Daylight image

If I asked you why 20% of respondents to a survey conducted in 2015 felt unhappy with their homes, would you hazard a guess at negative equity or rising interest rates? Noisy neighbours, perhaps? You’d be wrong in all cases. The answer is insufficient daylight. This may sound frivolous, but prolonged lack of sunlight can lead to serious health issues. There’s an economic impact, too; extended use of artificial lights wastes energy.

The solution lies in the introduction of effective daylighting. Let’s explore the issues around daylighting the modern home to preserve the well being of its occupants.

Light Matters

The same homeowners’ satisfaction survey rated lack of daylight ahead of more obvious categories such as the availability of amenities and access to work or schools. The bulk of complainants lived in homes less than a decade old. Analysis of the full results implied a need for government regulation around the daylighting of dwellings.
The Law Commission, in 2014, put forward changes to the so-called “Rights to Light.” Landowners already had a right to receive daylight within their structures. Regulations inhibited landowners from erecting structures in a manner that blocked light without consent from neighbouring landowners. On the other hand, it gave landowners comfort to know they would have continued access to natural illumination, thereby adding to the property’s utility and value.

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Daylighting Definition

Daylighting: the act of adding to homes (amongst other openings) windows, roof lights, and reflective surfaces. The practice introduces natural light into properties and allows it to permeate the interior.

Designers factor in such considerations as window dimensions, number, and position; the depth and layout of rooms; the prevalence of reflective surfaces as well as decoration; and the existence of exterior blockers of natural light (other buildings and trees perhaps). Designers consider these factors within the context of overall project energy efficiency.

Research has determined the impact on the daylighting of a range of specs for wall insulation. Results illustrated that, through the installation of enhanced insulation, the depths of sills and walls could be minimised, increasing internal daylight measurements without having to compromise on fabric performance.

Healthy Environments

The benefits to occupants of effective daylighting are considerable. Improving daylighting in hospitals, for instance, leads to substantially positive outcomes. Patients recover quicker from surgery, require less pain relief, bounce back more effectively from depressive episodes, and so on. The benefits of incorporating daylighting into buildings’ construction are documented at length in BS 8206-2:2008 (Lighting for buildings. Code of practice for daylighting).

Highlights include:

• The regulation of our circadian systems
• A reduction in SAD symptoms
• Optimal vitamin D levels
• A reduction in the prevalence of bacteria/ viruses known to cause winter respiratory infections

Building Standards

This recent interest in daylighting’s positive health effects has now extended to several essential building standards, such as the WELL building standard, which recognises owners’ right to light, window constructions that factor in daylighting, and the inclusion of circadian lighting into building design. The BREEAM assessments further acknowledge the reduction in artificial lighting requirements, which cut household costs.

Design Challenges

The challenges to designers in dealing with seasonal fluctuations in light levels are considerable. They must strike a balance between permitting excessive light into a building and having too little. Both extremes being problematic. An excess of light may lead to overheating; insufficient light creates dark and dingy living spaces.

Here, science comes to the rescue in the form of the average daylight factor (ADF). This allows for calculating the difference between the amount of inside light and the amount of external light. It is a measure of light reaching occupants.

An ADF calculation starts with estimating the height of a typical working surface and proceeds to work out how much light reaches this plane compared to its total area. The ADF for external comparison uses the light levels of a standard overcast sky, with the final ADF figure expressed as a percentage. This percentage is an average of light compared to the outdoor level.
A one percent ADF is, in effect, telling us that internal light, on average, is 1% of the that occurring outside.

ADF Calculations

A consensus exists amongst prominent authorities on lighting. As a rule of thumb, an ADF of two percent is the threshold at which designers may claim to have sufficiently Day-lit an interior. As expressed in BS 8206-2:2008, good practice requires rooms to have a day-lit appearance. They cite the two percent ADF figure as an appropriate target. To give so perspective, the standard would consider five percent ADF to define a room space that is adequately day-lit.

A deficit of naturally occurring light may elevate a building’s energy needs. Some codes set minimum ADFs for single rooms, depending on use (1 for bedrooms, 1.5 for living rooms, and 2 for kitchens.

Over and above these considerations lines the concept of ‘visual comfort.’ It’s one of the criteria in the BREEAM standard, which awards marks for the following aspects of design:

• Glare reduction
• Daylighting
• Outward view
• Inside lighting
• Outside lighting

The designer’s balancing act is to attain the optimal daylighting conditions without compromising too much on the recommended thermal U-values. If we look at this problem more carefully, we find that a particular volume of insulation will be needed to match the required U-Values under building regulations.

Countering this, the greater the number of windows and apertures a building has, the thicker insulation must be to reduce thermal bridging and minimised energy loss. This may negatively impact on window reveal measurements, requiring thinner insulation.


Industry research compared ADF for walls across two types of insulation: phenolic and mineral fibre. The facade layout with the best ADFs featured a single wide-spanning window for either type of insulation.

Percentage improvements in ADFs from external wall facades using phenolic insulation compared with fibre, regardless of layout, ranged impressively from 10.6 % to 63.2 %.
Results for bedrooms and living rooms using phenolic surpassed minimum standards for daylighting as set down in BS 8206-2:2008 The findings show that phenolic insulation offers a thinner solution, still compliant with the requirements of BS 8206-2:2008, and is undoubtedly more efficient than using fibre insulation.

Chris Nicholls
Commercial Director