What’s the U-Value of a Good Skylight?
By Regal Glass Group
If you’re shopping for a skylight and keep hearing the term U-value, you’re probably asking:
“What’s a good U-value for a skylight?”
Great question — and one that matters more than most people realise. A skylight’s U-value tells you how well it keeps heat inside your home. The lower the number, the better the insulation — and the more energy-efficient your home becomes.
At Company Brand Name, we often recommend our frameless fixed Regal Glass Skylights, which are triple-glazed and have some of the lowest U-values on the market. In this guide, we’ll break down what U-value means, what to aim for, and how to choose the best skylight for performance and design.
Table of Contents
💬 Questions
It’s a number that measures how much heat a skylight lets through. The lower the U-value, the better the insulation — helping you keep heat in during winter and reduce energy bills.
In the UK, anything 1.4 W/m²K or lower is considered energy-efficient. Triple-glazed skylights, like our Regal Glass Rooflights, often score 0.9 W/m²K or better — which is excellent.
Yes — a skylight with a low U-value helps reduce heat loss, which means your boiler doesn’t work as hard, and you save money in the long run.
Yes — especially the ones with triple glazing and thermally broken frames. Our frameless Regal Glass Skylights are designed with energy efficiency in mind.
Not directly — but larger skylights can increase heat loss if they’re not well insulated. That’s why it’s important to choose high-performance glazing, no matter the size.
🔍 U-Value Ratings Explained Simply
| U-Value (W/m²K) | Performance Level | What It Means in Real Life |
|---|---|---|
| ≤ 0.8 | Excellent (Passive standard) | Very well insulated, best for eco homes |
| 0.9 – 1.2 | Very Good | Typical of triple-glazed Regal Skylights |
| 1.3 – 1.4 | Acceptable (UK Building Regs) | Meets standard energy rules |
| 1.5 – 2.0 | Poor | Common with older skylights or basic models |
| 2.1 and above | Not recommended | High heat loss, cold spots, condensation risk |
💡 Tip: If you're upgrading an old skylight, you could cut your heat loss by 30–40% just by choosing one with a U-value under 1.0.
🧠 What Affects the U-Value of a Skylight?
1. Glazing Layers
The more layers of glass (double or triple glazing), the better the thermal barrier. Our Regal Glass Skylights use triple glazing as standard.
2. Gas Fills
Between glazing layers, noble gases like argon or krypton add insulation — reducing heat transfer.
3. Low-E Coating
Low-emissivity coatings reflect heat back into the room while letting daylight through — great for winter warmth.
4. Frame Design
Thermally broken aluminium frames (used in Regal Glass units) prevent cold bridging and condensation.
📐 Best Skylight Options Based on U-Value
| Skylight Type | Typical U-Value | Best Use Case |
|---|---|---|
| Regal Glass Fixed Skylight | 0.9 W/m²K | Flat roof extensions, modern builds |
| Vented Triple-Glazed Skylight | 1.1 – 1.3 W/m²K | Kitchens, bathrooms, lofts needing airflow |
| Basic Double-Glazed Rooflight | 1.5 – 2.0 W/m²K | Budget installs (not ideal for insulation) |
| Roof Lantern (Standard glazing) | 1.4 – 1.8 W/m²K | Larger rooms or high ceilings |
| Passive House Certified Skylight | ≤ 0.8 W/m²K | Ultra-low energy homes |
🛠 Design Tips for Energy-Efficient Skylight Installs
✅ Use a thermally insulated upstand or curb to prevent heat loss around the frame
✅ Position your skylight for winter sun if possible (south or southeast orientation)
✅ Avoid oversized rooflights unless they’re triple-glazed — bigger isn’t always better without insulation
✅ Seal everything properly — from flashing to the plasterboard reveal
✅ Choose self-cleaning, solar-control glass if your room gets hot in summer
💡 Bonus: Use our Skylight Layout Calculator to plan your setup — combining multiple frameless fixed rooflights while maintaining excellent U-values.
✅ Final Thoughts: What’s the Ideal U-Value to Aim For?
If you want a skylight that keeps your home warm, comfortable, and energy efficient — aim for a U-value of 1.2 W/m²K or lower. Ideally, 0.9 or better if you’re building a new extension or replacing old roof glazing.
And if you want both performance and aesthetics? Our frameless fixed Regal Glass Skylights are built with triple glazing, low-E coatings, and thermally broken frames, delivering U-values as low as 0.9 W/m²K — all with that seamless, modern look.
🛒 Shop now
🌡️ Looking for a skylight that keeps your home warm and efficient?
Explore our Frameless Flat Roof Regal Glass Skylights, designed with low U-values and sleek, modern styling. Or use our Layout Calculator to build your perfect rooflight setup.
👉 Browse Energy-Efficient Skylights
👉 Try the Skylight Layout Builder
