One of my good friends has the most stunning roof lantern in her kitchen extension – yet, she freely admits that, on a warm summer's day, she regrets ever having it installed. It makes the entire space uncomfortably hot to spend time in – and that's before any cooking begins.
With a kitchen remodel on the horizon, I love the idea of fitting a roof lantern into my new space as a way of bringing in more light, and creating a striking focal point – but I am really worried that doing so is just going to make the space unusable on a hot day.
Before ruling out the idea altogether, I decided to speak to some experts to get their opinion on the matter – and I was pleasantly surprised by what they had to say.
Do roof lanterns always cause overheating?
Andrew Madge, director at Vacuum Glazing UK, explains that, when designed and installed correctly, overheating from rooflights need not be an issue. "The aim should be a room that feels bright, comfortable and usable throughout the year. A good roof lantern should bring in the sky, not the full force of a heatwave.
"The glass needs proper thought because there is a big difference between a room filled with natural light and one that overheats every time the sun comes out," he continues. "You want daylight without discomfort, not a beautiful new kitchen extension that feels like a greenhouse with a dining table in the middle of July."
Edward Stobart, technical sales manager at IDSystems, agrees that overheating should not be something that just gets accepted as part of fitting a roof lantern. The key, he says, lies in getting the design right from the start. "Incorporating a roof lantern without creating overheating issues requires careful design from the outset."
Good news then, so what are the secrets to successful roof lantern installation?
Andrew Madge is Managing Director of Gowercroft Joinery and Director of Vacuum Glazing UK, the exclusive UK importer of LandVac vacuum glazing. Andrew has been involved in vacuum glazing since 2013, long before it became widely discussed in the UK retrofit market. At Gowercroft Joinery, he helped develop the UK’s first vacuum glazed replacement window range, created for heritage and retrofit projects needing a careful balance of traditional appearance and modern performance. Passionate about product development and innovation, Andrew has spent years researching vacuum glazing technologies, testing how different products perform and identifying the best solution for the UK market. Andrew is also President of the British Woodworking Federation.
A long serving member of the IDSystems team, for the past 20 years Edward has advised and supported self-builders and renovators to identify the most suitable glazing options for their project. He has recently finished a complete renovation and remodel of his own home, transforming a tired 1960s house into a stylish modern family home. His expertise ensures that each project is equipped with cutting-edge, bespoke glazing designs that enhance both functionality and aesthetics.
Start by thinking about G-values
While I understand the importance of looking at U-values when it comes to thermal performance, what I hadn't realised was that, in order to get a roof lantern that performs the way I want it to, I need to brush up on my G-values.
"Even though U-values are the most widely shared performance indicator for windows, when you are looking to keep your home cool, you need to be looking for G-values instead," explains Andrew Madge.
"A G-value tells you how much heat from the sun passes through the glass. The lower the number, the less solar heat enters the room. That is what helps stop a roof lantern from turning a bright space into an overheated one.
"That said, the answer is not simply to make the glass darker," points out Andrew. "People choose roof lanterns because they want natural light, so the skill is reducing unwanted heat while keeping the bright, airy feel."
Vacuum glazing is a wise choice
There have been so many glazing innovations in recent years that it can be hard to keep up with what to use in each type of application.
According to Andrew Madge, choosing rooflight units with vacuum glazing is a great idea. "Vacuum glazing can be a very good option for roof lanterns because it offers strong insulation in a very slim unit. Instead of filling the gap between the panes with gas, like many double or triple glazed units, vacuum glazing removes the air from a tiny cavity between the glass panes," he explains.
"Microscopic support pillars hold the panes apart, while the vacuum helps slow heat transfer. It works on a similar principle to a vacuum flask. That matters for roof lanterns because gas-filled units can behave differently when installed on a slope or horizontally," continues Andrew.
"Warm gas rises and cooler gas falls, which creates movement inside the cavity. In angled glazing, that movement can reduce the insulating benefit of the gas. Vacuum glazing doesn't have that problem because there is no gas moving around inside the cavity. There is nothing to rise, fall or circulate," he says.
"That means it can offer consistent performance whether the glass is installed vertically, horizontally or on an angle. Our glazing, for example, has centre pane U-values of 0.34 W/m²K – the lowest that we are aware of with any glass. The lower it is, the better it is at keeping heat inside, meaning your room will also stay cosy in the cold seasons," he notes.
Solar control glass can make a huge difference
Both experts I spoke to agreed that solar control glass, when used in a roof lantern, can make a huge difference and here's why.
"Opting for glazing with a solar control coating is an absolute must because it reduces the solar gain from direct sunlight," explains Edward Stobart. "The microscopic coating works by reflecting and absorbing the infrared heat while still allowing light to pass through."
"For larger, south-facing or exposed roof lanterns, I would usually look at solar control glass," agrees Andrew Madge. "This uses a more targeted Low-E coating to reduce solar heat gain while keeping useful daylight. With our solar control options, the G-value can be reduced to around 0.17 to 0.33, depending on the glazing set-up.
"That means far less heat entering from above during hotter periods," he explains. "It can also block up to 99% of UV rays, which helps protect flooring, furniture, artwork and fabrics from fading. That is especially useful in rooms with a lot of direct sunlight."
Tailor your roof lantern to the room it is being fitted in
As with all types of windows, the room that your roof lantern is being fitted in needs to lead the specification – in short, what works for one space could be all wrong for another.
"The best specification depends on the room," says Andrew Madge. "A small, north-facing roof lantern may benefit from maximising light and insulation. A large roof lantern over a south-facing kitchen or open-plan kitchen space will usually need stronger solar control."
"The proportions of light transmittance can be customised to meet the exact requirements of your home," adds Edward Stobart. "Roof lanterns installed in larger sizes or on south-facing elevations with no shade from trees may be better suited to glass with a lower transmittance, while areas with lower sun-exposure will likely benefit from glass that maximises the amount of light let in."
Fit a roof lantern with built-in ventilation
While opening windows is not always the best way to keep a house cool in summer, there are instances where having the ability to open up your roof lantern to allow a cool breeze in can be a good idea.
"When it comes to preventing overheating, it is important to consider whether a roof lantern with an opening vent may be a good idea," says Edward Stobart. "The opening vents allow warm air from inside the home to escape, as well as aiding natural ventilation.
"The opening vents are normally controlled electronically and can even be programmed to automatically open when the room below gets too warm – with a built-in rain sensor that closes again should the weather change," he explains.
Andrew Madge is keen to point out that shading – both inside and out – can help too. "External shading can reduce heat before it reaches the glass. Blinds can help with glare and comfort, but they should not be used to rescue the wrong glass choice. If the blinds need to stay shut all summer, the roof lantern is not doing its job properly."
FAQs
Should roof lanterns have Low-E glass?
When shopping around for roof lanterns or rooflights you may also hear the term 'Low-E' glass – something you could benefit from in the fight against overheating.
"Low-E stands for Low-Emissivity," explains Andrew Madge. "It is a microscopic metallic coating built directly into the glass, designed to control heat movement without making the glass look heavy or dark.
"Our standard Low-E coating gives 76% visible light transmittance," continues Andrew. "In plain English, it still lets lots of daylight through, so you keep the main benefit of the roof lantern. It also has a G-value of 0.58, which can work well for smaller, north-facing or shaded roof lanterns where overheating is less likely."
There are many different types of ceiling glazing and incorporating it into your home's design can be a great way to pull natural light in where standard windows won't work. Even small areas of ceiling glazing can have a transformative effect on previously gloomy spaces. Or, if you're considering a greater expanse of glass, take a look at these glass roof extension ideas for more inspiration.
