For decades, double glazing has been regarded as the definitive answer to energy efficiency. It is not. While two panes of glass dramatically improve performance compared with single glazing, the frame itself remains a major source of heat loss. In many cases, it undermines the benefit of the glass entirely.
The fact is clear: comfort, efficiency, and regulatory compliance require more than glazing alone. Without thermal breaks, homes continue to bleed heat through their frames. Across London, elegant period properties and modern residences alike are still losing warmth into the night, driving up energy bills and eroding comfort.
Sash Windows London specialises in solving precisely this problem. With a combination of advanced glazing and thermally broken frames, they deliver windows that preserve architectural character while meeting the highest performance standards.
The Heat Loss Problem
Every winter, London households spend thousands heating the air outside. The culprit isn’t always the boiler. It’s the windows.
A single-glazed sash has a U-value of around 4.8 W/m²K. That means heat pours through it almost unhindered. Put simply, nearly half of your home’s warmth is gone before it reaches the curtains.
The result?
- Rooms that never feel truly warm.
- Energy bills that climb every year.
- A home that looks beautiful but works against you.
It isn’t only glass to blame. Frames — especially aluminium without insulation — act like radiators, drawing precious warmth straight into the cold. The problem is structural, not cosmetic.
This is the battlefield Sash Windows London understands. For two decades, they’ve measured the invisible losses in heritage terraces and modern flats alike, showing homeowners exactly how much they’re giving away. The figures never lie. And once you’ve seen them, you can’t unsee them.
Double Glazing Explained
The idea is simple but powerful: two panes of glass separated by a sealed gap. Fill that gap with argon or krypton gas, and you slow the transfer of heat dramatically.
Compared with a single-glazed sash, which leaks at roughly 4.8 W/m²K, a modern double-glazed unit performs at 1.4–1.8 W/m²K. That’s a reduction of heat loss by more than half.
The benefits are immediate:
- Rooms warm up faster and stay warm longer.
- Energy bills drop by 20–30%.
- Outside noise falls to a murmur.
But double glazing alone isn’t perfection. In older properties, it can disrupt the proportions of heritage windows. That’s where skill matters. At Sash Windows London, slimline double glazing is fitted within traditional frames, preserving the Georgian or Victorian look while delivering twenty-first-century performance.
Glass is only part of the story. Without addressing the frame, even the best double glazing leaves a weak point. And that’s where thermal breaks step in.
Thermal Breaks Explained
Glass isn’t the only culprit. Frames are often the silent thief of heat. Aluminium frames, prized for their strength and slim sightlines, are also superb conductors. In winter, they behave like radiators in reverse — carrying warmth out of the home and drawing cold in.
The solution is a thermal break. It’s a slim, invisible barrier — usually a strip of polyamide — inserted between the inner and outer parts of the frame. This non-conductive layer interrupts the flow of heat, cutting frame losses by half.
The effect is striking:
- Interiors stay warmer, even around the edges of the window.
- Condensation risk falls, protecting frames and finishes.
- Overall, U-values improve, helping homes edge closer to Passivhaus standards.
It’s the kind of detail many homeowners never hear about until after the fact. But companies that work at the premium end of the market — Sash Windows London among them — specify thermally broken frames as standard. Not because it’s a gimmick, but because without it, even the best glazing is only doing half the job.
Why One Without the Other Fails
Double glazing without a thermal break is like wearing a down jacket with the zip undone. You’ve invested in warmth, but the cold slips straight through the gap.
The same holds in reverse. A thermally broken frame with single glazing is like building a fortress but leaving the gates wide open. One component works, the other undermines it.
We’ve walked into countless homes where proud owners had installed modern double glazing, only to discover icy draughts along the frame. The disappointment is real. The money spent feels wasted. And the home never reaches the comfort it promised.
The truth is plain: efficiency isn’t about glass or frame in isolation. It’s about the system. When glazing and frames work together, the effect is transformative. That’s why Sash Windows London doesn’t offer one fix without the other. Anything less is a half measure — and half measures cost more in the long run.
The Compliance & Prestige Argument
For many homeowners, building regulations sound like a burden. In reality, they’re a shortcut to reassurance — and a signal of quality.
Part L sets the standard for thermal efficiency. Part Q demands security that deters forced entry. Part K protects occupants from accidents and injury. They aren’t hurdles to clear; they’re guarantees that the windows in your home are safe, strong, and future-proof.
Luxury buyers know this instinctively. Compliance isn’t just about passing inspection. It’s about resale value, insurance premiums, and the quiet pride of knowing your home meets — or exceeds — the toughest standards. In today’s market, regulations double as selling points.
This is where Sash Windows London stands apart. They don’t treat compliance as a tick-box exercise. They weave it into the craft. Every thermal break, every slimline glazing unit, every locking mechanism is chosen with performance and regulation in mind. The result is a window that doesn’t just meet code — it elevates the property.
When your home can be described as “Part L compliant” or “Passivhaus ready,” you’re not just keeping warm. You’re investing in prestige.
Numbers That Sell
Good copy doesn’t rely on adjectives. It relies on facts. And the facts about glazing and thermal breaks are impossible to ignore.
- Single glazing: ~4.8 W/m²K. Heat pours through almost unhindered.
- Early double glazing: ~2.8 W/m²K. Better, but still wasteful by modern standards.
- Modern double glazing: ~1.4–1.6 W/m²K. A leap forward — but glass is only part of the story.
- Double glazing with thermal breaks: <1.2 W/m²K. Suddenly, the whole system works as one.
- Triple glazing with thermal breaks: often below 1.0 W/m²K — Passivhaus territory.
For a family in a London townhouse, that difference can mean hundreds saved each year on heating. Across a decade, it’s thousands — enough to offset the investment many times over.
And numbers don’t just speak to the bank account. They speak to comfort. A room that holds steady warmth. A sash that no longer streams with condensation. A frame that feels solid against draughts.
This is where Sash Windows London quietly excels. They don’t quote theory. They show homeowners their current U-values, model the improvements, and let the maths do the talking. The figures sell the story better than any flourish ever could.
Take the Next Step
Every month you wait is another month of heat slipping out of your home and money draining from your account. Double glazing helps. Thermal breaks help. But together, they transform the way your home holds warmth, meets regulations, and builds value.
This isn’t theory. It’s proven engineering, applied by craftsmen who know how to respect heritage detail while delivering twenty-first-century performance.
Sash Windows London has guided hundreds of homeowners through the process — from Chelsea townhouses to Hampstead villas — always with the same result: homes that look beautiful, feel comfortable, and perform far beyond expectation.
Now it’s your turn.
Book your window assessment today. See exactly how much heat you’re losing — and how much you could save. The sooner you act, the sooner your home works for you, not against you.
