Rooflights are well-known for their aesthetic appearance and ability to let an abundance of natural light into a room. Another key consideration when choosing a rooflight, however, is thermal performance. The Rooflight Co is consistently transparent about the thermal performance and energy efficiency of our range of roof windows.
Our flat rooflights as well as energy efficient roof windows for pitched roofs and heritage skylights are designed with longevity and energy efficiency in mind, which we take a closer look at below. Read on for a detailed look into what U-values are, how thermal performance fits into building regulations, ways to calculate it, and more.
A U-value is the calculation used to determine the amount of heat lost through building materials. Understanding U-values can be incredibly helpful in determining just how energy efficient a rooflight is. Understanding this can help you decide which rooflight style and size are best for your home or development project. With energy efficient windows becoming more and more important to homeowners, it’s good to know that there are ways you can measure the amount of heat retention rooflights offer.
U-values for roof windows will measure how effective the whole unit is as an insulator, and are the easiest way to determine how much heat passes through your roof windows, versus how much heat is retained in your home. This is important to understand since a roof window with poor U-values will result in higher energy bills and can also impact the environment.
The perfect rooflight for your home will create the perfect balance between keeping you cool in the summer and warm in the winter.
U-values can be calculated for rooflights and roof windows by understanding the thickness, thermal conductivity, and thermal resistance of both the glazing and the frame. They are also physically measured through a “hot-box” method that simultaneously exposes either side of the rooflight to hot and cold elements.
The value resulting from thermal resistance calculations will be expressed as watts per square metre per Kelvin, or W/m²K. In this way, 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. This means that the lower the U-value, the more heat is saved or kept from transmitting through the rooflight. A higher U-value will result in a roof window that is less thermally efficient and loses a larger amount of heat.
When considering rooflight U-values, be aware of the difference between whole unit U-values and centre pane U-values. A rooflight will have both measurements. However, some manufacturers only provide the lower, centre pane U-value of their rooflights.
While this value can be useful, especially when comparing glass or glazing separately, it does not tell the full story. Centre pane values will only measure the glazing’s thermal performance. These values tend to be lower than whole unit values since they don’t take into consideration the frame or edge sealing. The Rooflight Company discloses both whole window U-values and centre pane U-values for our energy efficient skylights and rooflights.
When viewing our rooflight datasheets, you will come across “Thermal Transmittance” and a reference to Uw and Ug values.
Building guidelines for flat roof windows and pitched roof windows are found in Part L of the UK’s building regulations. They specify what whole window U-values and centre pane U-values are appropriate for different types of projects and kinds of buildings.
In 2023, the regulations affecting the thermal performance of buildings were updated to redefine the energy efficient standard for ‘Future Homes’.
As part of the Government’s plans to improve building regulations, new U-value requirements were introduced. For those installing rooflights in new homes or existing dwellings, it is vital to ensure your rooflights and roof windows meet these updated regulations.
Good to know…
The 2023 building regulations stipulate that U-values for flat roof products must be declared on the horizontal axis, instead of a vertical axis, as required previously. These are also known as the horizontal and vertical plane.
Flat rooflight U-value limitations In England, the highest permitted U-value for flat rooflights is 2.2 W/m²K when measured on the horizontal plane. This aligns with the requirements set out in Approved Document L (2021 edition with 2023 amendments).
While some references still mention 1.7 W/m²K on the vertical plane, the building regulations now require the horizontal value to be declared. The 1.7 W/m²K figure aligns with guidance found in SAP 10 Appendix R, which adjusts thermal performance based on installation angle.
This means that flat rooflight U-values for new builds either needs to meet, or fall below 2.2 W/m²K. The same applies to existing homes. In other UK nations, the U-value for new builds may differ from those in existing homes, as this has not been finalised yet.
The rooflights we design and manufacture at the Rooflight Co meet the current 2023 standards required both vertically and horizontally, meaning there are no changes to any of our products.
In England, pitched roof windows being installed into existing homes will have to meet the new whole unit U-value standards of 1.4W/m²K. Pitched skylights into new builds will have a U-value upper limit of 1.6W/m²K.
There are exceptions to the above standards for listed buildings or homes in conservation areas. In cases where these U-value requirements cannot be met due to design or heritage constraints, building regulations allow some flexibility. In these situations, a centre pane U-value of at least 1.2 W/m²K is required.
Because energy efficiency has always been a priority in our rooflight development and design, all of the rooflights we currently manufacture already meet the updated 2023 regulations. For homeowners, the main change is how U-values are presented after purchase, but the performance of the product remains unchanged.
At The Rooflight Company, we are continuously innovating our products for maximum thermal performance, durability, and quality. A key factor in this has always been ensuring our rooflights and roof windows are energy efficient—keeping heat in during winter, and limiting solar heat gain during summer.
Our Conservation Rooflight is designed to fully comply to all conservation officer, English Heritage, and National Trust standards while maintaining a whole unit U-value of 1.4 W/m²K.
Building regulations ensure that U-values are low enough to provide sufficient insulation to a building. However, achieving U-values that are lower than the upper limit will ensure a more energy efficient rooflight. This is desirable for a number of reasons:
Rooflights use the sun to naturally light and heat a room. However, if your rooflight has a higher U-value, it wouldn’t be able to retain that heat. A lower U-value will keep a room nice and warm, especially during the cold winter months.
Having an energy efficient rooflight means that you will be reaching for the radiator and the air-conditioning less frequently throughout the year. The temperature will be much more stable and you will find yourself in a much more comfortable room.
Installing energy efficient rooflights can mean you won’t need to rely as much on energy sources to warm your home, which over time can generate substantial savings on bills.
G-values are different from U-values in that they measure the amount of solar transmittance that can enter through the glass of a rooflight. This is referred to as “solar heat gain”. G-values will often range between 0.2 and 0.7. Special coatings and glazing with a “solar control” can lower solar heat gain and achieve G-values of less than 0.5.
Although a large amount of solar heat gain can be pleasant in the wintertime, it may make the summer months unbearably hot. G-values become increasingly important the larger the roof windows are. There is a bigger risk of over-heating and creating a greenhouse effect in a building with large amounts of glazing.
Good U-values for a rooflight will vary depending on budget, building regulations, and the type of building or room it will be installed into. For example, rooflights with triple glazing may have the lowest U-values, but they will also be heavier and more expensive. Gauge what is most important for your building. Double glazing is often sufficient for U-values that will ensure an energy efficient and sustainable building.
If you have any questions about what U-value is best for your building, get in touch. A friendly member of our experienced team will be happy to advise you.
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