What is a U-Value (why do they matter for windows?)

U-values are a measurement for heat transfer applied to how building components conduct or transfer heat.

A U-value is calculated from the rate of energy transfer through one square meter of the structure divided by the difference in temperature at either side of the structure.

The value is quoted as watts per square meter per Kelvin (1°C = 1K) – W/m²K.

For a double glazed window with a U-value of 1.4, this means that 1.4 watts will be transmitted per square meter for every degree of temperature difference between the inside and outside of the window.

A higher U-value means there is more heat transfer and a low U-value is less heat transfer which means a higher insulated structure.

When applied to building components such as windows, the lower the U-value, the greater the energy efficiency. Choosing windows with low U-values will also help to reduce your carbon footprint.

U-values are used to measure different parts of a building structure including:

• Roof
• Walls
• Floor
• Doors
• Windows

Data source

Window U-values

For windows, there are two different U-values and it's important to compare like-for-like values:

• Ug measures just the glass (centre pane)
• Uw measures the whole window (glass and frame)

The Ug value for just the glass will always be lower than for the whole window. The whole window includes the frame, seals and spacer bar, and the thermal transfer rate of the frame and spacer bar is usually much higher than the glass alone.

Another thing to consider is that a U-value will actually increase (be less energy efficient) as the window size becomes smaller. Glazing is more energy efficient than most frame materials, so the more glass in a frame will reduce the ratio of the frame to window size; therefore there is less heat loss ratio for the window overall.
The factors that influence the U-value of a window are:

• Type of glass used (Low-e coated, laminated, standard float, etc.)
• The gas used between the panes (Argon etc.)
• The distance between the panes
• The thermal properties of the spacer bar
• Materials used in the frame
• The number of panes of glass used

What Is a Good U-value?

A good U-value would be anything that meets Building Regulations or lower – currently set at 1.4 for existing dwellings (see below).

European window manufacturers put more emphasis on U-values and have the lowest values of 0.80 on the market compared to the UK.

It is anticipated that before Future Homes Standard comes into effect in 2025, the U-value for windows will reduce and the standard for new homes could drop below 1.0 W/m2K.

Data source & Everest Data

Why U-values Are Important

The industry standard Window Energy Rating (WER) was introduced to help consumers compare the energy efficiency of different products.

The rainbow stickers are an easy-to-read rating that aligns with the stickers you see on large electrical appliances such as fridges and washing machines.

However, WER includes the G-value in its rating calculation. G-values measure the amount of solar gain captured through the glass and a coated glass or three panes of triple glazing will reduce the amount of light transmittance and make the G-value less efficient.

Different needs have specific requirements. For example, in cold rooms with no direct sunlight, reducing heat loss is more important than capturing solar gain. On the reverse, rooms that have direct sunlight in summer, need to reduce the solar gain to stop overheating.

It’s better to look at individual values separately for different needs. In the UK, the predominant need is for energy efficiency by reducing heat loss; therefore, the U-value is the critical value to consider.

The benefit of houses built with low U-value components are:

• Less heat loss from a room with a direct impact on room comfort
• Less energy use and saving money on energy bills
• Prevents surface mould growth by increased indoor surface temperature
• Healthy buildings with improved indoor thermal climates

Building Regulations for U-values 2023

Building Regulations 2023 edition, Approved Document L has been updated to cover limiting heat gains and losses. The new rules related to the standards for fabric performance of building elements came into effect in June 2022 and cover minimum U-value requirements as follows:

Data source

Currently, replacement windows in existing houses must have a maximum U-value of 1.4 W/m2K or less.

Everest Window U-values

At Everest, energy efficiency is at the core of what we do.

Everest triple glazing is a high performing glazing unit that is ultra energy efficient. Our double glazed windows also have low U-values. Choosing energy-efficient Everest windows can help lower your energy bills and reduce your carbon footprint.

*U-value White/White Woodgrain and Cream.

**U-value Dark Woodgrain, Golden Oak, Anthracite grey, Sage Green.