Rooflights, U-Values, and Energy Efficiency: Everything you need to know
August 2022

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 Company is consistently transparent about the thermal performance and energy efficiency of our range of roof windows.

Our flat rooflights as well as 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.


What are U values?

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.


How are rooflight U-values calculated?

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 1.2 W/m²K will transmit 1.2 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.


Whole unit U-values vs Centre pane U-values

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 rooflights and skylights.


U-values for flat rooflights and pitched rooflights

Building guidelines for flat rooflights 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.


Changes in building regulations and energy efficiency requirements for 2022

In 2022, the regulations affecting the thermal performance of buildings will be updated to redefine the energy efficient standard for ‘Future Homes’. As part of the Government’s plans to update the building regulations, there will be changes to U-value requirements. For those installing rooflights in new homes or existing dwellings, it will be vital to ensure your rooflights and roof windows meet these regulations.

Good to know…

The new 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.


2022 building regulations for flat rooflights: What you need to know
Flat rooflight U-value limitations

In England, the highest permitted U-value for flat rooflights will be 1.7W/m²K on a vertical plane. When calculated horizontally, this U-value is presented as 2.2 W/m²K .

This means that flat rooflight U-values for new builds either needs to meet, or fall below 2.2 W/m²K. The same applies for any refurbishments on 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 Company meet the new standards required both vertically and horizontally, meaning there are no changes to any of our products.


2022 building regulations for pitched rooflights: What you need to know
Pitched rooflight U-value limitations

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.


Pitched rooflights in conservation areas

There are some exceptions to the above figures. This is in particular reference to listed buildings or homes in conservation areas. For circumstances where the U-value guidelines cannot be achieved due to the importance of aesthetics and character, the building regulations afford some leniency. They are instead required to have a centre pane U-value of at least 1.2 W/m²K.

You can read more about this here.


Are the Rooflight Company’s rooflights compliant with the updated building regulations?

Because energy efficiency has always been a priority in our rooflight development and design, it means that all of the rooflights we currently manufacture meet the updated regulations. For homeowners, there will be a slight change in the way in which U-values are presented once the rooflight has been purchased, but there are no major alterations to the product itself.

At the Rooflight Company, we are consistently innovating our products for maximum performance, durability and quality. One of the key factors has always been ensuring our rooflights and roof windows are energy efficient, helping to keep the cold out and heat in, in the winter, and restricting the sun’s harmful rays in the summer.

Our conservation rooflight is designed to faithfully adhere to all conservation officer, English Heritage, and National Trust requirements while maintaining a whole unit U-value of 1.4 W/m²K.

Read more about rooflight installations and historical buildings in our guide: What is a conservation rooflight?


U-values and energy efficient rooflights

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:

Naturally provides light and heat

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.

A more comfortable living space

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.

Saves money

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.

Eco-friendly

Rooflights with low U-values are exceptionally eco-friendly. They are compatible with sustainable living and housing and can significantly lower your building's overall CO2 emissions.

The Rooflight Company’s neo Advance® offers a range of aluminium rooflights that reduces energy consumption by naturally lighting and heating a room. The modern elegant style blocks heat from passing through the aluminium frame’s thermal shield design. The neo Advance achieves a whole unit value of 1.06 W/m²K in the vertical plane and 1.56 W/m²K in the horizontal plane. These contemporary rooflights are double glazed and are some of the best on the market for energy efficiency.


What are G-values?

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.


Why are G-values important?

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.

Read more about the G-values in our rooflights in our technical rooflight FAQs.


What is a good U-Value for a rooflight?

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.

Shop our full rooflight range today or head to our blog for ideas and inspiration for your perfect rooflight.