Europe [23].

Figure 5 presents thermal performance improvements in hot-arid climates from individual case studies: a PCM dual-layer roof (India), envelope optimisation (Iraq), and envelope retrofit (Saudi Arabia). Percentages correspond to reductions in the primary performance metric reported in each study (heat flux, cooling load, or total energy demand) and are not directly comparable. The PCM-integrated dual-layer roof demonstrates the greatest impact, achieving an approximately 79.8% reduction in heat flux under experimental conditions. Envelope optimisation measures in Iraq have achieved cooling load reductions of up to 62%, primarily through improved insulation and glazing. Similarly, envelope retrofit strategies in Saudi Arabia report total annual energy consumption reductions of about 62%, highlighting the critical role of building envelope enhancement in achieving zero-energy targets. The percentages represent reported reductions in the cooling load, heat flux, or total energy demand under hot-arid climate conditions.

As shown in Figure 6, advanced installation methods significantly reduce thermal bridge intensity compared to traditional mounting [13]. Finite element method (FEM) simulations have become essential for evaluating thermal bridge effects in building envelopes. Studies using commercial 3D simulation software to estimate thermal losses demonstrate that the linear thermal transmittance of thermal bridges and associated heat flux loss can decrease by more than 50% when proper mitigation measures are implemented [27]. Optimising thermal insulation parameters through mathematical modelling enables the determination of optimal values based on energy-efficiency criteria, providing useful information for researchers, designers, and decision-makers [28].

Artificial intelligence approaches offer promising alternatives for thermal bridge analysis, with fuzzy systems demonstrating excellent performance in estimating linear heat transmittance coefficients. These AI-based approaches can reduce the need for time-consuming traditional calculations and expensive experiments while accurately determining ψ coefficients for cases not included in training data [29].

Collectively, these findings demonstrate that ψ-values represent a quantifiable and methodologically mature parameter whose exclusion from compliance frameworks is not due to technical limitations but regulatory simplification.

This study extends beyond literature synthesis through:

• Development of a structured regulatory comparison matrix across four major frameworks.
• Introduction of a climate-amplified ψ sensitivity model specific to GCC environments.
• Parametric quantification of ψ omission effects on cooling loads.
• Construction of a phased regulatory integration roadmap aligned with Vision 2030.

Unlike prior technical studies focused solely on thermal optimisation, this research integrates thermal physics modelling with governance architecture analysis, providing a dual-layer analytical framework.

2. Methodology

2.1. Research Design and Analytical Structure

This study adopts a comparative regulatory and deterministic steady-state analytical modelling based on standardised heat transfer formulations to investigate the structural omission of linear thermal transmittance (ψ) in fenestration-related energy compliance frameworks. The analytical modelling in this study does not involve finite-element simulation but instead uses standardised steady-state formulations consistent with ISO-based performance calculations to evaluate systemic regulatory implications. Although dynamic hourly simulations may yield different absolute values, proportional ψ-amplification trends remain consistent under equivalent ΔT conditions.

The methodology integrates regulatory evaluation, analytical heat transfer modelling, climate context assessment, and governance pathway development into a unified analytical framework. Rather than conducting a narrative literature synthesis, the study is structured around the following core analytical layers:

• Regulatory Structure Assessment—identifying formal treatment of ψ within energy codes.
• Thermal Performance Quantification—evaluating the mathematical implications of ψ omission using standardised heat transfer formulations.
• Climate Sensitivity Evaluation—assessing amplification effects in cooling-dominated climates.
• Regulatory Integration Modelling—constructing a phased governance transition framework.

The research workflow proceeds sequentially: (a) identification of regulatory variables, (b) development of a structured comparison matrix, (c) analytical scenario modelling, (d) climate amplification interpretation, and (e) governance pathway synthesis. This structured approach, as shown in Figure 7, ensures reproducibility and transparency in assessing the compliance gap.

Energy codes were selected based on:

• Direct applicability in the GCC (SBC) [9];
• Structural influence on SBC (IECC alignment);
• International benchmarking relevance (ASHRAE 90.1) [30];
• Mandatory ψ integration precedents (EPBD-aligned frameworks).

Documents were reviewed using the officially published SBC 601 and associated referenced standards, which remain the legally adopted energy conservation requirements in the Kingdom at the time of this analysis.

• 1 = Explicit requirement;
• 0.5 = Partial/implicit integration;
• 0 = Not addressed.

The aggregate governance maturity index was computed for comparative purposes.

Code documents were manually reviewed for:

• Explicit ψ references;
• ISO standard citation;
• Whole-window Uw formulation;
• Installation-level regulation;
• Verification stage requirements.

Cross-check validation was performed independently by both authors.

2.2. Structured Comparative Regulatory Matrix Development

A systematic comparative matrix was constructed to evaluate how major energy codes treat fenestration thermal performance and thermal bridge effects. Four frameworks were selected due to their relevance to GCC regulatory practice and international benchmarking, such as (a) Saudi Building Code (SBC 601), (b) ASHRAE 90.1, (c) International Energy Conservation Code (IECC), and (d) EPBD-aligned European codes.

Each framework was evaluated across multiple analytical dimensions, including:

• Explicit requirement for ψ-value calculation;
• Reference to ISO 10211/ISO 14683;
• Requirement for numerical 2D/3D modelling;
• Treatment of installation-level thermal bridges;
• Inclusion of ψ in the overall window U-factor (Uo) formulation;
• Recognition of thermal bridge impact on peak load;
• Climate adaptation orientation;
• Alignment with nearly zero-energy building (nZEB) targets;
• Verification stage (design vs. as-built).

Each dimension was categorised using a qualitative classification system: explicitly required/not required, implicitly embedded, and partially integrated. This matrix-based approach allows for structural comparison rather than descriptive commentary. The analysis focuses on governance architecture rather than performance thresholds, enabling identification of systemic regulatory omission patterns. The analytical structure of this comparative matrix and its integration with thermal physics modelling are illustrated in Figure 8.

The resulting comparative framework in the forthcoming setion serves as the analytical foundation for diagnosing the regulatory discontinuity.

2.3. Climate Amplification and Governance Gap Modelling

To assess the systemic relevance of ψ omission in hot-arid environments, a climate amplification framework was applied. Cooling-dominated climates are characterised by high cooling degree days, prolonged peak-load periods, elevated solar radiation, and large exterior–interior temperature gradients. Incremental heat gain attributable to ψ was evaluated using the steady-state heat transfer relation, as shown by Equation (2):

$$Q=U\cdot A\cdot \mathsf{\Delta }T$$

(2)

Under elevated ΔT conditions, even small increases in the effective U-value result in greater amplification of cooling demand. This relationship was interpreted at three scales:

• Component scale (window interface);
• Facade scale (cumulative interface length);
• Building scale (peak load implications).

To illustrate the thermal severity of cooling-dominated regions within Saudi Arabia, long-term average monthly ambient temperatures for representative cities across different climatic zones are presented in

Table 1. Long-term average monthly ambient temperatures (°C) in representative Saudi cities [31].

Month	Madinah	Riyadh	Guriat	Khamis Mushait	Jeddah	Dahran
Jan	18.3	15.1	7.5	13.5	23.9	15.0
Feb	20.2	16.5	10.0	16.0	24.8	16.5
Mar	24.4	22.7	12.0	17.0	26.4	20.0
Apr	28.2	26.6	18.0	20.0	28.8	24.9
May	34.1	32.5	23.0	23.0	31.8	30.5
Jun	37.8	36.0	27.0	26.0	32.8	34.8
Jul	37.9	37.4	30.0	26.0	33.9	35.9
Aug	37.3	36.5	30.0	25.0	33.9	36.2
Sep	37.0	33.9	27.0	23.0	32.7	33.9
Oct	31.0	27.5	23.0	22.0	31.0	29.1
Nov	24.9	20.4	15.0	17.0	28.5	23.9
Dec	20.5	16.1	8.0	15.0	26.1	16.8

. The selected cities reflect inland desert climates (Riyadh, Guriat), coastal humid climates (Jeddah, Dahran), and elevated southwestern climates (Khamis Mushait), thereby capturing national climatic diversity.

As shown in Table 1, peak summer temperatures exceed 37 °C in inland cities such as Madinah and Riyadh. In comparison, coastal regions such as Jeddah and Dahran exhibit persistently high ambient temperatures above 30 °C for extended periods. Even during winter months, several locations maintain moderate temperatures, indicating prolonged cooling demand seasons. The sustained high ΔT between exterior ambient conditions and conditioned indoor setpoints (typically 23–25 °C) amplifies envelope heat gain through both area-based and linear thermal transmittance components. Under such conditions, incremental ψ-related heat transfer becomes proportionally more significant in annual cooling load calculations.

For example, assuming an indoor setpoint of 24 °C, peak summer ΔT in Riyadh reaches approximately 13–14 °C during July–August, while in coastal Dahran, ΔT remains above 10 °C for five consecutive months. Such sustained temperature differentials intensify cumulative linear heat transfer across facade interfaces.

To directly link climatic data in Table 1 with ψ-related amplification, a simplified sensitivity analysis was performed using representative peak summer monthly averages for Riyadh (37.4 °C) and Jeddah (33.9 °C). Assuming an indoor setpoint of 24 °C, the resulting ΔT values are approximately:

• Riyadh (July): ΔT ≈ 13.4 °C;
• Jeddah (July): ΔT ≈ 9.9 °C.

Using Equation (2):

$$Q=U\cdot A\cdot \mathsf{\Delta }T$$

For the previously defined façade module (A = 90 m²) and the conventional installation scenario (Uw = 2.47 W/m²K), peak sensible heat gain becomes:

• Riyadh: Q ≈ 2.98 kW;
• Jeddah: Q ≈ 2.20 kW.

The 0.78 kW difference represents a 35% amplification in ψ-related cooling impact attributable solely to climatic severity. This quantitative comparison demonstrates that identical facade detailing results in disproportionately higher cooling penalties in inland hot-arid climates compared to coastal regions.

Parallel to thermal modelling, a governance gap assessment was conducted by comparing technological capability (availability of ISO-compliant ψ methods), regulatory requirement presence, market expansion indicators (facade complexity growth), and sustainability targets under Vision 2030. This dual-layer (technical + governance) evaluation enables identification of a structural regulatory–market integration gap. Having established the technical and climatic implications of omitting ψ, the following section evaluates how these parameters are formally addressed in prevailing energy codes.

2.4. Parametric Scenario Modelling of ψ Impact in GCC Cooling Climates

To strengthen the analytical contribution of the study, a representative parametric scenario was developed to quantify the potential impact of linear thermal transmittance (ψ) on cooling demand in hot-arid climates. A simplified but representative mid-rise residential facade module typical of urban Saudi construction was defined with the following parameters:

• Window size: 1.5 m × 1.5 m;
• Total window area (A_total): 2.25 m²;
• Glazing area (A_g): 1.89 m²;
• Frame area (A_f): 0.36 m²;
• Glazing U-value (U_g): 1.6 W/(m² K);
• Frame U-value (U_f): 2.8 W/(m² K);
• Glazing–frame interface length (L): 6.0 m.

Three ψ scenarios were evaluated (

Table 2. Linear thermal transmittance (ψ) scenarios evaluated in the parametric model.

Scenario	ψ (W/m·K)	Description
S1	0.02	Optimised installation
S2	0.08	Typical thermally broken frame
S3	0.27	Conventional installation

):

Whole-window U-values were calculated using Equation (1).

The results are presented in

Table 3. Calculated whole-window U-values under varying ψ conditions.

Scenario	Uw (W/m2 K)
S1 (ψ = 0.02)	1.79
S2 (ψ = 0.08)	1.95
S3 (ψ = 0.27)	2.47

.

The omission of ψ in simplified area-weighted calculations would yield:

$$U_{no\hspace{0.17em}\psi }=1.79\hspace{0.17em}\mathrm{W}/\left({\mathrm{m}}^2\mathrm{K}\right)$$

Thus, neglecting ψ may underestimate window heat transfer by:

• +6% (optimised case);
• +15% (moderate case);
• +46% (conventional case).

Assuming:

• Peak summer ΔT in Riyadh = 14 °C;
• Facade module repetition: 40 identical windows;
• Total window area = 90 m².

Peak sensible heat gain difference between optimised (S1) and conventional (S3): ΔQ ≈ 1.07 kW (per facade).

Over extended cooling seasons, this difference compounds into significant annual energy penalties. Parametric ΔT values were varied from 8 °C to 16 °C, representing coastal vs. inland climates. Results indicate ψ-related heat gains scale linearly with ΔT, amplifying performance gaps by up to 100% between mild and extreme conditions. This demonstrates that ψ omission becomes progressively more consequential in cooling-dominated environments characterised by sustained high-temperature gradients.

Sensitivity to window-to-wall ratio (WWR) was assessed by proportionally scaling the facade interface length and glazing area. Increasing WWR from 25% to 50% approximately doubles cumulative glazing–frame interface length, thereby proportionally increasing ψ-related conductive heat gain under identical ΔT conditions. Orientation effects further modulate ψ impact, with west-facing facades in hot-arid climates experiencing higher combined solar and conductive loads. While SHGC remains dominant for peak solar exposure, ψ-related transmission becomes increasingly relevant during extended evening and non-solar periods. The objective of the parametric scenario is not micro-scale junction optimisation but the systemic quantification of regulatory omission effects; therefore, steady-state analytical modelling is appropriate for governance-level assessment.

Although full finite-element modelling was not performed, the parametric scenario reflects a representative Saudi residential facade configuration using ISO-consistent steady-state formulations, thereby providing a quantitative ψ-based evaluation aligned with regional construction practice.

3. Regulatory Frameworks for Fenestration Assessment

The EPBD directive in 2002 and its 2010 recast have led to significant efforts in member states to improve building energy performance, with transmission characteristics playing a significant role in energy-efficient buildings. A review of thermal bridge treatment in energy calculation and compliance procedures across nine European countries reveals four main methods: detailed calculation based on linear thermal transmittance values, simple basic rules, default transmittance values, and mean U-values. Significantly, the review found that there are often no specific thermal bridge-related compliance procedures, with control mechanisms frequently ending at the building permit phase [23].

In the UAE, buildings consume more than 80% of total electrical generation, with cooling systems responsible for approximately 70% of buildings’ peak load [32]. Despite having similar climate conditions and construction practices, green building regulations in Dubai, Abu Dhabi, and Ras Al Khaimah have different threshold requirements. For example, the maximum thermal transmittance (U-value) for exterior walls varies from 0.57 W/m² K in Dubai to 0.32 W/m² K in Abu Dhabi to 0.48 W/m² K in Ras Al Khaimah (refer to

Table 4. Comparison of thermal transmittance requirements in GCC regions (source: [32]).

Region	Maximum Wall U-Value (W/m2K)	Thermal Bridge Requirements	Compliance Focus
Dubai	0.57	Not explicitly specified	Prescriptive
Abu Dhabi	0.32	Limited guidance	Prescriptive
Ras Al Khaimah	0.48	Not specified	Prescriptive
nZEB Examples	0.06–0.09	Detailed analysis	Performance-based

). Constructed nearly zero-energy buildings demonstrate U-values substantially lower than regulations, between 0.06 and 0.09 W/m²K, indicating significant gaps between minimum regulatory requirements and achievable performance [32]. A review of SBC 601 confirms that the code contains no explicit reference to linear thermal transmittance (ψ), no requirement for junction-level heat transfer calculations, and no reference to ISO 10211 or ISO 14683 methodologies. Window performance is evaluated using area-weighted overall U-factors (Uo) and SHGC limits without multidimensional interface modelling.

The general conclusion from international code analysis indicates that compliance frameworks need to be extended to assess as-built energy performance. This regulatory gap is particularly significant in the context of the rapid expansion of the facade market in the Gulf region, where sophisticated curtain wall and insulated glazing technologies are being deployed without corresponding regulatory provisions for multidimensional thermal assessment. The absence of explicit ψ-value requirements in prevailing codes, including the Saudi Building Code (SBC), ASHRAE 90.1, and the International Energy Conservation Code (IECC), creates a significant regulatory misalignment between available facade technologies and compliance methodologies [33,34,35].

Figure 9 provides a conceptual comparison of international regulatory approaches, highlighting the explicit inclusion of ψ-calculation in ISO-based methodologies and its omission in prescriptive frameworks such as ASHRAE 90.1, IECC, and SBC. This divergence illustrates the structural regulatory omission central to this study.

The comparative matrix (see

Table 5. Thermal bridge treatment in SBC, ASHRAE 90.1, IECC, and EPBD.

Dimension	SBC (Saudi Building Code)	ASHRAE 90.1	IECC	EPBD (EU Directive Framework)
Regulatory Philosophy	Prescriptive compliance largely aligned with the IECC structure	Hybrid (prescriptive + performance-based)	Primarily prescriptive, with the performance path option	Performance-oriented, lifecycle-based framework
Climate Context Orientation	Cooling-dominated (hot-arid focus)	Multi-climate (US climate zones)	Multi-climate (US climate zones)	Mixed climates (heating-dominated emphasis historically)
Window Performance Metrics Required	Area-weighted U-value + SHGC	U-factor + SHGC (via NFRC certification)	U-factor + SHGC	U-value + thermal bridge consideration (varies by member state)
Explicit ψ (Linear Thermal Transmittance) Requirement	Not explicitly required	Not explicitly required	Not explicitly required	Required in many member states under EPBD implementation
Reference to ISO 10211/14683	No direct reference	Not referenced	Not referenced	ISO 10211 is commonly referenced for 2D modelling
Indirectly Reflected in the Tested Assembly U-Values	No explicit junction-level ψ modelling required	Incorporated indirectly via tested assemblies	Not separately calculated	Often explicitly calculated at the junction level
Overall Window U-Factor (Uo), Including ψ	Not mandated	Not mandated	Not mandated	Often required in nZEB contexts
Numerical 2D/3D Thermal Modelling Required	No	No	No	Frequently required for compliance
Condensation Risk Assessment	Limited	Limited	Limited	Often mandatory (surface temperature factor)
Treatment of Installation Effects	Not regulated	Not explicitly regulated	Not explicitly regulated	Often included in a detailed calculation
Compliance Pathways	Prescriptive tables	Prescriptive + energy cost budget method	Prescriptive + performance path	Performance-based national implementation
Recognition of Thermal Bridge Impact on Peak Load	Not explicitly	Not explicitly	Not explicitly	Indirectly, through transmission loss calculations
Verification Stage	Design-stage compliance	Design-stage compliance	Design-stage compliance	Increasing emphasis on as-built performance
Alignment with Nearly Zero-Energy Building (nZEB)	Emerging	Partial alignment	Partial alignment	Strong alignment (mandatory under EPBD recast)
Market–Regulation Integration Level	Low (rapid facade market growth not matched by regulation)	Moderate	Moderate	Higher (advanced facade integration in EU markets)
Regulatory Treatment of Multidimensional Heat Transfer	Simplified 1D assumption	Simplified 1D assumption	Simplified 1D assumption	Recognises multidimensional heat flow
Cooling-Dominated Climate Adaptation	Emphasis on SHGC limits and area-weighted U-factor compliance	Balanced heating/cooling	Balanced heating/cooling	Historically heating-focused, expanding scope
Governance Maturity for ψ Integration	Early stage	Developed but ψ omitted	Developed but ψ omitted	Advanced (in several member states)

) reveals a structural divergence between ISO-influenced European regulatory frameworks and prescriptive North American-derived codes adopted in the GCC region. While SBC, ASHRAE 90.1, and IECC emphasise area-weighted U-values and SHGC as primary fenestration compliance metrics, none explicitly mandate linear thermal transmittance (ψ) evaluation. In contrast, EPBD-aligned national codes increasingly require junction-level ψ calculations using ISO 10211 methodologies, particularly in nearly zero-energy building (nZEB) contexts. This discrepancy illustrates the governance gap underlying the systemic exclusion identified in cooling-dominated climates.

This structural omission is not due to technical infeasibility, as ISO-compliant ψ-calculation methodologies are mature and internationally standardised, but rather reflects regulatory simplification inherited from the IECC-based framework adopted in SBC 601.

4. Thermal Performance Challenges in Cooling-Dominated Climates

4.1. Unique Requirements for Hot-Arid Environments

Thermal performance, energy demand, and environmental sustainability in hot-arid climates where cooling loads dominate are strongly influenced by facade design. Research demonstrates that climate-responsive passive facade strategies represent an effective approach to enhancing energy efficiency and sustainability in hot-arid environments, with experimental comparisons showing average heat flux reductions of 11.35% through detached curtain wall configurations. The parametric optimisation of building envelopes for energy efficiency in Taiwan’s subtropical climates demonstrates that optimised insulation strategies can achieve annual cooling energy savings of 1–3% when modelled with realistic HVAC systems, with climate projections indicating 5.12–11.15% increases in future cooling demand by 2080 [36,37,38,39].

Studies on hot-arid climates reveal that the solar transmittance (g-value) of windows plays a more important role than thermal transmittance (U-value) in reducing heat gain within interior spaces. In the UAE context, the choice of building materials significantly impacts a building’s passive performance and carbon footprint, with the incorporation of 50 mm expanded polystyrene insulation into external walls demonstrating substantial reductions in cooling requirements [40].

The interaction between climate severity, facade complexity, and regulatory simplification is conceptualised in Figure 10. In cooling-dominated environments, even marginal increases in envelope heat transfer can translate into disproportionately higher cooling loads, intensified peak demand, and grid stress. Although SHGC dominates peak solar gains, the ψ-related conductive heat transfer contributes to the cumulative cooling demand, particularly during non-solar load periods.

4.2. Window-to-Wall Ratio and Glazing Optimisation

Research examining the impacts of the window-to-wall ratio (WWR) on energy performance in Riyadh, Saudi Arabia, demonstrates that optimising WWR to 25% reduced the energy consumption by 5.6% and cooling loads by 7.5%. These findings, as shown in Figure 11, emphasise the critical role of building envelope design in enhancing energy efficiency, particularly in hot climates where balancing daylighting requirements with thermal insulation is essential. An evaluation of advanced glazing technologies for residential buildings in Jeddah reveals that 36 mm aerogel glazing (U = 0.9 W/m²K, SHGC = 0.3) reduces the annual cooling demand by 48.6% compared to single-pane glazing. Notably, 87% of these savings derive from the SHGC reduction, with only 3.02 percentage points attributable to U-value improvements [41].

4.3. Integration of Advanced Technologies

Double skin facades (DSFs) have received increasing attention as alternatives to conventional glazed curtain walls for their ability to effectively reduce thermal transmittance (U-value) and solar heat gain (G-value). DSF design comprises assessments of building geometric factors, glazing type, ventilation procedures, shading devices, daylighting, and maintenance expenses [42]. Research on high-rise residential buildings in Abu Dhabi demonstrates that optimal DSF configurations with a 35 cm cavity depth, comprising a double-glazed single-skin layer and a Low-E exterior layer, can reduce overall energy consumption by over 25% [43].

Phase change materials (PCMs) integrated into building envelopes have significant potential to reduce cooling loads in hot climates. Studies examining spherical PCM modules embedded within reinforced concrete roofs demonstrate average reductions in indoor surface temperatures of 10.2 °C and decreases in cooling loads of up to 69%. These findings highlight the practical viability of PCM-integrated systems as passive cooling strategies for buildings in hot climates [44]. Recent advances in radiation-assisted thermal regulation, selective emissivity surfaces, and thermal-diode-assisted cooling concepts offer additional pathways to reduce heat transfer irreversibilities in building envelopes and refrigeration systems. While these technologies primarily address radiative and active heat management mechanisms rather than conductive thermal bridges, their integration into future facade systems may further enhance overall thermal resilience in hot-arid climates.

While Section 4 outlines climate-driven amplification mechanisms, the following case studies demonstrate how targeted fenestration strategies can mitigate these effects in practice.

5. Case Studies: Fenestration Performance and Energy Savings

5.1. Window Installation Optimisation

Research on window mounting position effects shows that relatively small changes in the installation position can markedly reduce thermal bridging. The most effective strategy is to install windows within the insulation layer at an optimal depth of 7–12 cm, achieving reductions in the ψ-value from 0.27 W/(m·K) to 0.02 W/(m·K). With frame overlap and frame extenders, ψ-values can be further reduced to 0.005 W/(m·K) in optimal configurations. In case studies of historical buildings retrofitted to the Passive House standard, installing windows in the insulation layer reduced annual heating demand from 32 kWh/m² to 24 kWh/m² [13]. It should be noted that the referenced study was conducted in a heating-dominated climate, where a reduction in ψ decreases transmission heat losses. In cooling-dominated climates such as Saudi Arabia, the thermodynamic direction is reversed: a reduction in ψ decreases conductive heat gain rather than heat loss. While the magnitude of seasonal energy savings may differ due to the relative dominance of solar gains and internal loads, the proportional reduction in transmission-related load remains comparable under equivalent ΔT conditions. Therefore, ψ optimisation remains technically relevant in hot-arid climates, particularly during non-solar load periods and during extended high-temperature seasons.

5.2. Mitigation Technologies and Strategies

Cutting-edge technologies demonstrate considerable energy-saving potential for addressing thermal bridges in fenestration systems. As shown in

Table 6. Thermal bridge mitigation strategies for fenestration systems (synthesis from multiple studies).

Mitigation Strategy	LTT Reduction	Energy Savings	Implementation Complexity
Vacuum Glazing	U < 0.2 W/m2K	22–30%	High
Aerogel Frame Cavities	45% permeability reduction	15–25%	Medium
Optimal Window Positioning	Up to 90–95%	20–25% heating	Medium
Thermally Broken Frames	60–80%	15–20%	Low-Medium
Precise Installation	Up to 80%	10–15%	Low

, vacuum glazing with U-values as low as 0.2 W/m²·K and aerogel-filled frame cavities, reducing thermal permeability by 45%, represent significant technological advances. Furthermore, precise installation techniques can lower linear thermal transmittance (LTT) by up to 80%. A holistic approach integrating advanced glazing technologies, optimised frame materials, and meticulous installation methods offers a powerful solution to enhance window thermal efficiency.

Despite demonstrated mitigation potential, regulatory frameworks have not kept pace with technological advancements. Figure 12 illustrates the divergence between rapid growth in the facade market and continued reliance on prescriptive, simplified metrics, resulting in a performance–integration gap.

5.3. Retrofit and Near-Zero Energy Achievement

Achieving near-zero energy in hot climates requires comprehensive retrofitting approaches focused on building envelope optimisation. Research on residential villas in Riyadh demonstrates that optimising facade elements alone reduced annual energy consumption by 62%, from 60,641 kWh to 37,801 kWh. Integration of photovoltaic systems further decreased the net annual energy consumption to 9489 kWh, representing an 84% reduction compared to the base case. These findings align with the sustainability objectives of Saudi Vision 2030 and demonstrate the transformative potential of envelope-focused retrofit strategies [45,46,47,48].

6. Proposed Regulatory Integration Framework

6.1. Phased Approach to Code Revision

The transition from simplified prescriptive compliance toward physics-informed governance requires a systematic, phased approach. This framework should address the regulatory-market performance gap identified in current energy compliance systems while aligning with sustainability objectives under Vision 2030. The proposed pathway involves initial capacity building, followed by pilot implementation, and culminating in mandatory compliance requirements for thermal bridge assessment in fenestration systems.

To operationalise the transition toward physics-informed governance, a three-layer evaluation structure is proposed (Figure 13) that links component-level ψ calculations, assembly-level compliance metrics, and building-scale sustainability outcomes.

The phased implementation pathway illustrated in Figure 14 provides a structured regulatory transition model that progresses from voluntary ψ-reporting a full integration of performance-based compliance.

Research demonstrates that implementing government-funded building energy efficiency programmes is highly cost-effective, potentially reducing annual electricity consumption by 11,000 GWh and peak demand by 2500 MW, while creating over 4000 jobs during ten-year implementation periods. These macro-economic benefits support the case for regulatory advancement in the GCC context [49].

6.2. Technical Requirements and Standards Adoption

The integration of thermal bridge evaluation into regulatory frameworks requires the adoption of standardised calculation methodologies. ISO 10211 provides the foundation for numerical modelling approaches, while ISO 14683 offers simplified catalogue-based methods suitable for initial implementation. The application of building performance simulation tools integrated with BIM workflows enables a comprehensive assessment of envelope thermal performance, including thermal bridge effects [50,51,52]. To facilitate systematic adoption within national building codes, a structured implementation pathway is necessary. As summarised in

Table 7. Proposed phased regulatory integration framework for thermal bridge assessment.

Implementation Phase	Timeline	Key Actions	Compliance Level
Phase 1: Awareness	Years 1–2	Training, catalogue development	Voluntary
Phase 2: Pilot	Years 3–4	Large projects, data collection	Incentivised
Phase 3: Transition	Years 5–6	All new construction	Partially mandatory
Phase 4: Full Integration	Years 7+	All buildings, retrofits	Mandatory

, the proposed phased regulatory integration framework outlines a progressive transition from voluntary awareness-building measures toward full mandatory compliance. The staged approach allows for capacity development, industry training, and methodological standardisation to mature before full enforcement is introduced. Such a framework reduces implementation risk while ensuring long-term regulatory alignment with high-performance envelope standards.

Emerging AI-assisted estimation tools for linear thermal transmittance offer promising avenues for simplifying compliance procedures. Machine-learning-based ψ prediction models trained on validated simulation datasets can provide rapid estimates of common construction details, reducing reliance on time-intensive 2D/3D numerical modelling. Such tools could serve as intermediate compliance aids in phases 2 or 3 of the proposed regulatory roadmap, particularly for standardised residential typologies.

Preliminary cost implications of mandatory compliance with ψ-values are expected to be moderate for residential construction. In most cases, compliance does not require fundamentally new materials but rather improved installation detailing, thermally broken frames, or optimised mounting positions. The literature suggests that thermally broken frames increase window unit costs by approximately 5–10%, while installation optimisation primarily involves design-stage adjustments rather than substantial material cost escalation. When amortised over lifecycle energy savings in cooling-dominated climates, incremental construction cost is likely to be offset by reduced operational expenditures.

The widespread implementation of ISO 10211 may initially face capacity constraints among smaller design firms lacking advanced thermal simulation expertise. The proposed phased integration framework mitigates this challenge by introducing simplified ISO 14683 catalogue-based methods during early adoption stages, along with national training programmes and standardised detail libraries. This gradual transition reduces technical barriers while building institutional competence.

6.3. Alignment with Vision 2030 and Sustainability Goals

The building sector in Saudi Arabia consumes about 75% of total electrical energy, with unprecedented consumption growth driven by rapid population growth and urbanisation. Vision 2030 commitments to promote energy efficiency and renewable energy technologies create an environment that enables regulatory advancement. Research demonstrates that several green building concepts are crucial for design and operation in hot-dry regions, including thermal mass, daylight optimisation, natural ventilation, cavity walls, double-glazing, and solar panels.

Figure 15 conceptualises the alignment pathway between current prescriptive energy code metrics and broader national sustainability objectives under Vision 2030. Integrating ψ-value assessment acts as a bridging mechanism between envelope-level physics and macro-scale energy policy goals. Adaptive facade technologies, including double-skin facades, phase-change materials, and dynamic shading systems, can complement ψ-based compliance by reducing both solar and conductive heat gains. While the current regulatory proposal focuses on accountability for conductive thermal bridges, future code evolution could integrate adaptive facade performance metrics into performance-based compliance pathways, particularly in large-scale commercial or high-rise projects.

Façade-integrated photovoltaic systems represent a technically viable and environmentally beneficial renewable energy solution aligned with Saudi Vision 2030 sustainability objectives. A 5.6 kWp BIPV facade system analysis demonstrates an approximately 8200 kWh annual electricity production, with ventilated facade configurations reducing temperature-related efficiency losses while contributing to a reduced cooling demand and lower carbon emissions [53].

Figure 16 illustrates the parametric influence of linear thermal transmittance (ψ) on incremental peak cooling heat gain under representative GCC temperature gradients. The results are derived from Equation (2), where heat gain scales proportionally with both ψ and the external–internal temperature differential (ΔT).

As ψ increases from 0 to 0.30 W/m·K, the cooling heat gain rises linearly across all climate scenarios. The slope of each curve is directly proportional to ΔT, demonstrating that identical facade detailing results in substantially higher cooling penalties in inland GCC climates than in coastal conditions.

At ψ = 0.30 W/m·K, the extreme inland scenario (ΔT = 16 °C) exhibits approximately double the incremental cooling load of the coastal scenario (ΔT = 8 °C). This proportional amplification confirms that ψ omission in regulatory calculations becomes increasingly consequential under the sustained high temperature gradients typical of GCC environments.

The analysis demonstrates that, although ψ is a localised linear parameter, its cumulative impact at the facade scale can result in measurable peak cooling penalties. Therefore, the exclusion of ψ from code-based window performance calculations systematically underestimates the cooling demand in hot-arid climates.

6.4. Proposed Pilot Validation Framework for the Saudi Context

To enhance policymaker confidence, a pilot validation study is recommended for implementation in a representative public building in Riyadh. The proposed design would include:

• Instrumented facade sections with two installation configurations (optimised ψ vs. conventional).
• Embedded heat flux sensors and surface temperature probes at glazing–frame interfaces.
• Hourly indoor–outdoor temperature monitoring over peak summer months.
• Comparison of measured cooling load differences through HVAC energy metering.

Such a study would generate climate-specific ψ performance data, validate steady-state analytical assumptions, and provide empirical grounding for regulatory integration. The framework could be implemented within a government-funded demonstration building aligned with Vision 2030 sustainability initiatives.

Compared to EPBD-aligned European systems, the proposed GCC integration pathway emphasises staged enforceability rather than immediate full-scale numerical compliance. European frameworks often require detailed junction-level ψ calculations, supported by mandatory 2D modelling and condensation risk assessment, resulting in greater technical complexity but stronger enforcement mechanisms. The GCC pathway proposed in this study adopts a lower initial complexity through voluntary reporting and catalogue-based methods, progressively converging toward performance-based compliance. This approach prioritises regulatory feasibility and industry readiness while maintaining long-term alignment with international best practice. The present analysis is based on SBC 601. Future revisions of the Saudi Building Code may incorporate updated methodologies, and periodic review of code evolution is recommended.

7. Conclusions and Recommendations

This comparative regulatory and analytical study identifies a significant regulatory omission in the assessment of fenestration thermal performance in cooling-dominated climates. Current energy codes, including the Saudi Building Code (SBC), ASHRAE 90.1, and IECC, primarily rely on area-weighted U-values and solar heat gain coefficients without explicitly integrating linear thermal transmittance (ψ) evaluation. Published studies report that thermal bridges increase total building energy consumption by 5–30% and may elevate cooling loads by up to 25% in climate-sensitive facade configurations. The rapid expansion of sophisticated facade technologies in the GCC region amplifies the importance of addressing this regulatory gap. The combined regulatory comparison and analytical heat transfer modelling confirm that omitting ψ results in amplified load effects under the sustained high ΔT conditions typical of hot-arid climates.

Based on the literature synthesis, several recommendations emerge for regulatory advancement:

• Adopt Explicit ψ-Value Requirements: Energy codes should incorporate mandatory thermal bridge assessment for fenestration systems, following established ISO standards and utilising validated calculation methodologies.
• Develop Regional Catalogues: Context-specific thermal bridge catalogues should be developed for common construction details in hot-arid climates, providing practitioners with accessible compliance pathways.
• Implement Phased Integration: A graduated approach to regulatory implementation allows for capacity building while progressively advancing compliance requirements toward mandatory performance-based assessment.
• Support Technology Adoption: Incentive structures should encourage the adoption of advanced fenestration technologies, including vacuum glazing, thermally broken frames, and optimised installation practices.

Future research should focus on developing climate-specific performance data for thermal bridge effects in hot-arid conditions, validating simulation methodologies against field measurements, and establishing economic models for regulatory compliance costs and benefits [54]. Additionally, investigating emerging technologies, including AI-based thermal bridge analysis tools and advanced materials integration, offers opportunities to enhance performance assessment approaches. The integration of computational modelling with local building codes, construction practices, and affordability constraints remains essential for successful implementation in developing economies [51].

As facade systems become increasingly complex in cooling-dominated economies, energy codes must evolve from prescriptive simplification toward multidimensional thermal accountability. Integrating ψ-value evaluation represents a necessary step toward aligning regulatory architecture with multidimensional heat transfer behaviour in contemporary façade systems.