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. Everest triple glazing is 0.80 W/m2K.
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.
|Type of window||U-value|
|Old single glazing||>4.8|
|Old double glazing||>2.4|
|Double glazing + Low-e + Argon gas||1.22 – Good rating|
|Triple glazing + Low-e + Argon gas||0.80 – Excellent rating|
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:
|Standards for new thermal elements, windows and doors in existing dwellings|
Currently, replacement windows in existing houses must have a maximum U-value of 1.4 W/m2K or less.