Rethink the power of daylight: Redefining horizontal spaces with vertical light

Architecture goes beyond its fundamental function of defining spaces and providing protection, it shapes the user experience, influencing sensations of comfort, spaciousness and well-being. Among the many elements that make up a building, openings play a crucial role in connecting the interior and exterior, balancing privacy with transparency, and allowing the entry of natural light and ventilation. In particular, natural light transforms environments, defines atmospheres and enhances architectural details, making spaces more dynamic and inviting.

Windows, once simple openings in walls, have evolved through advancements in materials and technology, maximising efficiency and expanding their role in architectural design. If Gothic architecture showcased marvellous glazing through towering stained glass windows, modern architecture shifted toward horizontal forms and fully glazed facades, transforming how daylight is integrated into spaces. However, relying solely on facade glazing presents a limitation – natural light is often confined to the building's perimeter, leaving central areas in shadow. Overhead openings, such as skylights and flat roof windows, overcome this challenge by channelling daylight deep into interior spaces, reducing reliance on artificial lighting.

Beyond their functional benefits, these openings serve as powerful aesthetic tools, sculpting interiors with light and shadow to define central spaces. This dynamic interplay enhances spatial perception, adding depth and visual interest. Additionally, roof windows contribute to natural ventilation, improving air circulation and overall indoor environmental quality. As a result, they have become essential elements in contemporary design, seamlessly merging efficiency with architectural expression. Below, we explore some key aspects to consider when specifying flat roof windows, highlighting their benefits and strategic applications for different projects.

1. Understanding the importance of natural light for spaces and users

Beyond enhancing spaces and reducing dependence on artificial lighting, natural illumination plays a crucial role in user comfort and health by regulating circadian rhythms and improving the experience of performing household tasks. To ensure adequate lighting levels, specific regulations establish guidelines on the sizing and distribution of openings in buildings. The European standard EN 17037, for instance, recommends that at least 50% of occupied areas receive sufficient illuminance during half of the daylight hours of the year.

Illuminance is measured in lux (lx), an International System unit representing the amount of luminous flux incident on a surface, equivalent to one lumen per sqm. Its analysis can be performed through lux metre measurements, computer simulations, manual calculations and qualitative assessments, considering factors such as solar orientation, window size and positioning, glass light transmittance and impact on visual comfort. Different spaces require different lighting levels: kitchens and dining rooms demand 500 lux; bedrooms and bathrooms, 300 lux; home offices and study areas, 500 lux; while hallways and staircases need 100 lux for safe navigation.

2. Expanding new horizons with flat roof windows

In this context, flat roof windows stand out as efficient architectural solutions, maximising light entry and ensuring a more uniform distribution of illumination, even in deeper areas far from conventional windows. In buildings with flat roofs, traditional façade windows limit light penetration to about 3.5 m from the exterior wall. However, overhead windows overcome this limitation by allowing natural light to enter directly from above, ensuring consistent daylight throughout the space.

In addition to their lighting benefits, overhead windows also improve indoor air quality. By facilitating natural ventilation, they increase the air renewal rate from 1.5 times to up to 2.5 times per hour, reducing pollutant concentrations and providing greater thermal comfort. This enhanced airflow creates healthier environments, benefiting both occupant well-being and the building's energy efficiency.

3. Choosing the most suitable type for each project

VELUX, a Danish company specialising in roof windows and skylights, is known for its innovative solutions that enhance natural light and ventilation in residential and commercial buildings. While best known for its sloped roof windows, VELUX also offers a range of flat roof windows, modular skylights and smart home integration systems designed to improve indoor comfort and energy efficiency. Here are the main types available:

VELUX Curved glass rooflight: Its unique curved glass top ensures efficient water drainage, even on roofs with a 0° pitch, while the slim frame and invisible sash maximise daylight and outdoor views. For added comfort, optional blinds provide heat protection, blackout capabilities and insect resistance, making it a perfect addition to any space.

VELUX Flat glass rooflight: Featuring a slim frame and an invisible sash, it maximises daylight and outdoor views, even with blinds installed. Designed for roof pitches between 2-15°, this rooflight is available in fixed, electric and solar-powered versions. The electric model includes a seamlessly integrated motor, a pre-paired wall switch and a rain sensor for automatic closing. The solar version operates wirelessly, making it ideal for renovations without existing wiring, with reliable battery performance even on cloudy days. To enhance indoor comfort, optional blinds provide heat reduction, blackout capabilities and insect protection, ensuring a bright, comfortable and energy-efficient living space.

VELUX Dome with glazing: Suitable for roof pitches between 0-15°, it features an insulated base unit with a two-layer insulating glass unit and inner laminated glass. The top unit is available in clear or opaque acrylic or polycarbonate, offering either maximum daylight or increased privacy. Available in fixed, manual and electric versions, the dome ensures flexibility for different needs. The electric model includes a rain sensor and pre-paired wall switch, with smart home compatibility for seamless control. Manual operation is enabled via a telescopic rod for natural ventilation.

VELUX Dome: A durable and practical daylight solution for uninhabited spaces such as warehouses, garages, sheds and small industrial buildings with flat roofs (0-15° pitch). It features a 2- or 3-layer opaque polycarbonate or acrylic dome without glazing. Available in fixed, manual and electric versions, it provides flexible ventilation options. The 230V electric model includes pre-mounted brackets for easy motor installation and is compatible with standard electrical outlets and switches. For added safety, a fall-through grid is available.

4. Strategically placing skylights: where and how to install them

The effectiveness of flat roof windows depends on several factors, including roof thickness, positioning, glazing type and technical specifications, all of which directly impact natural light distribution, energy efficiency and thermal comfort. A single large skylight (e.g., 1000 mm × 1000 mm) generally provides more even and efficient illumination than multiple smaller ones, reducing shadow patterns and abrupt light variations. In open-plan spaces, this can enhance the perception of spaciousness while maintaining a comfortable light level throughout the day. Conversely, multiple smaller skylights can be beneficial in corridors or areas where localised light is preferred. Strategic positioning is equally crucial. Centrally placed skylights help achieve uniform light distribution, while task-oriented placement – such as above kitchen countertops, workstations or reading areas – improves functionality and visual comfort.

5. Understanding the sun's path

Understanding the sun's path is essential when determining the optimal placement of skylights, as it directly influences daylight quality, indoor temperature and energy efficiency. Unlike vertical windows, which have a fixed orientation, flat roof windows face all directions, allowing for more versatile light exposure. While overshadowing from surrounding elements can affect the amount of light received, the window itself is not limited by a specific orientation. The amount of direct sunlight a flat roof window receives is heavily influenced by latitude. In equatorial regions, flat roof windows will receive direct sunlight throughout the whole year. At higher latitudes, sunlight exposure changes with the seasons. A roof window can offset the limitations of vertical windows – for instance, if a room has a north-facing window (in the northern hemisphere) that receives minimal direct sunlight, adding a flat roof window can introduce natural light from the south at specific times of the year.

6. Opt for smart glass: enhancing comfort, efficiency and protection

Incorporating advanced glass technologies in skylights enhances thermal performance, energy efficiency and overall indoor comfort. One of the most effective solutions is low-emissivity (Low-E) glass, which features a microscopic coating that reflects heat while allowing natural light to enter, minimising heat gain in summer and helps retain warmth in winter, reducing reliance on heating and cooling systems. Additionally, skylights with UV protection and solar control coatings serve as a thermal shield, filtering excessive sunlight to prevent overheating and glare. These innovations create a more comfortable, energy-efficient and visually balanced indoor environment.

7. Simulating reality through computer modelling

Assessing whether a design meets daylighting expectations or requirements may not always be standard practice. To support this process, VELUX has developed a free, user-friendly software tool: VELUX Daylight Visualizer. This program accurately simulates light quality in spaces through photorealistic visualisations and mapping, allowing for detailed analysis of luminance, illuminance and light distribution across different times and climatic conditions. It also enables the simulation of solar control devices, such as light tubes and blinds, while supporting 3D modelling and BIM/CAD imports. Additionally, it generates automated reports to validate compliance with the European Standard EN 17037, ensuring precise, high-quality daylighting analyses.

8. Beyond illumination: a fundamental architectural element

Flat roof windows go beyond being simple light sources; they become essential elements in architectural design. Their strategic use maximises the benefits of natural light, contributes to defining the building's visual identity, enhances environmental comfort and reduces energy consumption. By integrating natural light as a design element, architects can create more sustainable, expressive and healthy spaces. Careful selection of the type, placement and applied technologies for overhead windows enhances both the functionality and aesthetics of buildings, providing brighter, more efficient and more pleasant environments for users.

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