What makes a building energy efficient?
When choosing products for a building project, it’s easy to concentrate on each item individually – costs, performance, availability – and lose sight of the bigger picture.
Throughout the UK, the sections of each country’s building regulations that might be termed ‘thermal regulations’ don’t just deal with heat loss through the building envelope. They aim to reduce the overall energy demand of the property and limit its impact on the environment through reduced carbon dioxide emissions.
Heat loss can be through the transfer of heat energy (expressed as figures known as U-values) through the floors, walls, roofs, doors and windows. It also occurs by warm air leaking from the inside of the building. Both are a significant contributor to building performance, but must be taken into account with the heating and hot water systems, ventilation and artificial lighting.
What U-values do rooflights achieve?
It’s all about getting a balanced and ‘whole house’ approach to the design and specification, whether the project is a new-build dwelling, or an extension to/refurbishment of an existing home. The role of rooflights in helping to achieve that balance is worth exploring.
In a high performance building specification, the roof structure might have a U-value as low as 0.15 W/m2K or 0.13 W/m2K. It’s not uncommon for a U-value of 1.4 W/m2K to be expected from glazing, and for a U-value of 1.0 W/m2K to be perceived as very low. Given the disparity in performance, the temptation might be to keep the inclusion of roof glazing to a minimum.
Designing a healthy building, however, means thinking beyond the headline thermal performance figures.
How important is natural light?
Rooflights provide up to three times more light than the same area of vertical glazing – so it’s no surprise they’re a popular feature. Natural light improves the quality of our living and working spaces, creating a pleasing, stimulating and more productive environment, and happier people.
There is a limit, however! Allowing too much light into the building can result in glare and, because of solar gains, overheating of the internal space. A reasonable starting point would be to aim for a rooflight area of around 15 to 20% of the floor area.
How do rooflights improve energy efficiency?
Although the U-values quoted for roofs and rooflights seem quite drastically different, the relatively small areas of roof glazing compared to the rest of the building fabric mean the small increase in heating demand is more than offset by solar heat gain and reduced use of artificial lighting.
The larger the area of roof glazing, the greater the number of hours each year that illumination of the building’s interior will be provided by natural light – and the less time that artificial lighting will need to be switched on.
Optimising the area of roof glazing to reduce artificial lighting demand without causing overheating has a massive impact on a building’s energy use and carbon dioxide emissions; the task of the designer is to find the sweet spot of the right amount of glazing.