The rising cost of energy prices and the recent introduction of more stringent building regulations (Part L) have pushed thermal efficiency to the top of the priority list for self-builders and renovators considering new glazing solutions for their homes.
These changes to energy requirements can be confusing, particularly for those trying to work out the best option for rooflights, roof windows, and skylights.
Peter Daniel of The Rooflight Company, explains the new regulations and gives his advice to self-builders considering the most thermally efficient option for their projects.
Rooflights and skylights are exceptional at letting natural light into the home and making the most of the living space inside but, as with all glazing systems, it’s important to choose the most thermally efficient option to avoid heat transfer from the inside outward, and vice versa.
A thermally efficient rooflight will help maintain a cool room in the summer and a warm one in the winter, while one with poor credentials will result in increased heating costs and have a negative impact on the environment.
Understanding u-values
The amount of heat lost through different materials is calculated using U-values. These tell you how energy efficient a product is so you can decide what is best for your project.
When it comes to glazing products, the thickness, thermal conductivity, and thermal resistance of the glass and the frame combined will determine how well it regulates heat. U-values are given in watts per square metre per Kelvin, or W/m²K, to show how much energy is lost. The lower the U-value, the more energy efficient the rooflight. For example, a rooflight with a U-value of 1.4 W/m²K will transmit 1.4 watts every square metre for every degree difference in temperature between the sides of the glazing (inside vs outside).
While this seems simple, there is one important pitfall to avoid. Two U-values are calculated for windows, including rooflights: the whole unit U-value and the centre pane U-value. Some manufacturers only publish the latter figure, which measures the thermal performance of the glazing only – a lower value. This is misleading as it does not consider the frame or edge detailing. By not considering the whole window U-value, renovators and self-builders could find themselves investing in a rooflight they believe is a superior product, but it doesn’t actually meet the current building standards.
The Future Homes Standard
In June 2022, the government updated Part L of the building regulations, as part of its strategy to make all new homes carbon neutral by 2025. These latest changes include new requirements for U-values.
Firstly, for flat roof products, U-values must now be declared on the horizontal axis, or horizonal plane, instead of a vertical axis, or plane.
Flat rooflights and pitched rooflights also have different U-value requirements. In England, flat rooflights installed either in new build projects or as part of renovation work on existing homes must meet, or fall below, a U-value of 2.2 W/m²K.
Pitched roof windows installed into existing homes must meet or exceed U-value standards of 1.4W/m²K, while the limit for new builds is 1.6W/m²K.
Conservation rooflights can help renovators achieve efficiency targets while retaining traditional features. For example, the Conservation Rooflight from The Rooflight Company has been specified by English Heritage and the National Trust due to its mix of modern design and traditional aesthetic. The Victorian style rooflight has an impressive whole window U-value of 1.4 W/m²K.
By choosing rooflights with the lowest whole window U-value, self-builders and renovators can confidently make their homes more sustainable for the future, while creating comfortable living spaces with a more stable temperature, to achieve substantial savings on energy bills over time.
Peter Daniel is the innovation director at The Rooflight Company