Guide to U-values and Energy Ratings for aluminium windows and doors

When building a new home or adding an extension, the look and feel of the house are naturally important – but this is also a great opportunity to make the home warmer, more comfortable and more energy efficient.

When the time comes to shop for windows and doors, it’s important to understand that not all offer the same levels of energy efficiency. When you look into it, you will find a number of different measures being used and you are likely to come across a certain amount of technical jargon. This article is a quick guide to help you make the right choices for you.

Sliding patio doors help bring the garden into this cool Swiss home


The U-value is a measure of heat transfer across a building material. It is typically used as a measure of heat loss from inside your home to the outside, and could be a measure of thermal performance for your walls, roof and floors as well as your aluminium windows and doors.

Put simply, the lower the U-value, the less energy is required to make conditions comfortable inside. Or to put it another way, the lower the U-value of the windows and doors you choose to fit, the less heat is lost and the better they will be at insulating your home.

Uf, Ug, Uw and Ud-Values

As a householder, you will be most interested in the U-value for the whole window or whole door, known as the Uw value and Ud value respectively. In each case these are calculated by combining the U-values for the window or door frame (known as the Uf value) and the U-value of the glass (known either as the Ug value or the ‘centre pane U-value’).

The important thing here is to make sure you are comparing like with like. Sometimes companies may quote the Ug value (because it can be very low). If the frame into which the glass is set is not very energy efficient, however, the U-value for the whole window or door may not be as good as you have been led to believe. As a general rule, try to compare U-values for the whole window and door.

Windows Energy Ratings

In recent years a national scheme for rating the efficiency of windows has been recognised within the Building Regulations to help consumers make an informed choice about energy efficiency.

The BFRC Window Energy Ratings system uses an A-G guide – similar to that used to rate white goods – to measure the energy efficiency of a window or door.

This system combines three different measures:

  • The U-value for the whole window or door
  • Heat loss due to air leakage
  • Energy gained from the sun.

Windows Energy Ratings are a great quick and easy guide, but it’s worth bearing in mind that even if your window has an A+ rating it might not necessarily be the right choice for your location.

A window or door with an A+ Energy Rating will usually be designed to maximise the energy gained from the sun. This is great for a north-facing room or a room with smaller windows that is not flooded with sunlight. But in a south-facing room with large windows and doors, a high Energy Rating may actually lead that room to become uncomfortably hot in the summer months. In such a room, it might be better to fit window and doors with a low U-value (to minimise heat loss in the winter), but not necessarily a high Energy Rating.

Need professional advice on design, specification or installation?

We have a network of carefully selected Partners who will help guide you through your home transformation process.