Influence of Roof Windows Area Changes on the Classroom Indoor Climate in the Attic Space: A Case Study

Abstract

Windows are a complex part of building design and provide a considerable benefit, including to school buildings. For the evaluation of the daylighting conditions prevailing in classrooms, the daylight factor (DF) was considered as the most appropriate parameter for indicating the quantity of admitted daylight. The DF values and CIE overcast sky were calculated using Velux Daylight Visualizer 3 software. The task of the paper is to compare various roof window openings in relation to the level of daylight in the attic, looking to optimize the use of the attic for teaching. The indoor air temperature has a general influence on comfort in the interior, in addition to daylight. In winter, the situation is not critical. The thermal insulation properties of packaging structures are sufficient. The situation is worse in summer, due to the fact that the heat-storage properties are undersized and there is excessive overheating of the indoor air. Four variants of roof windows and their influence on the overall microclimate in the attic are compared. The variant without roof windows is a suitable solution with regard to minimum overheating, but the worst situation for daylight. In order to receive even more light from the window (by moving windows to the top of the roof), we can use variant 2. Based on a combination of daylight calculations and summer temperature, a graphical dependence on window size prediction in terms of top and combined lighting is derived. This was hypothesized without shading the windows. Of course, the shading elements of these windows or cooling are expected in the summer. Finally, the energy required for cooling is compared depending on the size of the windows and achievement of the permissible temperature.

1. Introduction

Daylight systems should provide a sufficient level of daylight in the classroom for activities, such as reading and writing. The level and quality of daylight in the classroom is important for the health of students, and it also affects their academic performance and participation in the teaching process. Daylight is a critical design factor for those interested in global warming, carbon emissions and sustainable design—in addition to visual comfort. Creating an ideal classroom environment depends on so many different factors (see Figure 1), including the structure of the building, classroom size, windows, orientation to the cardinal, glare protection, interior equipment, color of surfaces, etc. [1].

Students who learn during the day can also experience health benefits such as improved immunity, growth, and vision due to sufficient daylight. The appropriate design of daylight constructions provides a connection between the indoor and outdoor environment, a suitable subjective perception of space, and ensures an adequate level of vision with good uniformity of lighting. Its application also saves energy on artificial lighting [2].

Because visual activities such as reading and writing are very important during the educational phase, it is essential to create comfortable visual conditions in school buildings that will contribute to these activities. Two elements of sustainable building design which have direct effects on student performance are natural daylighting and indoor air quality. Recent studies on the effect of natural daylight in schools revealed that, besides different health benefits, students perform better in daylighted classrooms [3,4].

The amount of energy consumption in a building mainly depends on its fenestration system. Windows play a significant role in the building by being able to provide daylight, to provide a suitable indoor environment, to allow visual contact with the outdoor space for building users, and, last but not least, to save energy. The design and selection of a suitable window system can be regarded as one of the most significant strategies for reducing the energy consumption effectively in a building [5,6]. Some aspects related to the measurement of window structures as well as adjacent structures, window sills, and the determination of conditions for simulations can be found in [7,8].

Proper daylighting in a school environment is very complex task. The energy consumption of classroom lighting can reach up to 50% of the school’s total electricity costs—this is because the energy used for lighting is consumed at a daily rate, and is expensive [9,10]. Adequate and high-quality lighting is a very important part of our daily activities.

Energy saving options without cost include [11,12]:

• ▪ Better utilization of daylight accompanied with proper management;
• ▪ Regular cleaning of windows and bulbs from dust; this can save energy in light;
• ▪ Incorrect lamp maintenance can absorb up to 50% of light through a thick layer of dust;
• ▪ When leaving the room, the last obligation is always to turn off the light.

The level of light incidences from above is higher. Lighting and brightness increases from horizon to zenith. Therefore, roof windows provide at least twice as much light as vertical windows of the same size, and three times that of dormers of the same size (see Figure 2). The roof window provides a greater possibility of combining lighting levels [13,14,15]. The daylight level is described by the daylight factor (see equation 5).

The minimum and average daylight factors are defined in the standard [16], STN 730580; Daylighting in buildings, Part-1 Basic requirements, and the standard [17] EN 12464-1:2012. Light and Lighting; Lighting of Work Places-Part 1: Indoor Work Places. Several practical exercises have been carried out to assess the impact of external outlook on well-being. The standard level was assessed through the analysis of many classes [18]. The quality of lighting assessment in university classrooms is the subject of [19]. The uniformity of lighting plays a very important role in regulations, and should be clear in every country. This has been performed, for example, through a standard design for classrooms at public schools in Malaysia [20].

Recently published contributions included several aspects of school light analysis, such as the daily intensity of high school buildings [21], the impact of glazing and window configuration on comfort and energy efficiency [22], daylight levels in classrooms adjacent to covered and exposed courtyards under clear skies [23], ways of using light in Portuguese school buildings, and the perception of user comfort [24].

It is possible to design an integrated daylight system, a combination of top and side lighting in the classroom, using the DaySim simulation tool [25]. When creating the architecture of the school building, it is possible to use simulation tools when designing daylight. In some schools it is also possible to consider top lighting. For example, [26] assesses the sustainability characteristics of a new skylight type when designing.

In the case of window structures, adjustments are important to ensure the lowest possible economic cost of improving the physical conditions of the indoor environment of school buildings [27]. Optimal conditions of the indoor climate at school can be designed by modeling the physical characteristics of the indoor environment, including daylight [28].

The orientation of the classroom to the cardinal influences the results of such modeling. It is always necessary to seek a compromise between daylight and heat gain in summer [29]. It is also important to optimize the design of the school building geometry in terms of daylight and energy efficiency in winter [30]. It all depends on different daylighting techniques, depending on the climate (latitude) and various complex shading systems [31]. This is described in a project to assess the energy efficiency and comfort of school buildings [32]. The principles and implementation of daylight systems in classrooms, in the field of energy management, with the impact of noise from urban transport and transport facilities are outlined in [33]. For the overall assessment of the quality of the environment in the auditoriums of the university building [34], the relationship between the physical conditions of the school building and the organizational commitment according to teachers’ perception [35] can be taken into account.

Last but not least, this affects the health and comfort of students. Such an impact on the health of schoolchildren in 54 classes of 21 Dutch school buildings was published in Building and Environment [36]. A comparative study of comfort indicators for school buildings in sustainability methodologies in the Amazon and South-East Brazilian regions was also conducted [37]. Here, optimum glazing configurations for visual performance are very important [38]. If these proposals do not meet the requirements, additional daylight methods should be used as a tool to improve the provision of daylight in existing education areas [39].

Currently, the focus is on intelligent methods of managing intelligent daylighting using artificial neural networks [40]. Given the higher demands on cooling compared to heating, emphasis is placed on reducing the energy efficiency of the school building in a hot environment [41] as well as the optimum window to wall ratio. Part of this work also deals with this issue. At present, attention is paid to the shading of transparent surfaces and also to the design of dynamic façade elements. The effect of two motion types on solar transmittance and daylight on dynamic facades is found in [42]. The impact of building accumulation in energy-efficient school buildings is significant [43]. One of the newest articles on classrooms is “Studying the Light Environment with the Phenomenon of Redirection of Daylight Prisms in Classrooms” published in Indoor and Build Environment [44].

K. Kondas dealt with the lighting of the attic space and ideas of designing new lighting technical standards in the laboratories of the Institute of Civil Engineering and Architecture of the Slovak Academy of Sciences, under the leadership of Dr. Darula [45]. The authors of [46,47] also dealt with the properties of sloping windows, i.e., roof windows, as well as the transmission of sunlight through the windows and the optical properties of glazing systems [46,47].

Pitched roofs have insufficient heat accumulation, which results in overheating in the attic space [48]. Skylights also create visual discomfort. The authors of this article have addressed similar issues in the past [49,50,51].

Computational procedures, needed to determine the lighting conditions in the outdoor environment, are included in the literature [52,53]. Recent research from 2020 examining daylight levels in terms of functional needs is described in [54]. In source [55], there is information about dimming daylight lighting through interior shading devices and the effects on the energy performance of buildings. Several authors deal with the overheating of school buildings in the summer. An analysis of summer overheating in an elementary school building is shown in [56]. There are several regulations for determining suitable temperatures for different rooms [57,58]. It can be said that these are different in each country. Many literary sources deal with various problems of overheating. Problems with indoor temperatures in new buildings, retrofitted and existing British dwellings are reported [59]. Problems related to the summer season in the UK are reported in several articles [60,61,62].

Overheating caused by passive solar elements in Tunisia is demonstrated in [63]. The use of phase change materials [64], shielding devices [65], and heat adaptation in hot and humid outdoor conditions [66] have also been demonstrated.

In the next part of the article, the authors would like to expand this knowledge by focusing on the current assessment of daylight and overheating of classes in the attic.

2. Materials and Methods

The subject of the paper is a classroom located in a school building. The school is in the northern part of Kosice, in Slovakia (see Figure 5). Although this is not a custom, the classroom is located in the attic. The intention behind this was to expand the teaching space and the reconstruct the attic, creating new possibilities. Due to the fact that there are roof windows in the attic, there is excessive lighting and the possibility of glare. The subject, goal and methodology of the research can be seen in Figure 3.

The aim is to find the optimal ratio of roof window area to floor area (WFR), or the ratio of roof window area to surrounding wall area (WWR). This WFR or WWR ratio affects the level of daylight, a reduction in the possibility of overheating but also a reduction in the level of glare.

The methodology is focused on in-situ measurements, calculations, the confrontation of measured and calculated facts, simulation of the whole process of classroom lighting, and prediction of overheating. The tested classrooms are illuminated by side windows and roof windows. As already mentioned, in this article, the following methodology is applied:

• ▪ Measurement of daylight value, Daylight Factor;
• ▪ Prediction of summer overheating;
• ▪ Calculated on the basis of simulation tools based on measured or predicted data;
• ▪ Search for optimization of indoor light and thermal climate.

In this study, four design variants were tested: the basic model (real situation), a model without roof windows, and two models with roof windows with different configurations (see Figure 4).

Figure 4a shows the current situation, (b) shows the illumination only using the strips of the associated windows in a vertical plane (without the roof windows). Configuration (c) shows the illumination only using skylights in one plane next to each other, and (d) shows the illumination by skylights in two planes. The mentioned configurations were chosen only on the basis of the design possibilities of the classroom in the attic space.

Further measurements and calculations are performed according to WFR and WWR factors, which express the ratio of the area of the windows to the floor area or to the area of the walls (Formulas (7) and (8)).

2.1. Daylight Metrics

Daylighting on the working plane in oriented attic classroom under overcast and clear sky was determined.

The investigation of the effects of direct sunlight and diffuse light on the size of the windows in the attic was carried out using a simulation program Velux Daylight Visualizer 3. It is a simple tool for designing and analyzing daylight under various standard conditions. It is designed to support the use of daylight in buildings and to help predict and document daylight and the appearance of the space before the construction project. The simulation program offers many outputs: surface brightness, illuminance values, and the distribution of the daylight factor on the reference plane as well as the animation of daylight/sunlight. Calculation of all radiations, globally, direct and scattered, at solar altitude on the ground γ, by the diffusion ratio E_vd/E_voh and the luminous turbidity factor T used, can usually be expressed as [52,53]:

$$E_{v,g}=E_{v,s}+E_{vd}\left(lx\right)$$

(1)

where

• $E_{v,g}$—is the global external horizontal illuminance [lx];
• $E_{v,s}$—is the direct external illuminance, recalculated on the horizontal plane [lx];
• $E_{vd}$—is the diffuse external horizontal illuminance [lx].

To calculate the direct component of the external illuminance, the next exponential formula, also recommended by [52,53], can be used:

$$E_{v,s}=E_{voh}·e^\left(-{\alpha }_V·m·T_V\right)\left(lx\right)$$

(2)

where

• $E_{voh}$—is the extraterrestrial illuminance expressed on the horizontal plane [lx];
• ${\alpha }_V$—is the luminous extinction coefficient [-];
• $m$—is the optical relative air mass of the atmosphere [-];
• $T_V$—is the luminous turbidity factor (T_V = 4 representing ISO/CIE Type 12 clear sky standard conditions and the environment in common cities areas).

The north-facing classroom was selected to demonstrate the optimization of interior lighting depending on the size of the windows in the roof plane. Direct radiation only falls into a certain part of the room. If we want to determine the total internal light intensity at the table plane, we must calculate the direct light intensity by the indirect internal light intensity obtained using a computer program. The direct illumination was calculated from the external direct illumination reduced by the loss of light transmission through the glazing material and impurities on the inner and outer glazing surfaces. The formula for calculating the contribution of sunlight to the internal light intensity E_i on the working plane for orientation is as follows:

$$E_{v,si}=E_{v,s}\left\{\frac{{\tau }_{s,\phi }}{{\tau }_{s,nor}}\right\}{\tau }_{s,nor}\left({\tau }_{z,e}·{\tau }_{z,i}\right)\left(lx\right)$$

(3)

where

• ${\tau }_{z,\phi }$—is the factor of light transmission at angle $\phi$ from the window normal [-];
• ${\tau }_{s,nor}$—is the factor of light transmission in normal direction [-];
• ${\tau }_{z,e}$—is the factor of dirt reduction for the outer side of the glass [-];
• ${\tau }_{z,i}$—is the factor of dirt reduction for the inner side of the glass [-].

The following daylight metrics are used in the study: lighting, daylight factor, and uniformity.

Illuminance is the amount of light falling on a surface per unit area, measured in lux. The recommended average value is 500 lux.

$$E=\frac{\varnothing }{A}\to \frac{lm}{m^2}\to \left(lx\right)$$

(4)

where

• $\varnothing$—is luminous flux [lm];
• $A$—is area [m²].

Daylight factor DF (%) is the simplest and the most common metric to quantity the daylight allowed by a window. Minimum and average daylight factor for classroom has been defined as 1.5% and 5%, respectively [16,17].

$$DF=\frac{E_{internal}}{E_{external}}·100 (\%)$$

(5)

where

• E_internal—illuminance of internal horizontal plane (lx);
• E_external—illuminance of external horizontal plane (lx).

Uniformity—is defined as the ratio of the minimum DF to the average DF within the space. Standards require a uniformity of 0.2.

$$U_0=\frac{D{F}_{min}}{D{F}_{average}}(-)$$

(6)

where

• DF_min—is minimum of daylight factor [%],
• DF_average—is average value of daylight factor [%].

Window–Floor Ratio

$$WFR=\frac{A_{WINDOW}}{A_{FLOOR}}·100(\%)$$

(7)

Window–Wall Ratio

$$WWR=\frac{A_{WINDOW}}{A_{WALL}}·100(\%)$$

(8)

where

• A_WINDOW—is area of windows [m²];
• A_FLOOR—is area of floor [m²];
• A_WALL—is area of walls [m²].

2.2. Summer Overheating

Let us take an example from measurements made in the attic when the temperature was measured at more than + 40.0 °C. The roof (according to the official methodology) heated the interior to a passive level only with low intensity.

However, when the sun heats the dark roof to 80 °C, the actual influx of heat into the interior is significantly higher. The room air temperature will be higher than the outside temperature. According to the Stefan–Boltzmann law, the radiant heat flux between a dark roof heated by the sun (80 °C) and safety waterproofing (40 °C) is 880 − 550 = 230 W/m² (for comparison: solar radiation intensity) is about 1000 W/m²). Just as the outside air cannot cool the glazed roof to “its” temperature by flow, the air flow in the gap cannot cool the safety waterproofing of the roof to the outside air temperature. Thus, in practice, the safety waterproofing is rapidly heated to a temperature of about 60 °C.

The optimal indoor temperature for a person depends on individual needs and other factors. According to these facts, the standard parameters of the indoor climate for classrooms are:

• In summer—from + 24 to 28 °C;
• In winter—from + 22 to 24 °C.

The regulation states the requirement should exceed the standard indoor air temperature (27 °C in non-production areas) in the Czech Republic. Exceeding this value is by maximum of 2 °C for a maximum of 2 h is standardly permitted, though only for residential buildings and with the consent of the investor.

Air conditioning, which is highly debatable from a health point of view, will help reduce the temperature in the attic. While cooling the room protects the body from overheating and subsequent nausea and exhaustion, on the other hand, the temperature in the air conditioner should be set to a maximum of six degrees below the outside temperature. Therefore, if it is 35 degrees, it is recommended to set the internal temperature to 29 degrees from a health point of view.

Computer programs for the energy assessment of buildings have long been one of the important tools that can facilitate the work of designing optimizations of project solutions. They can be divided into two main groups: programs using correlation methods and simulation programs. While correlation methods work with average data over a long period of time, the simulation works with dynamically changing boundary conditions, as it is in real life. Simulation methods are based on solving more or less accurate physical models in short time intervals.

One of the first widely used joint air ventilation models comes from the Lawrence Berkeley National Laboratories. This model, like others in its time, used the relationship between air exchange and the pressure difference on the building envelope. Its current form is as follows (ASHRAE 2005):

$$V=\frac{A_e}{1000}\sqrt{c_t· \left( {\theta }_{a,i}-{\theta }_{a,e}\right)+c_v·v^2}$$

(9)

where

• $V$—is the volume of infiltrated air [m³ /s];
• $A_e$—is the effective area of the orifice through which air can flow [m²];
• c_t and c_v are the outflow factors for the pressure difference caused by gravity and wind.

The effluent coefficients are taken from the table. These simple equations are suitable for the first estimate in a simple space (one zone) with an opening with bidirectional air flow. However, calculating the natural ventilation of interconnected spaces, where a much more complex airflow is created, requires more complex equations, or the use of a simulation tool such as multi-zone network models or CFD models. Multi-zone network models solve the equation of conservation of mass, energy and concentration based on several approximations. The most commonly used types of multi-zone network models use Bernoulli’s equations. Perfect mixing of the air within the zone is assumed. The model consists of nodal points and connections (airflow components) that create airflow in one direction for small openings, or both in the case of large ones. Then, in a multi-zone network model, the mass, energy and concentration conservation equations for each zone are solved in order to obtain a solution for specific boundary conditions.

This is essential when selecting and using weather data to simulate a proper understanding of the nature climate data.

This fact shows considerable variability within individual hours, with the following characteristics:

-

The air temperature changes relatively slowly, rarely rising or falling by more than a few degrees in an hour;

-

In the case of sunlight, large and rapid changes in values can occur within a few minutes. This is due to the very sensitive connection to cloudy skies;

-

The wind speed can take on multiple values in a few minutes. The record of this quantity is very variable, although it is possible to notice a certain character of increase or decrease during the day. Simplifying such a course of climatic factors into hourly values requires aggregation, which must necessarily omit a large number of sometimes very different records.

As for the number of hours the window is open, it increases significantly from the ambient temperature. Statistics show that at an outside air temperature of 20 °C, the windows open for 6 h, at 26 °C for 13 h. The increase in the opening time of the t_v (h) window here has an almost linear dependence on the outside air temperature θ_a,e (°C) according to the relation:

$$t_v=0.44·{\theta }_{a,e}+0.62$$

(10)

where

• $t_v$—is opening time of windows [h];
• ${\theta }_{a,e}$—is outside air temperature [°C].

3. Case Study—Classroom in Attic of School Building

The school building that was elected is located in the city of Kosice, north (see Figure 4a,b). The proposed attic of the school used in the case study is located on the fourth floor of this selected building.

3.1. Context

The situation of the selected case study building as well as views of the building from the exterior side and classroom spaces in the interior of the attic space can be found in Figure 5.

3.2. Daylight Rating

Measurements were performed in a classroom in school building in Kosice (see Figure 5e,f). A fourth floor north facing tested room was selected with interior dimensions 10.77 × 4.36 m × 3.03 m, and the height of the parapet was 900 mm. The area of the room was 46.96 m². The side windows had dimensions of 900 mm × 860 mm, and the roof windows of 780 mm × 1180 mm. The window–wall ratio ranged from 0.05 (without roof windows) to 0.12 (see

Table 1. Window–Wall Ratio WRW and Window–Floor Ratio WFR.

	Area (m2) Side Windows	Area (m2) Roof Lights	Area (m2) Windows (Side + Roof)	Area (m2) Wall	WWR (%)	WFR (%)
Variant 0	9.3	9.20	18.5	171	11	39
Variant 1	9.3	-	9.3	171	5	20
Variant 2	9.3	5.50	14.8	171	9	32
Variant 3	9.3	11.0	20.3	171	12	43

and Figure 6).

The fenestration systems are created using plastic frames and double glass. For the calculations, the following coefficients were considered: transmittance coefficient 0.8, maintenance factor of glazing on the exterior surface 0.9, maintenance factor of glazing on the interior surface 0.85, reflectance factor of ground 0.15 (dark ground). The walls and the ceiling were white in color with a reflectance factor of 0.7, and the floor had a reflectance factor of 0.45. The working plane was 0.80 m high (desk height). Light loss coefficient due to window construction was τ = 0.64. The neighboring objects at a distance did not shade the room. The classroom is usually occupied from 8:00 a.m. to 17:00 p.m.

The room being used for medium-precision activity was classified in III. Light—technical class. With the given lighting system, at the critical point of the functional place on the horizontal plane, the following values are required: minimum standard value of daylight factor DF_min = 1.5%, average daylight factor D_average = 5%, and uniformity of the illumination more than 0.3 for a given visual task [16,17].

An illumination of minimum 300 lx is recommended for most of the room area, meeting the target climate-based daylight factor and 500 lx for the areas where productive work is performed. On the selected day, the value of the outside light ranged from 5500–7000 lx in February this year. Measurement was performed at nine control points simultaneously with two lux meters (see Figure 7). An exterior horizontal illumination of 5000 lx was considered for the simulation program [16,17].

3.3. Daylight Measurement and Daylight Simulation

Daylight measurements were performed according to the Slovak standard “Measurements of day lighting.” The instruments used were two data loggers, ALMEMO 2690-10A, and illuminance sensor ALMEMO FLA 623VL with the production number 15061543, with an accuracy of 5%.

Daylighting simulations were performed with Velux windows with the model of the tested classroom and 3 variants. The measurement was performed similarly to the method of the authors in [54], who performed measurements of illuminance (lx) in various historic buildings in situ. The results of DF and other results can be seen in Figures 9 and 10. The results of the measured values and simulated values of DF (in points according to Figure 7) can be seen in

Table 2. Results of DF measured and simulated values—variant 0 (real situation), validation of model.

Daylight Factor (%)—Measured Values in the Points
Point	1	2	3	U0 (-)
A	12.2	5.4	2.3
B	12.7	5.0	2.8
C	10.6	3.1	1.8
				0.29
Daylight Factor (%)—Simulated Values in the Points Validation—According to Boundary Conditions Obtained from the Measurement
	1	2	3	U0 (-)
A	12.3	4.9	2.0
B	12.1	4.7	2.5
C	9.7	2.8	1.5
				0.26

and Figure 8.

Differences between variants with roof windows and without roof windows can be seen in

Table 3. Differences between variants with and without roof windows.

DF	DFmin (%)	DFmax (%)	DFaverage (%)	U0 (-)
Variant 0	50	47	64	−67
Variant 1	primary	primary	primary	primary
Variant 2	38	41	51	−25
Variant 3	58	48	68	−25

.

The results of the variant calculations of the daylight factor DF can be seen in Figure 9.

The results of luminance (brightness) for considered variants can be seen in Figure 10. The glare of the eyes is not high in the view of the window glass from the classroom in the cloudy sky. The brightness of the glazing is about 279 cd/m² in each case. The differences between the brightness of the background and the glazing are not large.

Figure 11 shows the situation of how the DF factor changes depending on the individual variants. In Figure 11a, the decreasing of the DF factor as a function of the distance from the windows, which are vertical, i.e., a distance from the perimeter wall, can be seen. Option 3 is the best, and it should be noted that all situations are better with respect to the minimum value of the DF factor. There will be a much higher DF value at all points. Even at the farthest points from the perimeter wall, there is a sufficient degree of illumination, unless the DF drops sharply to the minimum value. DF values can be found in parts (b) and (c) if we move the roof windows closer to the top of the sloping roof; i.e., they are shifted by a distance of 0.5 m; 1.0 m; 1.5 m; 2.0 m and 2.5 m in variants 2 (part b) and in variants 3 (part c). It can be clearly seen that the DF value decreases at the perimeter wall, but vice versa at the top of the roof, i.e., in distant places from the perimeter wall DF grows. Here, too, it is necessary to state that it is necessary to find a compromise and the geometry of the shape of the roof, to adjust the size of the windows so that we may approach the uniform lighting in the classroom. The higher the windows, the more discomfort there is, and it is necessary to pay attention to glare reduction using internal shading devices [55].

It is also important to defend against high heat input in the summer. The heat gains are such that internal discomfort arises.

3.4. Prediction of Indoor Climate in Summer Period

We addressed the issue of overheating in the summer not only for all considered variants (0,1,2,3), but also for cases without a vertical window strip (Figure 12) and for variable sizes of windows and variable distances from the window sill (peripheral wall). As only informative measurement was performed in the attic space, the situation can only be considered as a prediction. The measurement of daylight was performed in February under a cloudy sky. There was excessive lighting during the measurements and also during the simulations (not glare). It was stated that the daylight will be fine, but it will result in excessive heat gains through the skylights and at the same time through the roof covering, which has small (not suitable) heat accumulation. The evaluation of the overheating of the presented building was realized by the simulation tool WUFI Plus. Boundary conditions were considered as for daylight—only the date was changed. We considered the warmest months of June and July 2020 according to the IWEC forecast. This prediction contains “typical” weather files, suitable for use in simulating energy efficiency which can also be used as a prediction of people’s thermal sensations. Outdoor temperatures, global solar radiation and indoor temperature over the selected two months can be seen in Figure 13.

The analyzed time of maximum overheating was selected from 7 to 21 June 2020, which is 15 days. The composition of individual types of attic packaging structures can be seen in

Table 4. Composition of the structure—parapet masonry as a perimeter wall.

Nr	Material/Layer (from Outside to Inside)	ρ (kg/m³)	c (J/kgK)	λ (W/mK)	Thickness (m)
1	Cement lime Plaster	2000	850	1.2	0.02
2	Solid Brick Masonry	1900	850	0.6	0.45
3	Mineral Insulation Board	115	850	0.043	0.1
4	Air Layer 50 mm	1.3	1000	0.28	0.05
5	Gypsum Board	850	850	0.2	0.013

,

Table 5. Composition of the roof structure.

Nr	Material/Layer (from Outside to Inside)	ρ (kg/m³)	c (J/kgK)	λ (W/mK)	Thickness (m)
1	Roofing	2400	1000	0.5	0.0001
2	Air Layer 100 mm Low ventilated	1.3	1000	0.59	0.1
3	Mineral Insulation Board	113	850	0.040	0.18
4	Vapour retarder	130	2300	2.3	0.0001
5	Air Layer 50 mm	1.3	1000	0.28	0.05
6	Gypsum Board	850	850	0.2	0.013

,

Table 6. Composition of the ceiling structure.

Nr	Material/Layer (from Outside to Inside)	ρ (kg/m³)	c (J/kgK)	λ (W/mK)	Thickness (m)
1	Cement lime Plaster	2000	850	1.2	0.01
2	Steel-Concrete slab	2300	850	1.6	0.25
3	Mineral Insulation Board	115	850	0.043	0.06
4	Air Layer 70 mm	1.3	1000	0.4	0.07
5	Oriented Strand Board	630	1400	0.13	0.02
6	Oriented Strand Board	630	1400	0.13	0.02

and

Table 7. Composition of the partition structure.

Nr	Material/Layer (from Outside to Inside)	ρ (kg/m³)	c (J/kgK)	λ (W/mK)	Thickness (m)
1	Gypsum Board	850	850	0.2	0.013
2	Air Layer 120 mm	1.3	1000	0.723	0.12
3	Mineral Insulation Board	115	850	0.043	0.04
4	Gypsum Board	850	850	0.2	0.013

(three-zone model). It was a north-facing classroom, a corridor, and a south-facing classroom. Occupancy in the simulated zones was determined for 20 adults with easy work activity. Classrooms were staffed Monday through Friday from 7 a.m. to 6 p.m. with an hour-long break during lunch.

Considering the expected occupancy, this would mean a large increase in CO₂, which would be 750 ppm (1350 mg/m³), which is a lot. The air exchange rate was therefore set at 50 m³/h of fresh air per person. The IWEC weather limit was used to predict the simulation of the indoor climate in the Kosice Region for 2020. The simulation model prepared for this calculation was divided into three zones. There are two classrooms with north and south orientation and a corridor between of them, see Figure 4 and Figure 12.

The maximal internal superheat temperature must not exceed a value of 27 °C indoors according to prescription. The time period selected for the calculation represents the warmest weather, including days with high temperatures (day/night) combined with high sunlight.

From the temperatures, you can see how the accumulation capacity manifests itself. The maximum temperature in the outdoor environment is reached on June 19, but inside it is the maximum until June 20, there is an effect of the accumulation behavior of the envelope and its impact on the indoor environment of the attic.

It can be seen from the above that the temperature maximum in the indoor environment of the classroom is reached on June 20 for Variant 0 (real situation) 39.20 °C; for Variant 1 (without roof windows) 31.58 °C; for Variant 2 (one row of windows) 37.01 °C; and for Variant 3 (two rows of windows) 40.19 °C. The difference between these variants is about 8 K.

When the facade is oriented to the north, the effect of vertical windows on overheating in summer is irrelevant. Temperatures are simulated for individual variants—Variant (0), (2), (3) = (38.80 °C); (36.01 °C); and (40.10 °C). This means that without vertical windows, the resulting temperature in summer will be reduced by only about 0.5 K. Variant 2 (one strip of windows) seems to be most suitable for optimal lighting and small overheating.

The results for the considered two months are shown in Figure 13a, and those for the selected 15-day interval (7–21 June 2020) are shown in Figure 13b. Taking into account the boundary conditions as well as adjusting the air exchange to n = 6 h⁻¹ and orientation to the cardinal south–north.

The calculation of the need for air cooling was performed (see Figure 14) to achieve the optimal (permissible) indoor air temperature of 27 °C in the summer for the analyzed basic simulation variants (according to Figure 4). Based on the calculation, it can be stated that reducing the area of roof windows to the visual optimum (Variant 2) will reduce the need for supplied energy for cooling by a quarter compared to the original solution (Variant 0).

4. Discussion

This case study examines the effect of the size of skylights to maximize the use of daylight in an attic. It is used to find solutions to maximize the use of daylight, reduce the cost of artificial lighting, and minimize overheating. Real measurements and a series of computer simulations using a simulation tool from Velux Daylight Visualizer 3 and WUFI plus served as tools for determining the size of windows.

The results were evaluated in terms of daylight and classroom lighting levels according to daylight standards.

The daylight factor is the basis for assessing the illuminance in cloudy sky. Results obtained were compared to a standard DF value.

The degree of illumination at points at distances of 1 m, 2 m and 3 m from the window sill is shown in Figure 11b,c. Here a decrease and increase in the DF factor at a distance from the perimeter wall can be seen. As the roof windows are moved to the top of the slope roof, the level of illumination increases and the uniformity of the lighting in the classroom also improves.

Shifting the windows to the top does not affect the resulting temperature in summer or the energy for cooling the indoor air, but it improves the visual conditions presented by the level of daylight DF_average.

If we were to move this strip of windows to the center of the roof, i.e., at its top, the most suitable situation is when this strip is at a distance of about 2 m from the window sill—from the perimeter wall, (see Figure 15). In this case, the highest DF_average value is reached. An interesting result is that if the window strip moves more than 2 m away from the window sill, the average DF value decreases (see Figure 15b).

These results show that skylights successfully bring light deeper into the room. However, the number of light intensity levels is still below normal in some places. Good lighting intensity was analyzed based on class requirements for Slovak standards. The highest internal light intensity at the farthest point of the class is 160–200 lx, which is below the standard value of 500 lx. Skylights have a significant effect on daylight where the WFR value is exceeded and the WWR value is close to the required level (see Figure 6). WFR values reach a level higher than 10% in almost all variants.

A number of window resizing calculations were performed, as shown in Figure 16. This figure contains a graph from which it is possible to predict the size of the windows (relative to the floor), the WFR, and to find out what the level of daylight (DF%) and summer overheating (indoor air temperature °C) is predicted to be.

In a very simple way, we can preliminarily determine the size of windows under top and combined lighting. Based on the WFR from the graph in Figure 16, not only the value of DF (daylight factor) can be read, but at a given value of WFR (window–floor ratio), the temperature of the indoor air during overheating in summer also.

5. Conclusions

Students spend more time at school than anywhere else. Of this time, about 70% is spent in classrooms. Daylight has a significant impact on students’ educational outcomes. The proper use of daylight can improve the indoor environment and reduce energy consumption.

Windows allow natural light to penetrate to the interior. In the vertical plane of the perimeter cladding, in the window sill, they allow a visual connection with the external environment. Energy savings in daylighting are achieved by reducing the use of electric lighting. Daylight also improves passenger satisfaction and comfort. Windows are a complex part of building design and provide significant benefits to school buildings. The aim of the study is to evaluate the availability of daylight in the attic, which is used for teaching. The article compares three alternatives to skylights. Skylights are definitely needed to provide more light for remote areas. It is interesting that the movement of the roof window strip achieves maximum uniformity and achieves the required levels, even at distances from the perimeter walls.

In this study, simulation programs were used for model window situations. Based on the performed DF simulations and superheat predictions, the results were integrated closer to the optimal state. The results of this study show recommendations for the appropriate design of roof windows in the attic and their uses for similar studies. The whole process was realized in cooperation with light and thermal conditions. The measurement results show that the levels of interior lighting in the basic variant are below the level of the standard. Information measurements of the indoor temperature in the summer show that a high indoor air temperature has been reached, which causes discomfort. Daylight was not enough to ensure good lighting and the lighting was not distributed evenly, and the temperature was excessive despite the increased amount of ventilation. The analysis showed that there are many alternatives regarding the location of openings to improve daylight in the classroom. It was found that the combination of facade and roof windows (combined lighting) has better results, as well as the fact that vertical windows are not very important for calculating the temperature rise in summer.

The attic space is not common for use in the teaching process. Only a few classrooms are located in attics. The article shows such a case study, in which the school expanded the teaching space by raising the roof and creating new teaching spaces. There is an excessive amount of light in the classrooms; the problem is overheating in the summer. The prediction of the indoor climate in the summer showed that it is not possible to achieve the prescribed well-being with the proposed design solution. The shielding elements help to reduce overheating and reduce the indoor temperature, but not below the prescribed maximum indoor air temperature. The design solution will not solve the problem; cooling must be designed. For the evaluated period (June, July) in the given case, cooling is proposed, which will ensure a sufficient degree of reduction in the internal temperature in summer to the required level. The graphical dependence derived in the paper serves to predict similar spaces in conjunction with the achievement of the required DF and the estimation of the achievement of the indoor temperature in summer.