The increasing focus on sustainability across all areas of the construction industry, has led to a raft of marketing campaigns featuring product performance metrics and environmental credentials. The majority of these are factual and backed by testing and independent approvals and ratings. However, there are unscrupulous suppliers making false or exaggerated claims and others presenting confusing messages which blur the facts. Rooflight U-values are a case in point.

U-values (sometimes referred to as heat transfer coefficients or thermal transmittances) are used to measure how effective elements of a building’s fabric are in providing thermal insulation. In other words, how effective they are at preventing heat from transmitting between the inside and the outside of a building. U-values are measured in watts per square metre per Kelvin (W/m²K). For example, a rooflight with a U-value of 2.2, for every degree difference in temperature between the inside and outside of the window, 2.2 watts will be transmitted every square metre. The lower the U-value of an element of a building’s fabric, such as a rooflight, the more slowly heat is able to transmit through it, and so the better it performs as an insulator.

However, it is vital to note that thermal considerations are not the only considerations for overall energy consumption, and lighting energy demand is often the primary consideration. Use of daylight through rooflights and windows can dramatically reduce overall energy use, even if the glazing is less well insulated than opaque areas, because the saving in lighting energy is generally much greater than any increase in heating energy.

Independent research by De Montfort University has proved conclusively that compliant rooflights can save energy in many applications, and generally, the greater the rooflight area the greater the potential savings. This is reflected in Building Regulations, which include limits on overall energy use, based on calculations which reflect the benefits that increased areas of rooflights can offer. Nonetheless, whilst rooflight areas and light transmission remain the prime consideration, thermal insulation of rooflights is an important secondary consideration, and limiting U-values for rooflights are also included within Building Regulations

Generally, the better insulated the rooflight the better, although there is a law of diminishing returns. In some cases it may even be counter-productive to pursue very low values: if, for example, an excellent U-value is achieved by adding more layers of glazing, each additional layer reduces overall light transmission, and can reach a point where the resultant increase in lighting energy outweighs the savings in heating energy.

Care should also be taken to ensure rooflight areas are not so large that they create a potential for unwanted solar overheating.

Requirements of The Building Regulations

For all non-domestic applications, the worst acceptable standard for thermal performance (U-value) of rooflights in new build work is currently stated as 2.2 W/m²K. For refurbishment or domestic applications, this figure is reduced to 1.8 W/m²K.

U-values for flat or ‘in-plane’ rooflights

As the U-value is calculated by dividing heat loss through the rooflight by its area, this is straightforward for flat, ‘in-plane’ rooflights, the type most frequently installed on metal or fibre cement roofs on factories, warehouses and retail ‘sheds’. Therefore, manufacturers’ figures can generally be used to form the basis of calculations.

U-values for ‘out-of plane’ rooflights and roof windows

Many rooflights are ‘out-of-plane’ designs that sit proud of the roof. These include modular dome or pyramid rooflights, barrel vaults and glazing bar systems. Furthermore, rooflights may be mounted onto upstands, which can effectively be considered as part of the roof, or some rooflights (particularly dome and pyramid modular rooflights) can be supplied as an assembly with a pre-manufactured kerb. Building Regulations state that the worst acceptable U-values for rooflights are based on the developed area of the rooflight (not the area of the roof aperture, which is the true U-value). This is termed the U_d-value, and can be calculated for either a rooflight alone, or for a rooflight & kerb assembly.

U_d-values: the correct values for checking against limiting values in The Building Regulations

The U_d-value is calculated from the developed area of the rooflight. Where a rooflight-and-kerb assembly is being supplied, the rooflight supplier should be able to quote this value both for the rooflight only, AND for the entire assembly. To ensure Building Regulations compliance, both of these values must achieve the worst acceptable standard of 2.2 W/m²K for refurbishment or domestic applications, or 1.8 W/m²K for domestic applications. It is not acceptable to use an assembly of a rooflight with poorer thermal performance (such as double skin rooflights) on a kerb simply because the U_d-value for the rooflight-and-kerb assembly is better than the limiting values in the Building Regulations, unless the U_d-value for the rooflight alone also meets the limiting values.

Further articles and technical documents on rooflight specification are available to download from: www.narm.org.uk