Korniche Roof Lantern U-Value Explained: 1.2 W/m²K and How It Compares

The Korniche roof lantern delivers a centre-pane U-value of 1.0 W/m²K and an overall whole-product U-value of 1.2-1.4 W/m²K in its standard double-glazed specification (the larger the lantern the better the overall figure, because the glass-to-frame ratio is higher). That puts Korniche among the best-performing aluminium roof lanterns on the UK market in 2026. For homeowners specifying a roof lantern over a kitchen extension or open-plan living space, the U-value matters because the rooflight is one of the largest single sources of heat loss in the room. A poorly-specified rooflight at 1.8 W/m²K vs Korniche's 1.2-1.4 W/m²K is the difference between a comfortable winter kitchen and one that needs the heating turned up.

This guide explains what these U-value numbers actually mean in practice, how Korniche compares to other rooflights, and when (if ever) to upgrade to the triple-glazed Korniche Plus.

What U-Value Measures

U-value is the rate at which heat passes through a building element, measured in watts per square metre per degree Kelvin (W/m²K). Lower is better. A U-value of 1.2 means that one watt of heat is lost per square metre per degree of temperature difference between inside and outside.

UK Building Regulations Approved Document L 2022 sets a minimum U-value for replacement rooflights of 1.4 W/m²K. The Korniche standard specification beats this requirement out of the box.

For context, common building elements:

• Solid wall (uninsulated): ~2.0 W/m²K
• Single glazing: ~5.6 W/m²K
• Standard mass-market rooflight: typically 1.6-2.0 W/m²K
• Building Regulations minimum (replacement rooflight): 1.4 W/m²K
• Korniche standard double-glazed (overall): 1.2-1.4 W/m²K, with centre-pane 1.0 W/m²K
• Korniche triple-glazed (overall): 0.8-1.1 W/m²K, with centre-pane 0.7 W/m²K
• Passivhaus-grade rooflight: 0.8 W/m²K and below

For the underlying physics see What Is a U-Value: Windows Explained. For a deeper Korniche product review see Korniche Roof Lantern Review. That post covers the wider product (frame, glazing options, Korniche Plus triple-glazed SKU, longevity, project gallery). This post is a U-value-only deep dive.

Why Korniche Achieves 1.2 W/m²K

Three engineering choices combine to deliver Korniche's headline U-value:

1. Thermally broken aluminium structure

The Korniche frame and ridge are manufactured from extruded aluminium with a polyamide thermal break. This separates the inner and outer aluminium sections so heat does not conduct directly through the metal. The thermal break is the single biggest contributor to the whole-product U-value.

2. 40mm internal ridge — the slimmest in the UK

Korniche's 40mm internal ridge is the slimmest aluminium roof lantern ridge on the UK market. The thinner the ridge, the less aluminium running across the top of the lantern, and the smaller the heat-bridge path. Wider ridges (60-80mm on competitor systems) lose materially more heat at the apex. For more on the ridge specifically see our Korniche brand hub and Slim vs Standard Roof Lanterns.

3. High-spec double-glazed units as standard

The standard glazing specification is 28mm double-glazed units with:

• Soft-coat low-emissivity coating
• Argon gas fill
• Warm-edge spacer bar
• Toughened safety glass

This is meaningfully above the budget rooflight specification (typically 24mm units, hard-coat low-E, air fill) and explains why Korniche outperforms the standard rooflight market on U-value out of the box.

Where Korniche Sits in the UK Roof Lantern Market

Korniche's 1.2-1.4 W/m²K overall U-value places it in the top tier of the UK aluminium roof lantern market. Premium aluminium competitors with comparable positioning (such as Atlas) sit in a similar band. Mid-range and budget rooflights typically publish overall U-values from 1.4 W/m²K up to 2.0 W/m²K. Specialist Passivhaus rooflights can reach below 0.8 W/m²K but at a price step well above the residential aluminium market.

The gap matters most on extension rooflights specified at 3m × 2m or larger, where the rooflight area is significant relative to the room volume. For a head-to-head review against Atlas, Stratus and Skypod see Korniche Roof Lantern Review.

Should You Upgrade to Triple Glazing?

Korniche's triple-glazed option (Korniche Plus) drops the centre-pane U-value to 0.7 W/m²K and the overall U-value to 0.8-1.1 W/m²K. The upgrade is worth considering in three scenarios:

Scenario 1: Passivhaus or Future Homes Standard project

If the wider project targets Passivhaus certification or Future Homes Standard 2026 compliance, the rooflight needs to contribute to the building's overall thermal performance. Triple glazing on the Korniche is the right specification.

Scenario 2: Large rooflight area on north-facing aspect

A 4m × 3m Korniche on a north-facing kitchen extension is losing heat without solar gain to compensate. The 1.0 W/m²K vs 1.2 W/m²K upgrade is meaningful here.

Scenario 3: Very high-spec acoustic requirement

Triple glazing reduces noise transmission alongside heat loss. On properties under flight paths or close to motorways the acoustic improvement is the main reason to specify the upgrade.

For most standard residential extensions, the standard double-glazed Korniche at 1.2-1.4 W/m²K overall is the right specification. It comfortably beats Building Regulations, and the cost of upgrading to triple glazing is rarely recovered through energy savings alone in a 30-year service life.

Solar Control Glass — A Different Decision

U-value measures heat loss in winter. Solar control glass is a separate consideration that addresses solar gain in summer. Korniche offers two solar control options:

• Blue tinted solar control — reduces solar heat gain materially, with a blue tint visible from inside
• Neutral solar control — reduces solar heat gain with no visible colour change

South-facing or west-facing kitchen extensions usually benefit from solar control glass even on the standard Korniche specification. The blue tint is more effective on heat rejection but visible from inside; the neutral option is the right call where the visual purity of the lantern matters. Specific g-values for both options are confirmed at quote stage.

Pilkington Activ Self-Cleaning Glass

Korniche supports an upgrade to Pilkington Activ self-cleaning glass on the outer pane. The titanium dioxide coating reacts with daylight to break down organic dirt, which then washes away in the rain. On a roof lantern that is rarely accessible for manual cleaning, this is a meaningful long-term advantage.

The upgrade does not affect U-value materially (within the manufacturer's tolerance band).

What the U-Value Means in Practice

For a typical Korniche specification — 3m × 2m, standard double glazing, on a kitchen extension — the U-value translates roughly as follows:

• Heat loss at 0°C outside, 21°C inside: approximately 150 watts continuous through the rooflight (calculated from 1.2 W/m²K × 6 m² × 21K)
• Annual heating contribution of the rooflight: depends on the heating system, insulation elsewhere, and occupancy patterns, but typically modest in absolute terms

The point is not the running cost (which is small relative to the total heating bill), but comfort. A 1.2-1.4 W/m²K rooflight does not produce a cold zone underneath it the way a 2.0 W/m²K rooflight does. The room feels evenly warmed in winter rather than cold near the lantern.

Getting the U-Value Right at Survey

A few practical points the Vitrum Solutions survey team checks at every Korniche installation:

• Aperture preparation — the structural opening must be lined and insulated correctly so the U-value of the surrounding roof matches the rooflight tier. A 1.2 W/m²K Korniche bolted into uninsulated joists is wasted spec.
• Air-tightness sealing — the gasket between the lantern frame and the kerb must be continuous. Air leakage is invisible on the spec sheet but significant in practice.
• Cold bridging at the kerb — the timber or upstand kerb the lantern sits on needs its own thermal break detail. Easy to miss on first-time builds.

These details are not optional extras. They are the difference between the Korniche performing at its rated U-value in the real building and underperforming silently for 30 years. Every Korniche installation Vitrum Solutions completes is FENSA registered (Building Regulations compliant) with a 10-year CPA insurance-backed guarantee on the workmanship.

Frequently Asked Questions

What is the U-value of a standard Korniche roof lantern?

The standard double-glazed Korniche reaches a centre-pane U-value of 1.0 W/m²K and an overall whole-product U-value of 1.2-1.4 W/m²K depending on size (larger lanterns achieve a better overall U-value because the glass-to-frame ratio is higher). This is meaningfully better than the Approved Document L minimum of 1.4 W/m²K for replacement rooflights.

Can Korniche reach lower U-values for high-performance projects?

Yes. Korniche's triple-glazed option (Korniche Plus) drops the centre-pane U-value to 0.7 W/m²K and the overall U-value to 0.8-1.1 W/m²K. This puts triple-glazed Korniche at or near Passivhaus-grade thermal performance for the rooflight element of the building envelope.

Is the Korniche U-value good enough for Building Regulations?

Yes. The standard double-glazed Korniche overall U-value of 1.2-1.4 W/m²K meets the Approved Document L minimum of 1.4 W/m²K for replacement rooflights. Vitrum Solutions handles FENSA registration on every Korniche installation we complete, so Building Regulations compliance is documented automatically.

How does Korniche compare to a standard mass-market rooflight on U-value?

Korniche's overall U-value of 1.2-1.4 W/m²K outperforms a standard mass-market rooflight (typically 1.6-2.0 W/m²K). On a 3m × 2m extension rooflight, this is the difference between a comfortable kitchen in winter and one that needs supplementary heating under the lantern.

Should I upgrade my Korniche to triple glazing for the U-value gain?

For most residential extensions the standard double-glazed Korniche at 1.2-1.4 W/m²K overall is the right specification. Upgrade to triple-glazed Korniche Plus if the wider project targets Passivhaus or Future Homes Standard 2026 compliance, if the rooflight area is large on a north-facing aspect, or if acoustic performance is a priority.

Does Korniche U-value performance degrade over time?

The aluminium frame and thermal break are dimensionally stable and do not degrade. The argon fill in the double-glazed units depletes slowly over time (typically 1% per year). Korniche's manufacturer warranty covers the sealed glass units against seal failure for 10 years.

What is the difference between U-value and energy rating on a roof lantern?

U-value measures heat loss in W/m²K and is the universal building-regulations metric. Energy rating (A++, A, B etc) is a UK consumer-facing rating that combines U-value, solar gain and air leakage into a single letter grade. For roof lanterns the U-value is the more reliable comparison metric because the energy rating system was designed for vertical windows and translates imperfectly to rooflights.

Getting a Quote

Vitrum Solutions installs Korniche roof lanterns across Buckinghamshire, Berkshire, Oxfordshire, Surrey, Hampshire, Hertfordshire and West London. Request a free quote and we will specify the right Korniche tier for your project (standard double, triple-glazed, solar control, self-cleaning) based on aperture size, orientation and the wider thermal envelope.

For more on Korniche specifically see our Korniche brand hub and Korniche Slim Ridge child URL. For roof-lantern context across the UK market see Slim Roof Lanterns vs Standard and Korniche Roof Lantern Review.