The daylight factor

The benefits of natural daylight within homes are obvious. There are all manner of physiological, spatial, environmental, health and well-being advantages to having well-lit residential interiors and there are even obvious energy and economic benefits in terms of reduced reliance on artificial lighting. Which is why it may come as a surprise to some that the Code for Sustainable Homes calls for average daylight factors as low as 1.5% for living and dining areas and 2% for kitchens.

Access to daylight is an essential element for our homes. [It] affects the health and mental well-being of those people living in them

Angela Brady, RIBA

According to the RIBA and rooflight manufacturer Velux this just isn’t good enough. Last week the RIBA launched the Without Space + Light campaign to lobby for improved space and lighting standards within UK homes. A survey commissioned in conjunction with the campaign found 20% of respondents cited lack of natural light as a principal sources of dissatisfaction with their homes.

RIBA precedent Angela Brady explains the institution’s position with regard to domestic natural lighting as thus: “Decent access to daylight is an essential element for our homes. The importance of natural light in the household not only reduces energy costs and improves the appearance of your home but also affects the health and mental well-being of those people living in them.”

Moreover, Velux has recently called for the average daylight factor required in all new homes to be increased and has cited the early results from its pilot CarbonLight Homes scheme in Kettering, Northamptonshire, where a whopping average of 7.5% was achieved, as evidence of its proposal’s feasibility. Designed by architect HTA and developed in conjunction with Willmott Dixon and others, homes in the scheme are intended to “use nature in an intelligent way to maximise daylight and encourage a sustainable lifestyle”.

Velux suggests that not only should the daylight factors recommended in the code be increased, but that minimum daylight factors should be enforced by statute for the first time by being specified within the Building Regulations, a development that is already being considered in Scotland.

Now, a window manufacturing company essentially calling on the government to legislate for more windows may not exactly come as a huge surprise to the more cynically inclined. But Velux insists that increasing the amount of daylight in new homes is essential for occupier wellbeing and sustainable development.

“To be honest, even a factor of 2% or 3% is still fairly gloomy,” says Velux design manager Paul Hicks. “Four per cent begins to give the perception of being daylit, but it’s not till you get to 5% that you start to offer a well-lit space. We need to push the boundaries with regard to the amount of daylight we offer in our homes.”

Velux is particularly proud of the fact that the 7.5% ratio achieved on the CarbonLight project was realised with only a 25% glazing to floor ratio. This essentially means that generous daylighting was delivered without resorting to the obvious strategy of plastering windows across the external walls.

Hicks explains that, instead, more subtle design strategies were employed such as specifying glass balustrades and internal glazed partitions to increase the amount of reflected light, as well as painting internal walls white. Also, rooflights were used extensively, allowing the CarbonLight homes to benefit from the fact that facing the sky directly they deliver 40% more daylight than vertical windows.

Despite the lean 25% glazing ratio, the obvious environmental concern with any increase in window provision centres on the potential for heat loss and overheating, as well as glare. But Velux says the triple-glazed rooflights on the CarbonLight homes achieved a U-value of 1.0 W/m²k which is increased by an extra 0.1 W/m²k when installed blackout blinds are drawn.

This is achieved through an elaborate building management system (BMS) which includes computer-controlled window opening, automated internal, external and blackout blinds and a combination of natural and mechanical ventilation. While Hicks admits that this has a cost impact, he insists this is compensated by savings in energy costs. “No matter how sophisticated the energy-saving solution, you can’t legislate for how people behave. The BMS option allows user flexibility with no detrimental environmental impact.”

However, not everybody agrees that Velux’s vision of a super-lit home is desirable or even necessary. Max Fordham, founder of the environmental engineering practice of the same name, thinks the 7.5% daylight factor level is “probably a little too high. In any case, it’s minimum daylight factors that are more important than the average. It’s obviously a very good thing when natural light replaces artificial light, but more light means more windows, which obviously have a cost impact.

“But even more crucially, it’s important to consider that the human eye adjusts very quickly to light conditions. Also, while rooflights are obviously far more efficient at dispersing natural light than vertical windows, the moment a building rises over more than one storey, it becomes much harder to evenly distribute these benefits throughout the whole interior.”

Fordham also maintains that, when it comes to commercial buildings, the requirement for high light levels may actually have had a damaging effect. “Five-hundred lux [the minimum lighting level recommended by the CIBSE Code For Lighting] is generally too high for office interiors. This would mean a whopping daylight factor of 10% if delivered naturally through windows, so inevitably it tends to lead to excessive reliance on artificial lighting, which has a detrimental environmental impact.”

Despite reservations such as these, Velux and the RIBA remain committed to their call for increased daylight. Brady rebuffs any environmental accusations that may be levelled against an increased amount of windows by revealing that “the London Housing Design Guide includes a series of standards on natural light - such as minimum percentage of windows in habitable rooms and which habitable rooms need daylight - and the RIBA haven’t seen any evidence of environmental detriments in the homes being built using the guide.”

Hicks also believes that the CarbonLight homes project convincingly challenges misconceptions about the environmental impact of generous glazing.

“There are obviously huge benefits to Passivhaus but its perceived bias against lots of windows is indicative of a finite low energy/less daylight solution. CarbonLight presents a much more logical, flexible framework which optimises the benefits of daylighting in a way that balances energy use rather than dictates it.”