In terms of heat retention, the energy efficiency of any element of a building is quantified by the U-value. This is a measure of the rate at which heat energy, measured in Watts, passes through a square metre of that element for every degree Kelvin difference in temperature from inside to outside, or outside to inside for refrigerated buildings. The U-Value is usually expressed as W/m²K. The lower the U-value, the better the thermal performance.
Rooflights by their very nature must allow the passage of the maximum amount of daylight whilst inhibiting the flow of heat. The most simplest and common method is the inclusion of insulation sing multiple layers of materials with high transparency.
There are now a wide range of insulation layer specifications for in-plane rooflights available, from simple Building Regulation compliant values of 1.8W/m²K to very low U-values of around 0.8W/m²K that can still retain excellent levels of light transmission.
Independent research by De Montfort University, and published by The Rooflight Association (RA), demonstrates the savings that can be made by the introduction of rooflights, and at larger areas than might previously have been considered optimum areas even with the most modest U-value performance.
For further information on thermal transmission, looking into insulation options, condensation and how to determine surface temperature factor (f factor), you can download the section from our technical manual here.
The next chapter looks at another key specification consideration in sustainable building design; Solar Transmission (g-value).