U-Values Explained: What's a Good U-Value for Windows, Glazing and Roof Lanterns? (UK 2026)

Written by the Glazing Store team: we supply roof lanterns, rooflights and doors to homeowners and builders across the UK, and we answer glass and U-value questions on 01604 347222 every working day. Last updated: 6 November 2026

TL;DR: A U-value tells you how fast heat escapes through glass or a window, in watts per square metre per degree (W/m²K). Lower is better. Old single glazing is around 5.0. A modern double-glazed window is about 1.2–1.4, and a triple-glazed window is about 0.8–1.0. Building Regulations in England (Approved Document L) set a maximum of 1.4 for replacement windows in an existing home and 1.6 for windows in a new home. Rooflights and roof lanterns have their own limit of 2.2, because they're measured lying flat, where glass loses more heat. Korniche roof lantern glass comes in two grades: 1.2 W/m²K as standard and 1.0 W/m²K in the Premium range. Both sit well inside the rules. The thing to check is that you're comparing like with like: centre-pane glass figures always look better than whole-window or whole-lantern figures.

Contents

1. What is a U-value?
2. What is a good U-value for windows?
3. What U-value do Building Regulations require?
4. Is triple glazing worth it?
5. Why do rooflights and roof lanterns have a different U-value limit?
6. What U-value is a Korniche roof lantern?
7. Which Korniche glass should I choose?
8. How else can I cut heat loss through glazing?
9. FAQs

What is a U-value?

A U-value is the rate heat passes through a building element, such as a window, a wall or a roof lantern. It's measured in watts per square metre per kelvin (W/m²K): how many watts of heat escape through one square metre for every degree of difference between inside and outside.

The lower the U-value, the better the insulation. A window with a U-value of 1.2 loses about a quarter of the heat of a single-glazed window rated 5.0, in the same conditions.

Here's a simple way to picture it. On a winter day at 0°C outside and 20°C inside, each square metre of glass at:

• 5.0 W/m²K (single glazing) loses about 100 watts, roughly a bright old-style light bulb, all day.
• 1.2 W/m²K (modern double glazing) loses about 24 watts.
• 0.8 W/m²K (good triple glazing) loses about 16 watts.

Centre-pane vs whole-window U-values

This is where most comparisons go wrong. There are three different figures you'll see quoted:

Figure	What it measures	Usually written as
Centre-pane (Ug)	The middle of the sealed glass unit only	"Glass U-value 1.0"
Whole window (Uw)	Glass, spacer bar and frame together	"Window U-value 1.4"
Whole rooflight (Ud)	The whole rooflight over its developed (outer) surface, sometimes including the kerb	"Rooflight U-value 1.6"

Frames and the edges of the glass lose more heat than the middle of the glass. That makes the whole-window figure higher than the centre-pane figure for the same product. When one supplier quotes 1.0 and another quotes 1.4, check which figure each one means before deciding which is better.

What is a good U-value for windows?

For a replacement window in an existing UK home, 1.4 W/m²K or lower is the legal minimum standard, and 1.2 or lower is good. For a new build, aim for 1.2 or better. That's the figure the government uses for windows in its "notional dwelling", the benchmark home new houses are measured against. Anything under 1.0 is excellent, and usually means triple glazing.

Glazing type	Typical whole-window U-value (W/m²K)	Typical centre-pane U-value (W/m²K)
Single glazing	4.8–5.8	about 5.0–5.8
Older double glazing (air-filled, no coating, pre-2002)	2.7–3.3	about 2.8
Modern double glazing (argon, low-E, warm edge)	1.2–1.4	1.0–1.2
Triple glazing	0.8–1.0	0.5–0.7

Typical ranges only. The exact figure depends on the frame material, the glass coating, the gas fill and the size of the window. Always ask for the certified figure for the product you're buying.

U-value or Window Energy Rating?

Windows are also sold with a Window Energy Rating (WER) from A++ to E, calculated using the Glass and Glazing Federation method and issued by schemes such as BFRC. A WER balances heat lost (U-value) against free heat gained from the sun and air leakage. Building Regulations accept either a U-value of 1.4 or WER band B or better for replacement windows. The U-value is the better number when you're comparing how warm a room will feel. The WER is better for comparing overall energy balance.

From behind the counter: the most common mix-up we hear is someone comparing a centre-pane glass figure from one quote with a whole-unit figure from another. Ask both suppliers the same question: "Is that the glass U-value or the whole-unit U-value?" Once you know, it's a fair comparison.

What U-value do Building Regulations require?

In England, the U-values for new and replacement glazing are set in Approved Document L, Volume 1: Dwellings. Windows, rooflights and doors are "controlled fittings", so a new one must meet these limits whether it goes into an extension or replaces an old one.

Element	New home (max W/m²K)	Extension or replacement in an existing home (max W/m²K)
Window (including roof windows)	1.6	1.4, or WER band B
Rooflight or roof lantern	2.2	2.2
Doors more than 60% glazed	1.6	1.4, or Doorset Energy Rating band C
Upstand (kerb) built on site, under a rooflight	0.35	0.35

Source: Approved Document L, Volume 1: Dwellings, Tables 4.1 and 4.2 (2021 edition).

A few things worth knowing:

• These are maximums, not targets. A new home must also pass an overall energy calculation, so in practice new builds use much better glazing than the limits.
• Replacing only the glass doesn't count. Putting new sealed units into an existing frame isn't a "controlled fitting", so it doesn't have to meet these values. Replacing the whole window or rooflight does.
• Extensions have a glazing cap. Using the simple (elemental) route, the total area of windows, rooflights and doors in an extension shouldn't exceed 25% of the extension's floor area, plus the area of any windows or doors the extension covers up. Go over that and you'll need a calculation showing the extension performs no worse overall.
• A 2026 edition is coming. The government has published a 2026 edition of Approved Document L, which takes effect on 24 March 2027 for most work. It keeps the same window (1.6 / 1.4) and rooflight (2.2) limits.

Wales and Scotland have their own building regulations with similar but not identical figures, so check with your local building control.

Is triple glazing worth it?

Triple glazing has a third pane and two gas-filled cavities. That takes a typical window from about 1.2–1.4 W/m²K down to 0.8–1.0. It's quieter too, and the inside pane feels warmer to the touch, which cuts condensation.

It's worth it when:

• you're building to a very high standard, such as a new build or a deep retrofit aiming beyond the regulations
• the window faces north or onto a busy road
• the rest of the house is already well insulated, so the windows are the weak spot

It's often not worth it when:

• the walls and loft are poorly insulated, because you'll save more money there first
• the window faces south, because the extra pane cuts the useful free heat and light from the sun
• the frame is heavy or the opening is large, because triple units are about 50% heavier

For roof lanterns, double glazing with a good low-E coating and argon fill is the norm. Most lantern makers use double-glazed units to keep the weight on the frame and the roof down. Korniche, for example, uses 28mm argon-filled double-glazed units.

Why do rooflights and roof lanterns have a different U-value limit?

Glass loses more heat lying flat or at a shallow angle than standing upright. Warm air rises against the glass, and convection inside the glass unit gets stronger. The same sealed unit that rates 1.2 vertically can perform noticeably worse at a 25° roof pitch.

Because of that, Building Regulations treat rooflights separately:

• Windows are rated in the vertical position.
• Rooflights are rated in the horizontal position, and over their developed surface area: the full outer area of glass and frame, which is bigger than the hole in the roof.
• The limit is 2.2 W/m²K for rooflights, against 1.4–1.6 for windows. The government's own benchmark rooflight is 1.7 W/m²K (horizontal).

So a roof lantern rated 1.6–1.8 overall is genuinely good, even though the same number would be poor for a window. Detailed rooflight U-value methods are set out in BRE's BR 443 and the National Association of Rooflight Manufacturers' technical guidance.

Trade tip: the kerb (upstand) your lantern sits on counts too. If it's built on site, Approved Document L says its U-value should be no worse than 0.35 W/m²K. Insulate the inside face of a timber kerb before plasterboarding it, or the lantern edge becomes a cold spot. That's a common cause of winter condensation at the base of a lantern.

What U-value is a Korniche roof lantern?

Every Korniche roof lantern uses 28mm argon-filled double-glazed units with a warm edge spacer bar, and every glass option is self-cleaning. The glass comes in two U-value grades: 1.2 W/m²K (standard) and 1.0 W/m²K (Premium). These are the U-values of the sealed glass unit, so they're centre-pane figures. Ask for the whole-lantern figure if your building control officer needs it for a calculation.

The frame is built to keep heat in too. Korniche describes a polymer insulation core running right through the aluminium frame as a continuous thermal break, plus thermally isolated fasteners. That keeps the cold outer aluminium apart from the warm inner face.

There are six glass options. The difference between them is how much of the sun's heat they reflect and how much daylight they let through:

Korniche glass	U-value (W/m²K)	Solar heat reflected	Light let through
Clear	1.2	31%	77%
Neutral	1.2	59%	44%
Blue	1.2	73%	32%
Premium Clear	1.0	38%	70%
Premium Aqua	1.0	82%	28%
Premium Blue	1.0	83%	23%

Source: Korniche glass options, Made for Trade. Figures are for the sealed glass unit.

All six comfortably beat the 2.2 W/m²K rooflight limit at glass level. The Premium 1.0 range loses about 17% less heat through the glass than the 1.2 range.

Which Korniche glass should I choose?

U-value is about heat escaping in winter. In a roof lantern, the bigger issue is often heat coming in in summer. That's why the solar reflection figure matters as much as the U-value.

Your lantern faces or sits...	Main risk	Our suggestion
North, or shaded by the house or trees	Room feels dark and cold	Clear 1.2 or Premium Clear 1.0, for the most light
East or west	Morning or evening sun glare	Neutral 1.2 for a balance of light and comfort
South, in full sun	Overheating in summer	Blue 1.2, Premium Aqua 1.0 or Premium Blue 1.0
Over a kitchen or living room used all year	Winter heat loss and summer heat	Premium Clear 1.0 if you want brightness, Premium Aqua 1.0 if it gets sun

Korniche's own advice is the same: if the lantern faces south with direct sun, choose a glass with higher solar reflection and lower light transmission.

You can see the prices for each glass type on the Korniche roof lanterns page. Use our roof lantern size guide to work out the right size first. If you're still choosing between brands, our complete guide to roof lanterns compares the options.

How else can I cut heat loss through glazing?

The glass is only part of it. These make a real difference whatever U-value you buy:

1. Fit it properly. Gaps at the frame, a kerb out of square or missing insulation around the opening can undo good glass. Our Korniche installation guide covers the kerb in detail.
2. Insulate the kerb and the reveal. As above, an uninsulated upstand is a cold bridge.
3. Pick warm edge spacers. They cut heat loss and condensation at the edge of the glass. Korniche uses them as standard.
4. Don't oversize. A bigger lantern lets in more light but also loses more heat. Size it to the room, not the biggest the roof will take.
5. Use blinds or shading for summer. For south-facing glass, shading stops overheating without losing the view.
6. Think about rooflights for small spaces. Over a corridor or a small bathroom, a flat roof light can be a simpler, cheaper option. Our article on skylights vs roof lanterns explains the trade-offs.

FAQs

What is a good U-value for windows?

For a replacement window in an existing home, 1.4 W/m²K or lower meets Building Regulations in England, and 1.2 or lower is good. Under 1.0 is excellent and usually means triple glazing.

What is the U-value of double glazing?

Modern double glazing with argon gas and a low-E coating is usually around 1.0–1.2 W/m²K for the glass, and about 1.2–1.4 W/m²K for the whole window including the frame. Older air-filled double glazing is around 2.8.

What is the U-value of triple glazing?

A triple-glazed window is typically 0.8–1.0 W/m²K for the whole window, and 0.5–0.7 W/m²K for the glass alone.

What U-value do Building Regulations require for windows?

Under Approved Document L, a new or replacement window in an existing home must be 1.4 W/m²K or better, or Window Energy Rating band B. In a new home the limit is 1.6 W/m²K.

What U-value does a roof lantern need?

Rooflights and roof lanterns must meet a maximum U-value of 2.2 W/m²K under Approved Document L. It's measured in the horizontal position over the lantern's full outer surface area.

Is a lower U-value always better?

For keeping heat in, yes. But in a roof lantern, summer overheating can matter more. Look at the solar heat reflection figure too, especially if the lantern faces south.

What U-value is a Korniche roof lantern?

Korniche glass is 1.2 W/m²K as standard and 1.0 W/m²K in the Premium range. Both are 28mm argon-filled, self-cleaning double-glazed units.

What is the difference between Ug and Uw?

Ug is the U-value of the centre of the glass only. Uw is the whole window, including the frame and edges. Uw is always higher, so compare like with like.

Sources: Approved Document L, Volume 1: Dwellings (2021 edition incorporating 2023 amendments, and 2026 edition); Korniche roof lantern product information; Korniche glass options (Made for Trade); Glass and Glazing Federation; BFRC.