Buy Rooflights Online UK: Understanding U-values, G-values & Light Transmittance
Ready to buy a rooflight but confused by terms like U-value and G-value? You're not alone.
This guide will help you understand the key performance indicators to look out for when comparing rooflights for sale in the UK. We’ll explain what these terms mean in simple language, and how they affect your comfort, energy bills, and natural light – so you can make a confident, informed purchase.
What Is a Rooflight U-value and Why Does It Matter?
The U-value measures how well a rooflight prevents heat from escaping your home. The lower the U-value, the better the insulation.
In the UK, a low U-value helps keep your home warm in winter and reduces energy bills. It also ensures compliance with UK Building Regulations (specifically Part L).
- Low U-value (1.0 W/m²K or less) = Excellent insulation
- High U-value (2.0 W/m²K or more) = Poor insulation
All our rooflights meet or exceed UK insulation standards. See our triple-glazed rooflights with ultra-low U-values here.
What Is a Rooflight G-value?
The G-value (also known as solar factor) tells you how much solar heat enters through the rooflight as a percentage of the sun’s energy.
In simple terms: a higher G-value lets in more heat from the sun. This can help warm up rooms naturally in winter – but may lead to overheating in summer if not controlled.
- High G-value (e.g. 0.60+) = More solar gain, great for cold spaces
- Low G-value (e.g. 0.30–0.40) = Less heat gain, ideal for south-facing or glazed rooms
Many of our rooflights offer optional solar control coatings to balance heat and comfort. Browse solar-control rooflights with next-day delivery.
What Does Light Transmittance Mean in Rooflights?
Light transmittance refers to how much visible natural light passes through the glass, measured as a percentage.
For example, a rooflight with 70% light transmittance allows 70% of sunlight into your room. This affects how bright your space feels during the day.
- 70–80% transmittance = Brighter interiors, more natural daylight
- Below 50% = More shaded, ideal if glare is a concern
How Do U-values, G-values, and Light Transmittance Work Together?
These three values help you balance comfort, efficiency, and aesthetics. The best rooflight for you depends on where it’s being installed and how you use the space.
Here’s a quick guide:
- Loft bedroom? Low U-value + low G-value = energy efficient & cooler in summer
- Kitchen extension? Mid G-value + high light transmittance = bright and warm
- North-facing room? High G-value = maximise solar heat gain
Speak to our experts if you need help choosing the right performance spec for your rooflight.
Why Buy Energy-Efficient Rooflights from Skylights.co.uk?
We offer premium rooflights designed for UK homes – with performance, price, and peace of mind in mind:
- ✅ Skylights.co.uk's rooflight prices include VAT – no hidden fees
- 🛡️ Rooflight warranty UK – 10 to 20 years depending on model
- 🚚 Free delivery on selected ranges
- 📦 Next-day delivery skylights available across mainland UK
- ⭐ Customer reviews – rated 4.9/5 by UK buyers
- 📞 UK-based support to help you pre- and post-sale
Browse all energy-efficient rooflights for sale – or contact our team for personalized help.
FAQ: Buying Energy-Efficient Rooflights in the UK
For most homes in the UK, aim for a U-value of 1.2 W/m²K or better. The lower, the more insulating. Triple glazing usually provides the best performance.
Yes – some glass types are engineered to reduce heat while still allowing lots of daylight. These are ideal for living areas or kitchens with large roof glazing.
Absolutely. All our rooflights meet or exceed Part L and Part F of UK regulations for thermal performance and ventilation.
Yes – every rooflight we sell includes a UK-backed guarantee, usually 10–20 years depending on the brand and specification.
We offer next-day delivery on many popular rooflight models across mainland UK when you order before 12:30pm. Custom sizes may take 3–6 working days.
Standard sizes can be returned within 14 days if unopened and in resaleable condition. Bespoke rooflights are non-returnable unless faulty.
Yes – we offer exclusive trade pricing for builders, architects, and property developers. You can contact our team through email or phone.
Shop with confidence and get the perfect rooflight delivered fast – with expert support every step of the way.
