Rooflight manufacturers are right to publish and promote the U-values of their products and low U-values are indeed an important contributing factor to overall building energy efficiency. However, rooflight area is actually the more significant factor in achieving Part L compliance. Generally speaking the larger the roof light area, the greater the reduction in energy usage and subsequent CO2 emissions.
It’s obvious when you think about it. Artificial lighting accounts for almost 20% of global electricity consumption. So bringing more daylight into buildings will have a direct and positive impact on this.
It’s true to say that with larger rooflight areas, emissions from heating energy may increase slightly as the U-value of the roof lights is unlikely to match that of the surrounding roof.
However, when lighting energy is taken into account as well as heating, the picture changes very significantly. You can see an example of this in NARM Technical Document NTD06.2, in which a building with 12% roof light area is shown to emit around 40% less CO2 than a building with no rooflights. You’ll also find a detailed explanation of the graph, which illustrates the point clearly.
In fact, in many buildings, Part L compliance is impossible without roof lights.
One point to remember though: at larger rooflight areas, there may be a potential risk of overheating – so this should be considered in the design.