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The Complete Guide to Low-E Glass: Technology, Types, and Performance
The Complete Guide to Low-E Glass: Technology, Types, and Performance
If you have looked for new windows or doors recently, you have almost certainly encountered the term Low-E Glass. But what exactly is it?
- 🔥 Thermal Barrier: Low-E glass uses a microscopic coating to reflect heat back into your room.
- 📉 Lower Bills: It significantly improves the energy efficiency of your windows, lowering heating costs.
- 🛠️ Two Types: “Hard Coat” (older technology) and “Soft Coat” (modern, high performance).
- 🏆 Performance: Premium soft-coat glass can achieve a Centre-Pane U-value of 1.0 W/m²K.
Short for “Low Emissivity,” Low-E glass is the industry standard for modern energy efficiency. While we have previously discussed Toughened Glass for safety and Laminated Glass for security, Low-E glass is all about insulation. It is the invisible technology that stops your expensive heating from escaping through your windows.
Table of Contents
🎯 Interactive: Find Your Perfect Glass
Answer 2 questions to get our recommendation based on your room’s orientation:
1. Which direction does the window face?
2. What is your main problem?
🏆 Recommendation: Passive Low-E (Planitherm Total+)
Why: Your room lacks natural warmth. Passive Low-E glass is designed to let “free” solar heat IN and trap it there. It creates a net energy gain for your home.
🏆 Recommendation: Solar Control (Planitherm One/4S)
Why: Your room gets too hot. Solar Control glass has a specialised coating that reflects the sun’s heat AWAY while still keeping the warmth inside during winter.
What is Low-E Glass?
Low-E glass is glass that has been treated with a microscopically thin, transparent coating designed to reflect long-wave infrared energy (or heat).
This coating is thinner than a human hair and is placed on the inner surface of the glass pane inside a double or triple-glazed unit. It acts like a thermal mirror: it lets natural light in but bounces your home’s internal heat back into the room.
How It Works: The Science
To understand Low-E, you need to understand the two types of heat energy that hit your window:
- Short-Wave Energy (Sunlight): This is the heat and light coming from the sun. Low-E glass allows this to pass through, heating your home naturally (Solar Gain).
- Long-Wave Energy (Room Heat): This is the heat generated by your radiators, body heat, or furniture. Low-E glass reflects this energy.
In winter, the coating traps the long-wave energy inside, keeping your home warm. In summer, certain types of Low-E glass can also help reflect external heat to prevent overheating.
Hard Coat vs. Soft Coat
Not all Low-E glass is created equal. Historically, the industry relied on “Hard Coat” glass, but modern technology has shifted towards “Soft Coat” for superior performance.
⚠️ Durability Note
Because modern Soft Coat glass acts like a sponge to moisture, it must be sealed inside a double or triple-glazed unit. This protects the coating from oxidising, ensuring it lasts for the lifetime of the sealed unit (typically 20+ years).
Solar Control vs. Passive Low-E
Depending on the orientation of your windows, you might need different performance characteristics.
- Passive Low-E (Solar Gain): Designed to let heat in. Ideally suited for most UK homes, particularly for North or East-facing windows, to harvest “free” solar heat and retain it.
- Solar Control Low-E (Sun Protection): Designed to reflect the sun’s heat away. Essential for large south-facing windows or conservatories to prevent the “greenhouse effect” and overheating in summer.
💡 Designing for Orientation
Even with high-performance glass, large south-facing rooms can still overheat. We recommend combining Solar Control glass with adequate ventilation (like trickle vents) or internal shading for maximum comfort during heatwaves.
Pros & Cons (The Trade-offs)
While Low-E glass is an incredible technology, it is important to have a balanced view. Here is a transparent look at the advantages and potential trade-offs.
✅ Advantage: Supreme Insulation
Reflects heat back into your home, significantly lowering heating bills and improving comfort.
✅ Advantage: UV Reduction
Blocks significant harmful UV rays compared to standard glass, helping protect furniture and carpets from fading.
❌ Trade-off: Slight Light Reduction
All coatings reduce light transmission slightly. While modern Soft Coats are very neutral, they may let in ~5-8% less light than uncoated glass.
❌ Trade-off: External Condensation
Because the glass keeps heat in so well, the outer pane stays cold. On damp mornings, harmless condensation may form on the outside.
Confused by coatings?
We can help you choose the right glass for your specific room orientation during a free survey.
Book Free SurveyAchieving the Best U-Values (1.0 vs 1.2)
The performance of glass is measured by its “U-value”—the lower the number, the better the insulation.
Standard soft-coat double glazing typically achieves a Centre-Pane U-value of 1.2 W/m²K. By adding Argon gas and a Warm Edge Spacer bar, we can improve this to 1.0 W/m²K.
⚠️ Technical Note: Glass vs. Whole Window
Please note: The figures above refer to the Centre-Pane U-value (the performance of the glass only). The Whole Window U-value (Uw) will be slightly different as it includes the frame and spacer bar performance. However, upgrading the glass is the single most effective way to improve the overall Uw score.
Frequently Asked Questions (FAQs)
How it works: The microscopic coating acts like a thermal mirror. It allows sunlight (free heat) in but reflects your radiator’s heat back into the room.
Downsides: It can slightly reduce light transmission (a tiny tint) compared to standard glass, and its high efficiency can lead to harmless external condensation on cold mornings.
No. Most houseplants grow perfectly well behind Low-E glass. While the glass reduces UV rays and some heat, it lets through the visible light spectrum (PAR) that plants require for photosynthesis.
This is a positive sign of high efficiency. Old windows lost so much heat that the outer pane was warm, evaporating moisture. New Low-E windows keep heat inside so effectively that the outer pane remains cold. On clear, still mornings, dew forms on this cold surface, just as it does on your car. It will disappear as the sun rises.
Generally, no. Modern “Soft Coat” Low-E glass oxidises if exposed to air, so it must be sealed inside a double or triple-glazed unit. Older “Hard Coat” glass can technically be used singly, but it offers significantly lower performance.
In a standard double-glazed unit, the Low-E coating is usually placed on “Face 3”. That is the inner surface of the inner pane of glass. This protects the delicate coating inside the gas cavity.
📚 Explore Our Glass Knowledge Hub
Dive deeper into the specifics:
Mark Pearce is a highly experienced specialist in the double glazing industry, having built his knowledge from the ground up through hands-on experience in installation, manufacturing, sales, and business management. Beyond his role at KJM, Mark is a keen blogger, writer, and traveller, often sharing insights on the latest innovations in the home improvement industry.
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