Glazing Design Advice

Designing Glass to Reduce Light Spill

Colloquially referred to as ‘light pollution’, light spill is the term used to describe the artificial light that is emitted from a building or home when the interior lights are on. This ‘light spill’ is more noticeable at night when the light levels internally are much higher than they are externally.

Light spill from buildings as known and documented effects on both natural wildlife and the visibility of starlight in locality to the light pollution.

In recent years, there has been a notable rise in inquiries from architects seeking to minimise light spill in highly glazed rural buildings. This trend is largely driven by rural planning regulations that aim to protect night skies and local wildlife. Light spill tolerances can vary, with some projects achieving as low as 30% light transmission, which minimises light pollution but can make interiors appear dim even during daylight hours. For brighter interiors, around 60% transmission may be preferable, allowing more natural light while still reducing exterior light spill.

Dark Sky Policies to Reduce Light Spill from Glass

Many governments around the world have implemented ‘dark sky policies’ with the UK formalising its ‘Dark Sky Policy’ in 2020. This built on the local policies already in place within certain areas of the UK including the South Downs National Park (SDNP) which became an International Dark Sky Reserve in 2016.

The aim of the Dark Sky Policy (also known as the Dark Skies Movement) is to address the issue of light pollution and was a key component of the new regulations implemented under the National Planning Policy Framework.

These guidelines emphasise the reduction of light pollution through the thoughtful choice in building materials and lighting design, encouraging innovative solutions in architectural planning.

Under this framework, new projects are advised to integrate strategies for reducing obtrusive light, including stricter planning controls on exterior lighting, expanding the scope of planning permissions, and enhancing local authority powers to enforce nuisance lighting regulations.

Measuring Glass Light Spill (Light Transmission)

To understand what you need to achieve with your glass design to reduce light spill, you first need to understand a few technical terms associated with the performance of glass.

The amount of visible light that can travel through a glass unit is most often referred to as Light Transmission or LT for short. A more accurate name for this measurement is  Visible Light Transmission (VLT) but this is the same measurement.

Light transmission through glass is shown as a percentage value, informing you as to the percentage of visible light that will be able to pass through the glass.

The lower the LT percentage, the less visible light that will be able to pass through the glass.

Another thing to consider that is not always fully understood is that light transmission through glass works in both directions. The amount of light that is able to travel from inside to outside (light spill) is the percentage of light that will be able to pass through the glass to the inside.

Glass Options to Reduce Light Spill

As above, when you change the light transmission of a glass unit it also changes the amount of light that can enter a building. Therefore, designing glass for reduced light spill needs to be done with consideration to ensure that the internal atmosphere of the building is not negatively affected.

When you are given guidance to ‘reduce light spill’ within an architectural design there is often very little additional guidance provided, including specific light transmissions to achieve.

If specific performances have been provided for your glass design the team at IQ can help to achieve these by modelling the performance of the glass, calculating the optimal glass specification for the light transmission you need to achieve and providing guidance as to the best solutions.

In these instances, the percentage of acceptable light transmissible through the glazing varies from project to project.  The requested light transmission for glass in these areas typically ranges from 30% to 60% depending on the reasoning for the request and the area.

In some instances, the request is received to ‘reduce light spill’ with no specific performance. In these cases, the team at IQ can assist with balancing the design requirements of the glazing with reducing the light transmission through the glass design.

Glass Coatings to Reduce Light Spill

Variations of solar control coatings can be applied to glass units to change the light performance of the glass. These coatings have a neutral design so are the most suitable glass options to maintain a clear, unchanged design to the glass whilst reducing the light transmission. The other advantage of using a glass coating to reduce light spill is that it will also reduce the solar gain through the glazing, perfect for adhering to overheating regulations for the building.

The application of solar control coatings can provide a glass unit with light transmission from 59% down to 30%. Lower light transmission values have a slightly higher level of internal and external reflectance to consider. For example, a double-glazed unit with a high-level solar control coating applied could provide a light transmission of 30% with 24% reflectance outside and 24% reflectance inside (ρv levels). Whereas a more neutral solar control coating applied to the same glass specification could provide a light transmission of 59% with an 18% external reflectance and 20% internal reflectance.

Using coatings such as these are the most common and effective way to reduce the light transmission of a glass unit in a controlled way that minimises impact on design.

Body Tinted Glass to Reduce Light Spill

Body tinted glass is a glass pane that has had melt colourants added to the float glass during manufacturing to create a slight tint or colouration to the glass itself. There are various colours and tints available for body tinted glass and it will depend on the architectural design and performance required as to the best option for your project.

The most common colours for tinted glass are Grey, Green, Bronze, and Blue.

Tinted glass provides a similar level of light transmission as possible with coated glass but with lower levels of internal and external reflectance. For example, a grey tinted glass unit (with no other coatings) could provide a light transmission of 45% with an external reflectance of 6% and an internal reflectance of 5% (ρv).

However, as the colouration of the glass is within the material itself (rather than an applied coating) colouration differences will be noticeable across different thicknesses or specifications of glass. For example, 4mm grey tinted glass will look different to an 8mm grey tinted glass which will look like a darker grey.

If body tinted glass is being used on a project it is important that careful consideration of the glass specification throughout the project is taken to ensure an even colouration across all elevations.

The Myth of ‘One Way Glass’ for Reduced Light Spill

Light transmission works in both directions. If you change the amount of natural light that can escape a building through the glass (reducing the light spill) then you will also reduce the amount of light that can enter a building through the glass.

Highly mirrored glass or glass coatings can create the appearance of ‘one way’ glass in the daytime. When the light levels are much higher outside than they are inside, the highly reflective external surface reflects the external light away, creating that mirrored appearance.

However, at night, when the light levels inside are much higher than they are outside that external reflectance is no longer in play. Light will be able to travel through the glazing and the interior spaces will be visible from the outside.

This concept is shown to great effect at the IQ Glass head offices in Buckinghamshire. The right-hand side of our front elevation is clad in highly reflective glass. This is achieved through a high specification mirrored solar control coating applied on the glass.

During the day – when we are working in the upstairs offices – the light levels are higher outside and the glass has a mirrored appearance externally. From the inside the light levels are reduced but the offices still have natural light and views to the outside.

At night when the lights are on inside, the light inside is much higher than outside so that external reflectance is not as noticeable and you can see through the glazing.

For this reason, this type of ‘one way glass’ is most often used in office spaces that are more likely to be occupied during daylight hours and empty at night. Our Pentonville Road case study is a great example of this where we used highly reflective glass on the top floor glass extension to an office building.

Specifying Glass to Reduce Light Spill

As you can probably tell from above, there are lots of ways to change the light performance of your glass design to achieve the light transmission you need for planning.

The easiest way to make sure your glass design complies with the planning requirements is just to speak to the team at IQ Glass. Our team can take the performance requirements and do the hard work for you, providing you with the best options for the glass design to achieve both the design and performance needed.

Contact the IQ Glass team by phone by calling +44 1494 722 880 to speak to the technical team straight away or send us an email to hello@iqglassuk.com.