Designing the Building Skin for Daylight in Cold-Climate Classrooms: An Orientation-Specific Comparison of Façade and Roof Glazing Strategies

1. Introduction

Glazing is a critical component of the building skin because it mediates daylight, solar gains, heat transfer, and visual connection to the exterior. These interactions are particularly consequential in cold climates, where seasonal daylight variability and winter heat loss place competing demands on envelope design. Window-to-Wall Ratio (WWR) is therefore an important design parameter, but glazing area alone does not determine luminous performance: orientation, window height, distribution, and solar-control strategies can substantially alter how daylight is admitted and distributed within a space [1,2,3,4].

In educational buildings, these decisions have particular importance because daylight quality has been associated with student well-being and learning outcomes [5,6]. Climate-based daylight metrics provide a more informative basis for assessing such conditions than static daylight factors. Spatial Daylight Autonomy (sDA) and Annual Sunlight Exposure (ASE) characterize daylight sufficiency and potential overexposure [7], while Useful Daylight Illuminance (UDI) distinguishes between insufficient, useful, and excessive illuminance ranges over occupied hours [8]. Recent classroom studies further emphasize that these metrics should be interpreted together with spatial distribution and visual-comfort considerations rather than as isolated compliance targets [9,10].

Roof glazing adds another dimension to building-skin design. Skylights can reinforce daylight in deep or poorly served zones and improve minimum illuminance, but their performance depends not only on glazed area but also on number, position, and geometry [11,12]. Similar observations apply to façade openings: studies of window geometry and distribution show that increasing opening size does not necessarily improve daylight penetration or uniformity [4,13]. Consequently, WWR and Skylight-to-Roof Ratio (SRR) are useful descriptors of envelope transparency, but they do not fully characterize the daylighting strategy.

This distinction is especially relevant in Quebec school construction, where long winters, low seasonal solar altitudes, and substantial variations in daylight availability make orientation-sensitive design important [14]. Previous research in this context used parametric optimization to investigate the position of skylights and light shelves while maintaining predefined glazing conditions, demonstrating that computational methods can improve daylight uniformity and compliance with climate-based metrics [15]. The present study builds on that work by comparing a broader set of practical façade and roof glazing configurations rather than seeking a single optimum glazing ratio.

The research asks how realistic façade and roof daylighting configurations perform across eight orientations in a cold-climate classroom, and what orientation-specific design guidance can be derived from their climate-based daylight performance. Two related configuration series are examined: four façade strategies that vary window area, sill and head height, and upper-level glazing distribution; and five skylight arrangements that vary the number, orientation, and distance of roof openings from the rear wall. WWR and SRR are used to describe these configurations, but they are not treated as fully isolated experimental variables. Performance is evaluated through sDA, ASE, UDIa, mean and minimum illuminance, and daylight uniformity.

The analysis shows that the effectiveness of additional glazing depends strongly on orientation and on where transparent area is placed within the building skin. Southern and western conditions generally reach daylight sufficiency with the baseline façade strategy, whereas northern orientations benefit more from increased façade glazing or top lighting. The results also indicate that high-level façade glazing and rear-positioned skylights can improve daylight penetration and minimum illuminance without relying solely on larger total glazing areas. The objective is therefore not to establish a universal optimum WWR or SRR, but to develop an orientation-specific decision framework for early-stage building-skin design in cold-climate classrooms.

2. Materials and Methods

2.1. Research Design and Scope

This study uses a simulation-based comparative design to examine how practical façade and roof daylighting configurations perform across different orientations in a cold-climate classroom. The analysis is organized as two related configuration series evaluated against a common reference condition: a roof-glazing series that varies the number and position of skylights while retaining the baseline façade, and a façade-glazing series that varies window size and vertical distribution without skylights. Window-to-Wall Ratio (WWR) and Skylight-to-Roof Ratio (SRR) are therefore used as descriptors of the tested configurations rather than as fully isolated independent variables.

Each configuration was simulated across eight orientations and assessed using climate-based daylight metrics together with illuminance-based indicators. The purpose is not to determine a universal optimum glazing ratio, but to identify orientation-specific performance patterns and to determine when changes in opening area, vertical distribution, skylight number, or skylight position produce meaningful daylight benefits. The study does not include heating or cooling-load simulation; thermal properties of the glazing systems are reported to characterize the envelope components, while the quantitative analysis is limited to daylight performance.

Figure 1. Research framework. Façade and roof daylighting configurations are evaluated as two related series across eight orientations using climate-based daylight metrics, then synthesized into orientation-specific design guidance.

Figure 1. Research framework. Façade and roof daylighting configurations are evaluated as two related series across eight orientations using climate-based daylight metrics, then synthesized into orientation-specific design guidance.

2.2. Reference Classroom and Envelope Conditions

A 75 m² side-lit reference classroom was used for all simulations (Figure 2). The plan dimensions are 10,040 mm × 7,475 mm, with a ceiling height of 4,200 mm. The baseline façade contains two 3,900 mm × 2,000 mm windows, positioned symmetrically with a 600 mm parapet and separated by 840 mm. This baseline geometry corresponds to a WWR of 36.41%.

For the South, South-West, and South-East orientations, the original design condition includes a horizontal light shelf 1,000 mm in width, positioned at 1,950 mm above floor level, to reduce direct-sun exposure and redistribute daylight. This light-shelf condition is retained consistently across the tested configurations for those orientations. Consequently, comparisons among configurations within a given orientation isolate changes in the tested façade or roof strategy, whereas comparisons between southern and non-southern orientations should be interpreted as comparisons between orientation-specific design conditions rather than as a pure orientation effect.

Interior finishes were represented by matte white walls, a pale grey concrete floor, a light-coloured wood ceiling, and light grey aluminium mullions. Interior finishing descriptions were held constant across all simulations. Façade glazing was modelled as Solarban 72 solar-control low-E glazing and the skylight system was modelled using Kalwall 70 mm Air Crystal White glazing (Table 1).

2.3. Daylighting Configuration Series

Two configuration series were studied separately. The roof-glazing series compares the effect of skylight number and position while retaining the baseline façade configuration. The façade-glazing series compares four window arrangements that vary opening area and vertical distribution. Because area and geometry change together in several scenarios, the configurations are interpreted as realistic design alternatives rather than as a factorial experiment in which WWR or SRR is isolated from geometry.

2.3.1. Roof-Glazing Configurations

The roof-glazing series comprises a baseline condition without skylights (Class 0) and five skylight arrangements (Figure 3). Classes 1 and 2 contain one skylight and correspond to an SRR of 4.85%. Classes 3, 4, and 5 contain two skylights and correspond to an SRR of 9.70%.

At each glazing level, skylight position was varied to assess its influence on daylight distribution. The tested arrangements include skylights oriented perpendicular or parallel to the rear wall and positioned at different distances from that wall.

Class 4 is a positional variation of Class 3 in which the two skylights are moved closer to the rear wall. Classes 2 and 5 similarly place the single or paired skylights parallel to and near the rear zone. These variants allow the study to distinguish, within the tested set, the effect of increasing roof-glazing area from the effect of relocating top lighting toward the portion of the classroom that is least effectively served by façade daylight.

The SRR values are therefore reported as characteristics of the roof configurations. Performance differences are interpreted in relation to both skylight area and placement, rather than attributed to SRR alone.

2.3.2. Façade-Glazing Configurations

Four façade configurations were evaluated (Figure 4). Class 1 is the baseline configuration, with two 3,900 mm × 2,000 mm windows, a 600 mm parapet, and a WWR of 36.41%. Class 2 lowers the parapet to 200 mm and increases the window height to 2,400 mm, producing a WWR of 43.70%. Class 3 retains the 600 mm parapet but extends the windows upward to 2,300 mm, producing a WWR of 41.88%.

Class 4 maintains a total glazed area close to the baseline (WWR approximately 36.79%) but redistributes it vertically: two 3,000 mm × 1,600 mm windows are combined with a continuous 8,800 mm × 700 mm upper strip window. The façade series therefore intentionally compares both opening area and opening geometry. Classes 2 and 3 alter WWR and vertical position simultaneously, whereas Class 4 provides an approximately constant-area comparison that helps reveal the effect of placing a larger proportion of glazing higher in the façade.

2.4. Orientation and Comparison Logic

Each configuration in both series was evaluated for eight façade orientations: North, North-West, West, South-West, South, South-East, East, and North-East. The two series share the same baseline façade condition but were analyzed separately; a full factorial combination of all façade and roof configurations was not simulated. Accordingly, the study evaluates orientation-specific responses within each strategy family and does not claim interaction effects between every WWR and SRR combination.

2.5. Simulation Environment and Boundary Conditions

Annual daylight simulations were conducted in ClimateStudio [16], a Radiance-based environment for climate-based daylight modelling. The Mirabel, Quebec Typical Meteorological Year weather file, CAN_QC_Montreal-Mirabel.Intl.AP.719050_TMYx.2004-2018, was used. This weather file corresponds to a cold humid continental climate (Köppen Dfb; ASHRAE climate zone 6), with an annual mean temperature of 11 °C and total annual solar radiation of 1331 kWh/m². The occupied period was defined as weekdays from September through June, from 07:00 to 16:00, with Daylight Saving Time applied. This schedule reflects the school-use assumptions of the original study and is used consistently for all configurations.

The surrounding context was modelled as unobstructed so that differences among simulations are attributable to the classroom envelope and orientation rather than to external shading. The analysis surface covered the usable classroom area at desk level. Sensor points were distributed on a 0.6096 m (2 ft) grid, with a perimeter exclusion band of approximately 0.30–0.46 m (12–18 in.) from room boundaries, and the analysis plane was set at 0.762 m (30 in.) above the finished floor. Geometry, material properties, weather data, occupancy schedule, and analysis settings were held constant within the simulation set except for the daylighting configuration and orientation being evaluated.

Table 2. Main simulation parameters and boundary conditions.

Parameter	Value
Simulation platform	ClimateStudio (Radiance-based daylight simulation environment)
Weather file	CAN_QC_Montreal-Mirabel.Intl.AP.719050_TMYx.2004-2018
Climate classification	Köppen Dfb; ASHRAE climate zone 6
Occupied period	Weekdays, September–June, 07:00–16:00, with Daylight Saving Time applied
External context	Unobstructed site, without surrounding buildings or trees causing external shading
Analysis area	Usable classroom area
Sensor grid spacing	0.6096 m (2 ft)
Perimeter exclusion band	Approximately 0.30–0.46 m (12–18 in.) from room boundaries
Analysis plane height	0.762 m (30 in.) above finished floor level

Table 2. Main simulation parameters and boundary conditions.

Parameter	Value
Simulation platform	ClimateStudio (Radiance-based daylight simulation environment)
Weather file	CAN_QC_Montreal-Mirabel.Intl.AP.719050_TMYx.2004-2018
Climate classification	Köppen Dfb; ASHRAE climate zone 6
Occupied period	Weekdays, September–June, 07:00–16:00, with Daylight Saving Time applied
External context	Unobstructed site, without surrounding buildings or trees causing external shading
Analysis area	Usable classroom area
Sensor grid spacing	0.6096 m (2 ft)
Perimeter exclusion band	Approximately 0.30–0.46 m (12–18 in.) from room boundaries
Analysis plane height	0.762 m (30 in.) above finished floor level

2.6. Performance Metrics

Daylight performance was evaluated using climate-based metrics and complementary illuminance indicators (Figure 5). Spatial Daylight Autonomy (sDA300/50%) was used to quantify the proportion of the analysis area receiving at least 300 lux for at least 50% of occupied hours. Annual Sunlight Exposure (ASE1000,250h) was used to identify areas receiving more than 1,000 lux of direct sunlight for more than 250 occupied hours. For the comparative compliance assessment, the study uses sDA300/50% ≥ 75% and ASE1000,250h ≤ 10%, consistent with the criteria adopted in the original Quebec school analysis.

Useful Daylight Illuminance in the autonomous range (UDIa, 300-3,000 lux) was used to quantify the proportion of occupied time during which daylight levels remained within the target range. Mean illuminance was calculated as the average illuminance over the analysis surface and occupied hours. Minimum illuminance was used as an indicator of the least-served portion of the room, and daylight uniformity was calculated as the ratio of minimum to mean illuminance. Mean illuminance, minimum illuminance, and uniformity are supplementary indicators rather than LM-83 compliance metrics and are used to interpret spatial distribution alongside sDA, ASE, and UDIa.

2.7. Comparative Analysis and Decision Framework

The analysis combines compliance screening with within-orientation comparison of the tested configurations. The baseline façade configuration (Class 1 for the façade series and Class 0 for the roof series) serves as the reference condition. Absolute metric values are retained as the primary evidence, and percentage changes relative to the baseline are used only to describe the magnitude and direction of configuration effects. No composite optimization score or universal ranking is assumed.

2.7.1. Compliance Assessment

For each orientation and configuration, sDA and ASE are first assessed against the adopted daylight sufficiency and overexposure criteria. This identifies whether a configuration satisfies the minimum daylight requirement without exceeding the adopted direct-sun exposure limit.

2.7.2. Within-Orientation Comparative Analysis

Configurations are then compared within each orientation using UDIa, mean illuminance, minimum illuminance, and uniformity together with the sDA and ASE results. For the roof-glazing series, comparisons focus on the combined effects of skylight number, SRR, orientation, and position relative to the rear zone. For the façade series, comparisons focus on the combined effects of glazed area and vertical distribution, with Class 4 providing a near-constant-area geometry comparison.

2.7.3. Synthesis Into Orientation-Specific Design Guidance

The final synthesis identifies recurring performance patterns rather than a single numerically optimal configuration. A strategy is considered advantageous when improvements in daylight sufficiency or useful daylight are accompanied by acceptable ASE and do not substantially degrade spatial distribution. The resulting guidance is therefore heuristic in the architectural sense: it translates the comparative simulation results into orientation-specific early-design recommendations while preserving the limitations of the tested configuration set.

3. Results

The results are presented as comparisons among the tested daylighting configurations rather than as a factorial optimization of WWR and SRR. The façade and roof series are therefore reported separately, with the common baseline used to establish the orientation-dependent daylight condition. Results are interpreted within each orientation because the southern orientations include the horizontal light shelf described in Section 2.2.

3.1. Baseline Performance Across Orientations

The baseline condition consisted of the Class 1 façade configuration (WWR 36.41%) without skylights. Under this condition, the south, south-east, south-west, east, and west orientations achieved the adopted sDA daylight-sufficiency criterion. The north, north-east, and north-west orientations did not meet the sDA criterion, although ASE remained negligible in the northern orientations. In the southern orientations, where the baseline included the horizontal light shelf and solar-control glazing, the adopted ASE criterion was also satisfied.

This baseline establishes two distinct daylight conditions within the tested model. Orientations with higher solar exposure already provided sufficient daylight with the baseline façade, whereas the northern orientations remained comparatively underlit and therefore had greater potential to benefit from additional or redistributed glazing.

3.2. Façade-Glazing Configurations

3.2.1. Mean and Minimum Illuminance

Across the eight orientations, Class 3, in which the windows retain the 600 mm parapet and extend upward to 2,900 mm above floor level, produced the highest mean illuminance values. Classes 2 and 4 were typically 30-150 lux lower than Class 3, depending on orientation. Notably, Classes 2 and 4 produced similar mean illuminance values despite a difference of approximately seven percentage points in WWR (43.70% and approximately 36.79%, respectively). This comparison shows that the recorded daylight response cannot be explained by opening percentage alone and is strongly affected by the vertical distribution of the glazing.

Minimum illuminance followed a similar pattern. Configurations with glazing extending higher in the façade, particularly Classes 3 and 4, generally produced the strongest minimum-illuminance values. Increasing window area by lowering the sill in Class 2 also increased mean illuminance relative to the baseline, but the effect was generally less pronounced than that obtained by extending or redistributing glazing upward.

3.2.2. Uniformity and Useful Daylight

Because mean and minimum illuminance tended to change in the same direction across the four façade configurations, daylight uniformity varied less strongly than the absolute illuminance values. In the south, south-east, and south-west orientations, larger window configurations tended to reduce uniformity because mean illuminance increased more rapidly than the minimum value. In the northern orientations, larger or higher glazing produced a modest improvement in uniformity.

UDIa differentiated the façade configurations more clearly. Class 3 achieved the highest values across the orientation set, generally between 65% and 70%. Class 2 followed, with values of approximately 61-69%, while the baseline Class 1 reached values as low as 56%. Class 4 remained between Classes 1 and 2 despite having a total glazed area close to the baseline. Its comparatively strong performance therefore reflects the contribution of the high-level strip window and supports the importance of glazing position as well as total area.

3.2.3. Orientation-Dependent Response

The effect of modifying the façade depended on the baseline daylight condition. In the south, south-east, and south-west orientations, the baseline already achieved the adopted sDA and ASE criteria under the light-shelf condition. Increasing window size raised illuminance but did not consistently improve UDIa or uniformity, and the largest configurations increased direct-sun exposure in some cases. In the east and west orientations, where the baseline also achieved the sDA criterion, increasing or redistributing façade glazing produced comparatively limited improvements in UDIa and uniformity.

The northern orientations showed a different response. Because the baseline did not achieve the adopted sDA criterion, larger and higher façade openings produced meaningful increases in mean illuminance, minimum illuminance, UDIa, and sDA, while ASE remained very low. Within the tested set, façade modification therefore had its clearest daylight benefit in the north, north-east, and north-west orientations.

Figure 6. Daylight performance of the four façade-glazing configurations across the eight orientations, showing minimum and mean illuminance, sDA, UDIa, and uniformity.

Figure 6. Daylight performance of the four façade-glazing configurations across the eight orientations, showing minimum and mean illuminance, sDA, UDIa, and uniformity.

3.3. Roof-Glazing Configurations

3.3.1. Mean and Minimum Illuminance

The roof-glazing series produced a clear increase in mean illuminance as skylight area increased. Configurations with two skylights (Classes 3-5, SRR 9.70%) averaged approximately 150 lux more than the one-skylight configurations (Classes 1-2, SRR 4.85%), corresponding to increases of roughly 12-30% depending on orientation. In the south orientation, mean illuminance averaged approximately 1379 lux with two skylights compared with 1228 lux with one; in the northern orientations, the corresponding values were approximately 760 and 600 lux.

Skylight position had a stronger influence on minimum illuminance than on mean illuminance. In both the one- and two-skylight groups, configurations positioned parallel to and closer to the rear wall (Classes 2 and 5) produced the strongest minimum-illuminance results. In the Class 5 configuration, minimum illuminance was approximately 1.6 times the baseline value in the south, south-east, south-west, east, and west orientations and nearly doubled in the northern orientations.

3.3.2. Uniformity and Skylight Position

Uniformity results similarly favored placement near the rear of the room. Across the orientations, a single skylight positioned parallel to the rear wall (Class 2) produced higher uniformity than two centrally located perpendicular skylights (Class 3), despite the lower total skylight area. In several orientations, moving the perpendicular skylights closer to the rear wall (Class 4) also improved uniformity. These comparisons indicate that skylight placement can influence spatial distribution independently of the number of openings.

The northern orientations exhibited relatively high uniformity values even in the baseline condition. These values should be interpreted together with absolute illuminance, because the high minimum-to-mean ratio partly reflects the comparatively low mean illuminance of the northern baseline rather than a generally superior daylight condition.

3.3.3. Useful Daylight and Daylight Sufficiency

UDIa improved with the introduction of skylights in all orientation groups, although the magnitude varied. Relative to the no-skylight baseline, average UDIa increased by more than 30% in the north and north-west, by more than 25% in the north-east, by more than 20% in the south-west and west, and by approximately 10-15% in the remaining orientations. The highest UDIa values occurred in the north-east and east orientations, while the south-west produced the lowest values because a greater proportion of the near-window area exceeded the upper 3000 lux UDIa limit. Skylights also increased sDA in the northern orientations, where the baseline condition was insufficient, without introducing meaningful ASE. In the orientations that already satisfied the sDA criterion, the principal contribution of roof glazing was not simply increased daylight quantity but improved rear-zone illuminance and, when positioned appropriately, more even distribution.

Figure 7. Daylight performance of the roof-glazing configurations (Class 0 to Class 5) across the eight orientations, showing minimum and mean illuminance, sDA, UDIa, and uniformity.

Figure 7. Daylight performance of the roof-glazing configurations (Class 0 to Class 5) across the eight orientations, showing minimum and mean illuminance, sDA, UDIa, and uniformity.

3.4. Orientation-Specific Synthesis

Taken together, the two configuration series show that the daylight value of additional or redistributed glazing depends on both orientation and placement. In orientations where the baseline façade already achieved daylight sufficiency, increasing façade area yielded comparatively small benefits, whereas strategically positioned skylights could improve rear-zone conditions. In the northern orientations, where baseline daylight was insufficient, both façade modification and roof glazing produced larger improvements in sDA and UDIa. Table 3 summarizes these observed patterns without assigning a composite optimization score, and Figure 8 translates the same findings into a qualitative decision matrix.

4. Discussion

4.1. Orientation-Dependent Daylight Response and Strategy Selection

The results show that the value of a daylighting intervention depends strongly on the luminous condition established by orientation and by the associated façade strategy. In the south, south-east, and south-west cases, the baseline façade with solar-control glazing and a horizontal light shelf already satisfied the adopted sDA and ASE criteria. The east and west baselines also achieved daylight sufficiency. In these orientations, the design question is therefore less about increasing daylight quantity than about improving useful daylight and spatial distribution without increasing direct-sun exposure. By contrast, the north, north-east, and north-west baselines remained below the adopted sDA criterion, so interventions that increased daylight admission produced a clearer benefit.

This distinction is important for early-stage design because it shifts the emphasis from a single preferred glazing ratio toward orientation-specific needs. Where daylight sufficiency is already achieved, larger façade openings may increase mean illuminance without improving UDIa or uniformity and can intensify near-window contrast. Where the baseline is insufficient, as in the northern orientations, larger or higher façade openings and roof glazing can increase sDA and UDIa while ASE remains low. These patterns are consistent with previous studies showing that façade orientation and solar exposure substantially modify the daylight response of glazing strategies [1,2,4,13].

The southern cases require a specific qualification. Because the original design condition includes a horizontal light shelf for the south, south-east, and south-west orientations, the study compares complete orientation-specific daylighting strategies rather than identical envelopes rotated around the compass. The favorable baseline performance in these orientations therefore cannot be attributed to orientation alone. This condition is nevertheless representative of the design logic being investigated: solar-control measures and glazing geometry are selected together as components of the building skin.

Accordingly, orientation should not be interpreted here as a direct predictor of an optimal WWR or SRR. It establishes the environmental boundary condition within which different façade and roof configurations become useful. This interpretation is more consistent with the comparative design of the study and with climate-based daylight assessment, where sufficiency, overexposure, and useful illuminance need to be considered together [7,8].

4.2. Geometry and Position as Envelope Design Variables

One of the clearest findings is that glazed area alone does not explain daylight performance. The façade series combines changes in opening area with changes in sill height, head height, and vertical distribution. Class 3, which extends the glazing upward, produced the highest mean illuminance and UDIa values across the tested set, while Class 4 achieved comparatively strong performance with a total glazed area close to the baseline by introducing a high-level strip window. These results support previous work showing that window geometry and the vertical position of openings influence daylight penetration and distribution independently of total opening area [4,13].

This also defines an important boundary on interpretation. Because glazing area and geometry were not varied independently, the differences between Classes 1-4 should not be attributed to WWR alone. The study instead demonstrates the performance of realistic façade configurations in which size and position are coupled, as they normally are during architectural design. The resulting design implication is not that one ratio is intrinsically superior, but that additional glazing is more effective when its position responds to the spatial daylight deficit.

The roof-glazing series reinforces this point. Skylight position had a stronger effect on minimum illuminance and uniformity than on mean illuminance. A single skylight positioned parallel to and near the rear wall could outperform two centrally located perpendicular skylights in terms of uniformity, despite having less roof glazing. Moving two skylights closer to the rear zone likewise improved distribution in several orientations. The location of roof openings is therefore at least as relevant as their number when the objective is to reinforce the deepest part of the classroom.

For building-skin analysis, WWR and SRR consequently remain useful descriptors of how much of the envelope is transparent, but they are insufficient as stand-alone design variables. Height, position, distribution, orientation, glazing properties, and solar-control elements all mediate the daylight effect of a given ratio. A configuration-based description provides a more architecturally meaningful basis for comparing envelope strategies than ratio alone.

4.3. Complementary Roles of Façade and Roof Glazing

The two configuration series should be understood as complementary rather than as direct competition between vertical and horizontal glazing. The study was not structured as a factorial WWR × SRR experiment, nor were façade and roof openings matched by equivalent glazed area. Instead, each series tests a set of plausible architectural strategies against the same classroom archetype and climate-based metrics.

Within this comparative framework, façade and roof glazing address different daylighting needs. Façade modifications primarily change the quantity and vertical admission of side light. Their strongest benefit was observed in the northern orientations, where the baseline daylight level was insufficient. Roof glazing, by contrast, was particularly effective in increasing minimum illuminance and reinforcing the rear zone. In orientations where the baseline façade already provided sufficient daylight, the principal value of a well-positioned skylight was therefore redistribution rather than simply raising average illuminance.

Across the tested cases, increasing skylight area generally increased mean illuminance, but placement determined how effectively that additional daylight reached the darker part of the room. This is consistent with previous skylight research emphasizing the role of top lighting in deeper-plan daylight distribution [11,17]. The results therefore suggest a simple design distinction: when the primary deficit is overall daylight sufficiency, façade enlargement or additional top lighting can both be effective; when the primary deficit is rear-zone illumination, the position of roof glazing becomes especially important.

This complementary reading also avoids a false hierarchy between façade and roof glazing. A skylight is not inherently more effective than a window, nor is a larger window inherently preferable to top lighting. Their usefulness depends on what aspect of the daylight field requires correction and on the orientation-specific condition already established by the façade.

4.4. From Glazing Ratios to an Early-Design Decision Framework

WWR and SRR are attractive early-design indicators because they are simple to calculate and are closely related to envelop quantity. The present results, however, show why ratio-based guidance benefits from being combined with geometric and orientation-specific information. Two façade configurations with similar total glazed area can perform differently when one admits daylight higher in the wall, and two roof configurations with the same SRR can produce different minimum illuminance and uniformity when their openings occupy different positions.

The proposed heuristic should therefore be understood as a sequence of design questions rather than as a universal optimization index. The first question is whether the baseline configuration satisfies daylight sufficiency and limits overexposure for the orientation under consideration. If it does not, the designer can consider increasing or redistributing façade glazing or introducing roof glazing. If sufficiency is already achieved but the rear zone remains weak, redistributing glazing upward or positioning skylights toward the rear may be more appropriate than simply increasing total transparent area. The final choice must then be tested against the complete envelope, including solar control, thermal performance, structure, constructability, and cost.

In this sense, the contribution of the study is not a prescriptive optimum WWR or SRR. It is an orientation-specific comparative framework that helps distinguish when additional area is useful, when redistribution may achieve a similar daylight objective, and when a different daylighting mechanism is warranted. This scale of guidance is particularly suited to early architectural design, before a project has sufficient definition for full multi-objective optimization.

4.5. Thermal Implications, Scope, and Limitations

The study evaluates daylight performance only. Heating and cooling loads were not simulated, so the tested configurations should not be interpreted as energy-optimal building envelopes. Nevertheless, the amount and location of glazing are consequential for thermal performance in cold climates, and previous research has shown that increased façade and roof glazing can alter heating, cooling, and overall energy demand [1,2,12,17]. The daylight results can therefore inform a subsequent integrated analysis by identifying configurations that achieve the required luminous function without assuming that more transparent area is automatically beneficial.

Several limitations define the scope of the findings. The simulations use a single 75 m² classroom archetype, one Quebec weather file, fixed glazing products and interior surface properties, an unobstructed exterior context, and a specific academic occupancy schedule. The tested façade and roof options are discrete design configurations rather than continuous parametric ranges. In addition, the south, south-east, and south-west cases include a light shelf while the remaining orientations do not, meaning that the orientation comparison represents different design packages rather than a controlled rotation of an otherwise identical envelope.

The study also does not isolate WWR and SRR from opening geometry, nor does it test every possible combination of façade and roof glazing. It therefore cannot establish generalized ratio thresholds, saturation points, or causal effects attributable to glazing ratio alone. The findings should instead be read as comparative evidence for the specific configuration families studied and as a basis for a more explicit orientation-responsive design framework.

Future work should extend the configuration set to systematically sampled glazing ratios and positions, couple daylight simulation with heating and cooling analysis, test additional classroom depths and surrounding contexts, and evaluate alternative shading strategies. Empirical validation in occupied classrooms would further clarify how the simulated daylight patterns translate into visual comfort and operational performance. Within its stated scope, however, the present study provides a defensible basis for selecting and refining façade and roof daylighting strategies before more detailed building-performance analysis is undertaken.

5. Conclusions

This study compared a set of practical façade- and roof-glazing configurations across eight orientations in a representative cold-climate classroom. Rather than seeking a universal optimum Window-to-Wall Ratio (WWR) or Skylight-to-Roof Ratio (SRR), the analysis examined how changes in glazing area, vertical distribution, skylight number, and skylight position affect annual daylight performance under the climatic conditions of Quebec. The results confirm that the usefulness of a daylighting strategy depends strongly on orientation and on the geometric distribution of transparent areas within the building skin.

For south-, south-west-, and south-east-facing classrooms, the baseline façade configuration combined with solar-control glazing and a horizontal light shelf already satisfies the adopted daylight-autonomy and sunlight-exposure criteria; increasing façade glazing provides limited additional benefit and can reduce uniformity. East- and west-facing classrooms also achieve daylight sufficiency with the baseline façade configuration, while roof glazing can improve useful daylight and rear-zone illumination. In north-, north-, east-, and north-west-facing classrooms, where baseline daylight availability is lower and Annual Sunlight Exposure remains limited, both larger or higher façade openings and top lighting can substantially improve sDA, UDI, and minimum illuminance.

The comparison also shows that glazing geometry and position can be as consequential as glazing area. Raising the upper extent of façade glazing improves daylight penetration more effectively than simply enlarging openings downward, while skylights positioned closer to the rear of the classroom improve minimum illuminance and spatial distribution more effectively than centrally positioned alternatives with comparable roof-glazing area. WWR and SRR are therefore useful descriptors, but they are insufficient on their own to guide daylight-oriented envelope design. The main contribution of the study is an orientation-specific decision framework that helps designers select and refine façade and roof daylighting strategies during early design, before more detailed whole-building analysis is undertaken.

The study is limited to climate-based daylight performance and does not quantify heating, cooling, or whole-building energy use. The tested configurations also combine changes in glazing ratio and geometry and do not constitute a factorial experiment capable of establishing universal ratio thresholds or causal effects attributable to WWR or SRR alone. Future research should extend the configuration set to systematically sampled opening ratios and positions, couple daylight and energy simulation, examine additional classroom geometries and surrounding contexts, and validate the findings in occupied buildings. Within these limits, the results support a shift from maximizing transparency toward strategically locating and distributing glazing in response to orientation and spatial daylight needs.