Part O Building Regulations Explained

What is Part O?

Part O deals with overheating in new residential buildings in England.

This regulation has become increasingly important because modern homes are highly insulated and airtight and often incorporate large areas of glazing.

The same glass that provides daylight and attractive views can allow substantial solar energy into a building.

If that heat cannot escape, internal temperatures can become uncomfortable or potentially dangerous.

Where Does Part O Apply?

Approved Document O applies to new residential buildings.

It should not be casually presented as a requirement applying to every replacement window.

However, overheating remains a sensible design consideration for extensions and refurbishment projects with large areas of glazing.

Why Windows Matter

Windows affect overheating in two main ways:

Solar gain

Sunlight passing through glazing brings energy into the building.

Ventilation

Opening windows can help remove excess heat.

Part O therefore creates a balancing act.

Designers need sufficient glazing for daylight and amenity while controlling excessive solar gains and providing an effective means of removing heat.

What Is a G-Value?

The g-value describes the proportion of solar energy passing through glazing.

A higher g-value allows more solar energy through.

A lower g-value reduces solar gain.

For example, a large west-facing glazed screen can produce significant afternoon heat gain.

Specifying lower-g-value solar-control glass can reduce that gain.

U-Value vs G-Value

These are frequently confused.

U-value: heat passing through the window because of the temperature difference between inside and outside.

G-value: solar energy entering through the glazing.

You can therefore have a window with an excellent U-value that still contributes significantly to summertime overheating.

Both need to be considered.

Large Bifolds and Sliding Doors

Part O is particularly relevant when architects specify:

• Large bifold doors
• Floor-to-ceiling glazing
• Large sliding doors
• Glazed gables
• Rooflights
• Multiple glazed elevations

A six-metre south- or west-facing slider can introduce substantially more solar energy than a conventional window.

The glass specification needs to be considered during design rather than after installation.

Solar Control Glass

Solar-control glazing can reduce the amount of solar energy entering the building.

Depending on the product, it may also affect:

• Light transmission
• External appearance
• Internal colour
• Reflection
• U-value

The goal isn’t necessarily to select the lowest g-value available.

The correct specification balances overheating risk, daylight, appearance and thermal performance.

Window Opening Area

Part O isn’t purely about glass specification.

The ability to remove heat is also important.

Opening windows may contribute to the overheating strategy.

But simply making every window open as widely as possible can introduce other risks.

Part O vs Part K

Large openings can create a falling risk.

This is particularly relevant where windows have low cill heights or are positioned above ground level.

The design may require:

• Guarding
• Restrictors
• Different opening configurations

Part O vs Part B

Opening windows may also be part of a fire escape strategy.

Changing opening sizes or fitting restrictors without understanding the fire strategy can therefore cause problems.

Part O vs Security

Opening windows for summertime ventilation can potentially introduce security risks.

Designers may need to consider how ventilation is achieved while maintaining appropriate security.

Part O vs Noise

This is another major real-world issue.

Imagine a new apartment next to a busy road.

The overheating strategy relies heavily on occupants opening windows at night.

Thermally this might work.

Acoustically it may be unacceptable.

This is why overheating, ventilation, acoustics and glazing specifications should be considered together.

Shading

Overheating can also be controlled using shading.

Examples include:

• External shutters
• Overhangs
• Brise soleil
• Balconies
• Building orientation
• Other designed shading measures

External shading can be particularly effective because it intercepts solar energy before it enters through the glass.

Common Part O Mistakes

Choosing glass based only on U-value

Excellent insulation does not automatically mean good overheating performance.

Ignoring orientation

North-facing and west-facing glazing do not experience the same solar exposure.

Ordering glazing before the overheating assessment

This can lead to expensive replacement units.

Assuming “solar glass” is one specification

Solar-control products have different g-values and light transmission characteristics.

Ignoring opening restrictions

A ventilation strategy that relies on opening windows needs to consider safety and security.

Real-World Example

A developer designs a new home with a 5-metre west-facing sliding door.

The original quotation uses standard low-emissivity double glazing.

The overheating assessment subsequently requires reduced solar gain.

If the frames have already been manufactured and the sealed units ordered, changing the glass specification can become expensive.

The correct approach is to obtain the required:

• U-value
• g-value
• Light transmission

before manufacture.

What Builders Should Put on RFQs

When requesting quotations for large glazed areas on new residential projects, include:

• Required whole-window/door U-value
• Required glass g-value
• Light transmission where specified
• Safety glass requirements
• Acoustic requirements
• Opening requirements
• Part Q/PAS 24 requirements where applicable

This dramatically reduces specification errors.

Frequently Asked Questions

Does Part O apply to new homes?

Yes. Approved Document O applies to new residential buildings in England.

Does Part O apply to every replacement window?

No.

What is a glass g-value?

It indicates how much solar energy passes through the glazing.

Is a lower g-value better?

For reducing solar gain, yes, but the correct specification needs to balance daylight, appearance and other performance requirements.

Can solar-control glass help?

Yes.

Can opening windows solve overheating?

They may form part of the strategy, but safety, security, acoustics and practical use must also be considered.

Are bifolds affected?

Large areas of glazing can have a significant influence on overheating calculations, so bifolds and sliders deserve particular attention.

Related Guides

• G-Values Explained
• Solar-Control Glass
• Part L Explained
• Part F Explained
• Part K Explained
• Acoustic Glass
• New-Build Window Regulations

Need Help With a Glass Specification?

Send Trade Tech the project’s glazing schedule or overheating requirements.

We can help identify suitable glass specifications and window or door systems before manufacture.

Important: Part O compliance is a building-design issue and cannot be determined solely from a window specification. Always follow the project’s overheating assessment and current Approved Document O.