The lowdown on U-values

Our highest-performing triple-glazed windows have U-values as low as 0.64 W/m²K. Two panes of heat-reflective Low-E glass per window allow warming infra-red rays into your home but reflect heat back into the room that would otherwise be lost to the outside.

A single-glazed window has a U-value of around 5.0, meaning it loses around 8 times more heat than our best triple-glazed window – a difference that really does matter. In most houses, 20 to 25% of heat is lost through windows and doors, so 75% of heat loss originates from other elements of a structure.

The best way to maximise performance is a holistic approach to the thermal envelope – the walls, floor, roof, windows, doors, roof windows and roof lights. That said, investing in windows with low U-values still has a drastic impact on the thermal efficiency and comfort of your home, targeting up to a quarter of all heat lost.

The International Passive House Association describes the Passive House Standard as "the only internationally recognised, performance-based energy standard in construction".

To become Passive House certified, a new building must meet strict requirements for thermal insulation, windows, ventilation heat recovery, airtightness and thermal-bridging control. The end goal is a comfortable, affordable and impressively low-energy building.

For a window to meet Passive House requirements, the entire unit – glazing and frame – should have a U-value of 0.80 W/m²K or less, a specification easily met by most of our triple-glazed range.

Triple glazing provides homeowners a marked increase – 40 to 45% – in thermal performance over double-glazed alternatives, meaning greater comfort, lower heating bills and an uplift in property value.

These windows and doors are durable, secure and aesthetically pleasing, and they are not just for Passive Houses. When choosing, take into account your whole-building thermal efficiency, budget and noise concerns to make an informed decision. High-performance double glazing can provide a useful alternative for those who feel triple glazing does not suit their project.

When you choose alu-clad timber triple-glazed windows and doors, you benefit from the charm and warmth of wood and the sleek, low-maintenance needs of aluminium. Norrsken double and triple-glazed windows are engineered for exceptional thermal performance, keeping your home comfortable and energy-efficient year round.

U-values are a measure of heat transfer through a material. They are used to evaluate the effectiveness of building components as insulators.

The more effectively insulated a structure is, the lower (and better) the U-value will be. They are measured in watts per square metre per degree Kelvin (W/m²K).

U-values are calculated by summing all the thermal resistances (the R-values) in the materials of the target component, and finding the reciprocal of that total.

Thermal resistance is a measure of a material's insulation capacity, calculated by dividing the thickness of the material by its thermal conductivity (k-value).

UK Building Regulations require windows to achieve a U-value of 1.2 W/m²K or lower. The best double glazing can achieve around 1.2 W/m²K.

Passive House windows must achieve 0.80 W/m²K or lower. Our highest-performing triple-glazed windows achieve 0.64 W/m²K for exceptional insulation, warmth and comfort.

Double-glazed windows generally achieve U-values ranging from 1.2 to 3.0 W/m²K.

Our double glazing ranges from approximately 1.24 to 1.29 W/m²K, sitting at the highest end of the double-glazing performance spectrum and well within UK Building Regulation requirements for windows.

Most triple-glazed windows have a U-value of less than 1 W/m²K.

Our triple-glazed windows range from 0.64 to 0.94 W/m²K, and several models are suitable for use in Passivhaus and ultra-low-energy projects with exceptional U-value performance of under 0.80 W/m²K. Learn all you need to know about triple glazing here.

Thermal resistance (denoted by an R-value) is a measure of the heat-insulation capacity of a material.

R-values measure how much heat is preserved across a material's thickness, whereas the U-value is how much it loses per square metre across a 1-degree temperature difference between the inside and outside surfaces.

Sometimes called the centre-pane U-value or Ug-value, the U-value of a glass package depends on several factors: whether it is double or triple-glazed, the spaces between glass panes, the insulating gas used and the thickness of the glass panes.

Because it is measured away from the edges of the window where more heat is lost, it is usually lower than the whole-window U-value.

Thermal insulation is usually considered the most important requirement in the design and building of a Passive House.

All opaque surfaces of a building must have U-values of 0.15 W/m²K or less. Passive House windows must achieve 0.80 W/m²K or less to meet the requirements.

No. U-value = 1 / (R₀ + R₁ + … + Rₙ), the reciprocal of the sum of all the requisite R-values in a component.

The R-value of each material = thickness of the material / conductivity of the material. So the U-value = thermal conductivity / thickness, averaged out across all the materials in a component.

U-values can be calculated for any building material. This allows effective, standardised comparison of all the component parts of your structure based on their thermal performance.

This in turn allows you to catch the areas where heat will be lost, eliminate poor insulators, and achieve the best overall level of thermal efficiency for your project. For windows, new builds require 1.2 W/m²K or better and replacements 1.4 W/m²K or better.