8 January 2025

No Sky Line (NSL) Explained

The No Sky Line test is an essential method outlined in the Building Research Establishment (BRE) guidance to evaluate daylight distribution within buildings.

This test determines adequate daylight penetration into interior spaces, promoting energy efficiency, occupant comfort, and compliance with modern design standards.

Understanding and implementing this test is crucial for architects, developers, and planners aiming to create sustainable and well-lit environments.

What is the No Sky Line Test?

The No Sky Line test determines areas within a room where no part of the sky is visible.

These areas, marked on the working plane—typically 0.85m above the floor in residential spaces and 0.7m in offices—are considered devoid of direct daylight. The no sky line serves as a visual and quantitative marker of daylight access within a room.

Significance of the No Sky Line Test

This test plays a critical role in assessing daylight quality and distribution by:

• Identifying areas within a room that require supplemental lighting due to inadequate daylight.

• Enhancing the design of buildings for optimal energy efficiency and occupant well-being.

• Supporting planning applications by ensuring compliance with daylighting guidelines.

Guidance from BRE on the No Sky Line Test

The BRE guidance provides a comprehensive framework for applying the No Sky Line test, especially in :

• New Developments : To assess the impact of proposed buildings on the daylight availability in adjacent properties.

• Design Refinement : To optimize room layouts and fenestration for improved daylight access.

• Post-Development Validation : To confirm that as-built conditions meet daylighting standards.

How to Perform the No Sky Line Test

Set the Working Plane : Define the horizontal plane at the specified height (e.g., 0.85m in residential spaces).

Determine Sky Visibility : Identify areas on the working plane that have a clear view of the sky.

Draw the No Sky Line : Mark the boundary between zones with and without direct skylight.

Analyze Results : Calculate the proportion of the working plane beyond the no sky line. BRE guidance suggests that if over 20% of the working plane falls beyond the line, the space may require improved daylighting measures.

Improving Daylight Distribution

The BRE guidance provides several strategies for improving the daylight distribution indicated by the No Sky Line test :

Increase Window Sizes

Larger windows allow more daylight to enter a space. However, raising the height of window heads is particularly effective in improving both the amount of daylight entering and its distribution. This is especially important for basement spaces or heavily obstructed rooms.

Optimize Room Layouts

Rooms that require more daylight, such as living areas and kitchens, should be positioned away from obstructions. Locating these rooms on upper floors or along building facades with minimal obstructions can enhance daylight access and improve the No Sky Line.

Use Reflective Interior Surfaces

Light-colored walls, ceilings, and furnishings increase the reflection and dispersion of daylight within a room. Reflective surfaces amplify light penetration and improve brightness throughout the space.

Incorporate Dual-Aspect Designs

Dual-aspect rooms with windows on two sides significantly improve daylight distribution by allowing light to enter from multiple directions. This approach is particularly beneficial for deep-plan buildings or urban settings with narrow spacing between structures.

Reduce External Obstructions

Minimizing obstructions such as overhangs, balconies, or nearby structures enhances the angle of visible sky (θ) from windows. Staggered balconies or unobstructed bay windows are practical design solutions to maximize light penetration.

Leverage External Surface Reflectance

Light-colored external surfaces, such as paving and adjacent building walls, reflect more light into windows. Regular maintenance of these surfaces is necessary to preserve their reflective qualities.

Utilize Light Wells or Skylights

For spaces with limited wall openings, introducing light wells or skylights can bring daylight deeper into the building’s interior. This is particularly effective for areas like stairwells or basements.

No Sky Line Conclusion

The No Sky Line test is a cornerstone of daylighting analysis in modern building design, as emphasized by BRE guidance. It ensures that spaces are not only visually appealing but also sustainable and comfortable for occupants.

By proactively addressing areas beyond the no sky line, designers and developers can contribute to creating environments that are brighter, healthier, and more aligned with contemporary standards of living.

Expert Guidance on Daylight for Planning Applications

Anstey Horne have a team of experienced experts that will assess what levels of light a local authority are likely to accept based on their experience of working on similar projects within that area.

We work closely with project teams to develop a strategy in order to effectively utilise alternative NSL targets in dense urban areas that fall outside the default BRE recommendations. This often involves speaking with the local authority at an early stage (such as pre-application) for instance.

Engaging with local authorities early in the process – such as during pre-application discussions – can help establish agreed-upon No Sky Line targets that respect the existing context. This proactive approach ensures compliance and streamlines the planning process, reducing the risk of delays and additional costs associated with justifying NSL levels at the planning submission stage.

For more advice on how we can help support a planning application with a daylight & sunlight assessments please give us a call. If you would rather we contacted you please fill in our Contact Form and we will be in touch.

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