Glass G-Values Explained

What Is a G-Value?

A glazing g-value describes the proportion of solar energy that passes through the glass into a building.

Put simply:

Higher g-value = more solar heat enters.

Lower g-value = less solar heat enters.

This becomes particularly important with large glazed areas.

Example

Imagine two large west-facing sliding doors.

One has glazing with a g-value of approximately 0.70.

The other has solar-control glazing with a g-value around 0.40.

The lower-g-value product allows substantially less solar energy into the building.

That can make a major difference to summertime overheating.

These numbers are illustrative – always use the actual declared values for the proposed glass.

G-Value vs U-Value

These measurements answer different questions.

U-value

How readily heat passes through the window because of temperature differences.

G-value

How much solar energy enters through the glazing.

A window can therefore have an excellent U-value while still allowing significant solar gain.

This is why Part L and Part O need to be considered together on applicable new-build projects.

Why Large Sliders Matter

A traditional bedroom window might contain 1–2m² of glazing.

A large sliding door could contain 10m² or more.

Even with the same glass specification, the total solar gain can therefore be dramatically different.

Particular attention should be paid to:

  • Large sliders
  • Bifolds
  • Glazed gables
  • Floor-to-ceiling glazing
  • Rooflights
  • South-facing elevations
  • West-facing elevations

Part O

Approved Document O applies to new residential buildings in England and sets standards for mitigating overheating risk.

The design may use either the applicable simplified or modelling approach.

Glazing specification can become an important part of that strategy.

Don’t Guess the Glass

If an overheating assessment specifies:

Maximum g-value 0.40

do not order glass described vaguely as:

“solar-control glass.”

Ask the glass supplier for the declared solar factor/g-value of the proposed build-up.

Light Transmission

Reducing solar gain can also affect daylight and appearance.

Two solar-control glasses with similar g-values may have different:

  • Light transmission
  • Colour
  • Reflectivity
  • External appearance

Therefore specify both performance and appearance.

Rooflights

Roof glazing deserves particular attention because it receives significant solar exposure.

Don’t automatically use the same glazing specification as vertical windows without considering the design requirements.

Solar-Control Coatings

Modern coatings can reduce solar heat gain while retaining useful daylight.

The optimum product depends on:

  • Orientation
  • Glass area
  • Building use
  • Overheating calculation
  • Appearance
  • U-value
  • Light transmission

Common G-Value Mistakes

Confusing g-value with U-value

They’re different.

Ordering standard glass before the Part O assessment is complete

Expensive.

Assuming every solar-control glass performs the same

It doesn’t.

Ignoring light transmission

A low g-value is not the only consideration.

Looking only at individual windows

Overheating is a building-design issue.

Real-World Example

A new-build property has a 6m wide west-facing aluminium slider.

The window schedule states the dimensions and colour but no solar performance.

The contractor obtains a standard glazing quotation.

Later, the overheating assessment requires a significantly lower g-value.

The sealed units now need changing.

The correct process is:

Overheating assessment → required performance → glass selection → manufacture.

Not the other way around.

Frequently Asked Questions

Is a lower g-value better?

It reduces solar gain, but “better” depends on the design objective.

Is g-value the same as U-value?

No.

Does Part O apply to replacement windows?

Part O applies to new residential buildings. It does not generally apply to later extensions and conservatories added to existing residential buildings.

Do bifolds need solar-control glass?

Not automatically. It depends on orientation, area and building design.

Does solar-control glass look tinted?

Some products have more noticeable colour or reflection than others. Modern neutral products can be relatively subtle.

Related Guides

  • Part O Explained
  • Solar-Control Glass
  • U-Values Explained
  • New-Build Window Regulations
  • Double vs Triple Glazing

Trade Tech Technical Support

For new-build projects with substantial glazing, send us the required:

  • U-value
  • G-value
  • Light transmission
  • Glass build-up
  • Acoustic requirements

before manufacture.

Important: Part O compliance relates to the building as a whole. A glazing g-value alone cannot demonstrate compliance.