Understanding U-Value &
Solar Heat Gain Coefficient
U-value and Solar Heat Gain Coefficient (SHGC) are two important measurements used to describe how a window manages heat transfer and solar heat gain. Understanding them makes it easier to compare window specifications and choose the right performance for your home.
U-Value
Describes how readily heat passes through the complete window system. In general, a lower U-value indicates better resistance to heat transfer.
Solar Heat Gain Coefficient
Describes how much solar heat passes through the window. A lower SHGC generally means less solar heat enters through the glazing.
What Is U-Value?
U-value describes how readily heat passes through a window system. It is one of the measurements used when comparing the thermal performance of different window specifications.
Think of U-value as heat transfer
A window separates the indoor environment from the conditions outside. Heat can move through the glass, frame and other components that make up that window.
The U-value helps describe how readily that heat is transferred through the complete window system.
In general, a lower U-value indicates better resistance to heat transfer, while a higher U-value indicates that heat passes through the system more readily.
Lower or higher — which is better?
When comparing otherwise appropriate window specifications, the basic U-value principle is simple:
When talking about U-value, lower generally means better resistance to heat transfer.
What Is Solar Heat Gain Coefficient (SHGC)?
Solar Heat Gain Coefficient describes how much solar heat passes through a window. It helps explain how strongly a window contributes to heat gain from the sun.
How do you read an SHGC number?
SHGC is expressed as a value between 0 and 1. The basic principle is:
Lower SHGC = less solar heat gain.
Is a lower SHGC always better?
Not necessarily. Unlike U-value, where a lower value generally indicates better resistance to heat transfer, the most appropriate SHGC depends on what the building needs.
A lower SHGC can be useful where reducing unwanted solar heat gain is important. In other situations, some solar heat gain may be beneficial.
Orientation, glass specification, window size, shading and the way the room is used should all be considered when choosing an appropriate glazing specification.
Local conditions include strong sunshine as well as cool winter periods, so SHGC should be considered together with U-value and the complete window specification rather than treated as a stand-alone number.
How Glazing Changes U-Value and SHGC
To make the numbers easier to understand, here is a like-for-like example from our technical data: a 600 × 1200 mm side-hung window shown with single glazing and double glazing in both EcoLine and UltraLine.
EcoLine Side Hung Window
Example size: 600 × 1200 mm
In this example, double glazing reduces both the U-value and SHGC. That indicates improved resistance to heat transfer together with lower solar heat gain for this particular window configuration.
Explore the EcoLine Range →UltraLine Side Hung Window
Example size: 600 × 1200 mm
The same pattern is visible here: the double-glazed UltraLine example has a lower U-value and lower SHGC than the corresponding single-glazed configuration.
Explore the UltraLine Range →U-Value and SHGC Should Be Read Together
U-value and Solar Heat Gain Coefficient describe different aspects of window performance. Looking at both gives a more useful picture than considering either number on its own.
U-Value
Tells us about heat transfer through the complete window system. A lower U-value generally indicates better resistance to heat passing through the window.
SHGC
Tells us about solar heat entering through the window. A lower SHGC means a smaller proportion of solar heat is transmitted through the window.
The Right Specification Is About Balance
There is no single U-value or SHGC that is automatically right for every home. The appropriate specification depends on factors such as orientation, window size, glazing, shading, room use and the performance requirements of the building. The two measurements should therefore be considered together as part of the complete window specification.
Choosing the Right Window Performance for Your Home
U-value and Solar Heat Gain Coefficient are useful because they help us understand how a complete window system manages heat transfer and solar heat gain. But neither figure should be considered on its own.
U-Value
Look at how effectively the complete window system resists heat transfer. In general, a lower U-value indicates better thermal insulation.
Solar Heat Gain
SHGC helps describe how much solar heat passes through the window. The appropriate level depends on the building, glazing and exposure to the sun.
The Complete Window
Frame design, glazing, seals, hardware, window size and correct installation all contribute to the performance of the finished window.
What does this mean for homes in Johannesburg?
Johannesburg homes experience strong sunshine, warm summer days and cool winter nights. Choosing a window is therefore not simply about finding the lowest number on a specification sheet. The aim is to select a combination of uPVC frame, glazing and window design suited to the property.
Where greater thermal performance is required, double glazing can be combined with an insulating uPVC frame to further reduce heat transfer and improve year-round indoor comfort.
Learn more about double glazed uPVC windows →Need Help Choosing the Right uPVC Windows?
You don't need to work out U-values, SHGC figures and glazing specifications on your own.
Tell us about your project and we can help you consider the appropriate uPVC window system, glazing and configuration for your requirements and budget.
Tell us about your project.
Whether you're replacing existing windows or specifying windows for a new project, send us the basic details and our team can assist with the next step.
- EcoLine & UltraLine Choose a system suited to the project and budget.
- Glazing Options Single and double glazing options are available depending on the window system and requirements.
- Window Configuration Window style, dimensions, glazing, seals and hardware all form part of the complete system.
Request a uPVC Window Quote
Complete the form below and tell us a little about what you need.