Solar-Control Glass Explained

What Is Solar-Control Glass?

Solar-control glass is glazing designed to reduce the amount of solar energy entering a building.

It is particularly useful where large areas of glazing are exposed to direct sunlight.

Typical applications include:

• Large sliding doors
• Bifold doors
• Floor-to-ceiling windows
• Glazed gables
• Rooflights
• South-facing glazing
• West-facing glazing
• New residential developments

The Quick Answer

If a room gets uncomfortably hot because of sunlight, changing only the window U-value is unlikely to solve the problem.

You need to look at solar gain.

The key glazing figure is usually the g-value.

In general:

Lower g-value = less solar energy passes into the building.

But selecting the lowest possible g-value is not automatically the best answer because solar-control coatings can also affect:

• Daylight
• Glass colour
• Reflectivity
• External appearance

The correct glass needs to balance all of these.

What Is Solar Gain?

Sunlight contains energy.

When it passes through glazing and is absorbed by surfaces inside the building, that energy contributes to internal heat.

This is useful during colder weather but can cause overheating during summer.

Large glazed areas can make the effect substantial.

What Is a G-Value?

The g-value describes the total solar energy transmission of the glazing.

It is normally expressed as a decimal between 0 and 1.

For example:

g = 0.70

means substantially more solar energy passes through than:

g = 0.35

These figures are illustrative.

Always use the declared value for the actual proposed glass build-up.

Part O G-Values

For Approved Document O calculations, the government’s guidance confirms that the referenced g-values are centre-pane g-values and should not include the frame effect.

This is an important distinction from whole-window U-values.

When an overheating assessor specifies a maximum g-value, make sure the glass supplier understands exactly which value is being requested.

G-Value vs U-Value

These two figures are constantly confused.

U-Value

Measures heat transfer due to the temperature difference between inside and outside.

Lower is generally better for reducing heat loss.

G-Value

Measures solar energy entering through glazing.

Lower reduces solar gain.

Therefore a window can have:

Excellent U-value + high solar gain.

This can produce a well-insulated home that becomes very hot in summer.

Why This Matters More in Modern Homes

Modern homes tend to have:

• Better insulation
• Better airtightness
• Larger glazing
• More bifolds and sliders

Once solar energy enters, improved insulation can make it harder for that heat to escape.

Overheating therefore needs to be considered during design.

Part O

Approved Document O deals with overheating mitigation in new residential buildings in England.

It considers both:

• Limiting unwanted solar gains
• Providing means to remove excess heat

Solar-control glass can be one tool used within that strategy.

It is not the only solution.

Don’t Treat Solar Glass as a Substitute for the Part O Assessment

A builder may say:

“We’ll just put solar-control glass in.”

That does not demonstrate Part O compliance.

The assessment considers the building and may also involve:

• Orientation
• Glazing area
• Shading
• Window openings
• Ventilation
• Occupancy
• Location

Use the required performance from the overheating design.

South-Facing Glazing

South-facing glazing receives substantial solar exposure.

Depending on building design, mitigation may involve:

• Solar-control glazing
• External shading
• Overhangs
• Reduced glazing area
• Other design measures

West-Facing Glazing

West-facing glazing can be particularly uncomfortable because it receives strong low-angle afternoon and evening sun when external temperatures may already be high.

Large west-facing sliders deserve particular attention.

North-Facing Glazing

North-facing elevations generally experience less direct solar gain.

This demonstrates why applying one glass specification to every elevation isn’t always the optimum solution.

The architect or overheating consultant may specify different glazing performance by orientation.

Large Sliding Doors

Modern sliders can provide enormous areas of glass.

A six-metre wide glazed opening can introduce far more total solar energy than several conventional windows.

When quoting large sliders for new builds, ask early:

“Has the Part O / overheating assessment been completed, and is there a maximum g-value?”

That one question can prevent replacement sealed units later.

Bifold Doors

Exactly the same issue applies to large bifolds.

The more glass, the more important solar performance can become.

Don’t automatically quote the same standard double-glazed unit used in upstairs casement windows.

Rooflights

Rooflights can receive intense solar exposure.

A large rooflight can contribute strongly to summertime overheating.

Solar-control glazing is therefore commonly considered for:

• Flat rooflights
• Lantern roofs
• Large overhead glazed areas

The required glass should be coordinated with the rooflight manufacturer’s approved construction.

Light Transmission

Solar-control performance isn’t the only number.

Another important value is light transmission, sometimes abbreviated LT or Tv.

This indicates how much visible light passes through the glass.

A glass with very aggressive solar control may reduce daylight more than the client expects.

Therefore an appropriate specification may include both:

Maximum g-value

and

Required/minimum light transmission

Neutral Solar-Control Glass

Modern coatings can provide useful solar control while maintaining a relatively neutral appearance.

This is often desirable on residential projects where strongly tinted or reflective glazing would be visually unacceptable.

However, “neutral” does not mean invisible.

Always consider samples where appearance is important.

Tinted Glass

Tinted glass can also reduce solar transmission, but it affects the appearance of the façade.

Colours may include:

• Grey
• Bronze
• Blue
• Green

depending on the product.

Don’t specify tint solely for solar performance without checking aesthetics.

Reflective Glass

More reflective solar-control products can provide strong solar performance but may substantially change the appearance of the building.

Planning or architectural design considerations can become relevant.

Coating Position

Modern insulated glass units use coatings on particular glass surfaces.

The coating position depends on the glass product and sealed-unit design.

Do not casually reverse or substitute panes.

Follow the glass manufacturer’s processing and unit-construction requirements.

Toughening Requirements

Solar-control coated glass may also need heat treatment depending on:

• Pane size
• Thermal stresses
• Safety-glazing requirements
• Manufacturer processing rules

The glass processor should confirm the appropriate construction.

Solar Control and Safety Glass

A pane may simultaneously need:

• Low g-value
• Safety classification

For example, a floor-to-ceiling west-facing sidelight could need solar-control performance and Part K safety glazing.

The final build-up must satisfy both.

Solar Control and PAS 24

A new-build slider might require:

• PAS 24
• Part O performance
• Part L U-value
• Part K safety glass

Don’t order security glass first and then decide to replace it with a solar-control alternative.

Find a glass build-up compatible with all required performance.

Solar Control and Acoustic Glass

Properties on busy roads can create another multi-performance problem.

You may need:

• Acoustic laminate
• Low g-value coating
• Safety performance
• Low U-value

This is why “28mm double glazing” is not a useful complete specification.

The glass build-up matters.

Internal Blinds vs Solar-Control Glass

Internal blinds can improve occupant comfort and glare control, but they do not intercept solar energy before it passes through the glazing as effectively as external measures.

For Part O design, don’t assume an ordinary internal blind automatically substitutes for specified solar-control glazing or designed shading.

External Shading

External shading can be very effective because it intercepts sunlight before much of the energy passes through the glass.

Examples include:

• Brise soleil
• External shutters
• Overhangs
• Balconies
• External blinds

Part O allows overheating mitigation to be considered through building design rather than glass alone.

Part L and Summer Solar Gain

The 2026 Approved Document L also contains guidance on limiting the effects of solar gains in summer, reinforcing that energy-efficient design is not solely about preventing winter heat loss.

Common Solar-Control Glass Mistakes

Mistake 1 – Confusing g-value and U-value

They measure different things.

Mistake 2 – Ordering before the overheating assessment

Potentially very expensive.

Mistake 3 – Asking only for “solar glass”

Specify a performance requirement.

Mistake 4 – Ignoring light transmission

The room may become darker than expected.

Mistake 5 – Ignoring appearance

Coatings can change colour and reflection.

Mistake 6 – Using one specification on every elevation without checking

Orientation matters.

Mistake 7 – Changing glass after PAS 24 has been agreed

Security evidence may be affected.

Real-World Example

A developer designs a rear elevation containing:

5.5m × 2.4m aluminium sliding doors.

The opening faces west.

The original quote uses standard high-performance low-E glass.

The Part O assessment later requires:

g-value ≤ 0.40

The builder now has three choices:

• Change the glass specification before manufacture
• Introduce other acceptable mitigation
• Redesign the glazing

If the standard glass has already been ordered, the change could cost thousands of pounds.

The correct sequence is:

Part O design → required g-value → glass specification → quotation → manufacture.

What Should Builders Put on the Enquiry?

For large glazed areas, provide:

• Maximum g-value
• Required U-value
• Light transmission if specified
• Orientation
• Safety requirements
• Acoustic requirements
• PAS 24 requirements
• Glass appearance requirements

Don’t simply write:

“28mm clear double glazing.”

Frequently Asked Questions

What is solar-control glass?

Glass designed to reduce solar heat gain through glazing.

What is a g-value?

A measure of total solar energy transmission through the glazing.

Is a lower g-value better?

It reduces solar gain, but the optimum value depends on the building design and daylight requirements.

Is g-value the same as U-value?

No.

Does Part O apply to all replacement windows?

No. Approved Document O applies to new residential buildings in England.

Do south-facing bifolds need solar glass?

Not automatically. The design and overheating assessment determine the required performance.

Are west-facing sliders at risk of overheating?

Large west-facing glazing can experience significant afternoon solar gain, so it deserves early assessment.

Does solar-control glass look dark?

Some products are darker or more reflective than others. Neutral products are available.

Can solar-control glass also be toughened?

Suitable products can be heat treated where required and permitted by the manufacturer.

Can solar-control glass be laminated?

Suitable laminated/coated constructions are available depending on the required specification.

Related Guides

• G-Values Explained
• Part O Explained
• Window U-Values Explained
• Toughened Glass Regulations
• Laminated Glass Guide
• PAS 24 Explained
• New-Build Window Regulations

Trade Tech Technical Support

For large bifolds, sliders, rooflights and glazed gables, ask for the overheating specification before ordering the glass.

Send us:

• Maximum g-value
• U-value
• Light transmission
• Safety requirement
• Acoustic requirement
• Security requirement

and we can work with the appropriate glazing specification rather than guessing.

Important: Solar-control glass is one part of an overheating strategy. Part O compliance is determined at building level, not simply by installing low-g-value glazing.