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Aluminum double glazed windows use an insulated glass unit, or IGU, made from two panes of glass separated by a sealed cavity.

The cavity, coating, spacer and frame all contribute to the final thermal performance.

That means “double glazed” is not a complete specification.

Two windows can both use two panes of glass but perform very differently depending on:

• Low-E coating
• Cavity width
• Argon fill
• Spacer
• Frame geometry
• Thermal break
• Window size

For UK projects, the useful figure is the whole-window U-value, not simply the centre-pane glass value. Current English Approved Document L guidance states that window U-values should be assessed for the whole element, including the glazing and frame. It also allows calculation using the actual window size and configuration under the specified calculation standards.

The key rule: compare the complete IGU + frame configuration, not the phrase “double glazing” on the quotation.

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Quick Specification Summary

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What Is an Insulated Glass Unit?

An IGU consists of two panes separated by a spacer and sealed around the perimeter.

The basic construction is:

Glass → sealed cavity → glass

The cavity reduces heat transfer compared with a single pane, while the Low-E coating can further reduce radiative heat transfer.

The IGU also needs a reliable seal system.

A typical insulated glass unit contains:

• Two glass panes
• Spacer
• Primary seal
• Secondary seal
• Air or inert gas cavity

The spacer keeps the panes apart, while the seal system maintains the cavity and protects the gas fill.

This is why an IGU is a manufactured system rather than simply “two pieces of glass.”

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The Four IGU Specifications That Matter Most

Low-E Glass

Low-E means low emissivity.

The coating reduces long-wave radiative heat transfer and can be selected for either stronger winter insulation or greater solar control, depending on the product.

For a UK building, the best Low-E configuration depends on:

• Orientation
• Heating demand
• Solar exposure
• Window-to-wall ratio
• Glazing area
• Building energy strategy

Do not specify simply:

“Double Low-E glass.”

Request the actual glass make-up and performance data.

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Argon Gas

Argon is commonly used instead of air in insulated glazing because its thermal properties can reduce heat transfer through the cavity.

However, the benefit depends on:

• Cavity width
• Glass configuration
• Low-E coating
• Temperature conditions
• Spacer design

Therefore, avoid presenting a universal percentage improvement such as “argon always improves performance by 10–15%.”

The better procurement question is:

What Ug does this exact IGU achieve with the specified gas fill and Low-E coating?

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Warm-Edge Spacers

The edge of an IGU is often thermally weaker than its centre.

A conventional metal spacer can create a conductive path around the perimeter.

A warm-edge spacer uses a lower-conductivity construction to reduce this thermal bridge.

This can improve:

• Edge thermal performance
• Interior surface temperature
• Condensation resistance
• Overall window performance

For a high-performance aluminum window, the spacer should therefore be specified together with the glass and frame.

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The Aluminum Frame

A good IGU can still underperform if it is installed in a poorly designed aluminum frame.

Aluminum conducts heat readily, so thermally broken construction can be important in heated or cooled buildings.

The final window performance depends on the relationship between:

Ug + Uf + spacer + frame geometry + window size

That is why Uw, the whole-window U-value, is more useful than looking at Ug alone.

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Double Glazing vs Triple Glazing

Triple glazing adds another pane and another cavity.

That can reduce thermal transmittance further, but it also increases:

• Glass weight
• Frame and hardware loads
• Product cost
• Manufacturing complexity
• Installation weight

So the question is not:

“Is triple glazing better?”

The better question is:

“Does the additional performance justify the additional cost and weight for this project?”

There is no universal rule that every UK project needs triple glazing.

The correct choice comes from the building energy model and complete-window performance.

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Why U-Value Must Be Compared at Whole-Window Level

One of the most common mistakes in window quotations is comparing:

Ug from Supplier A

against:

Uw from Supplier B.

Those are not equivalent values.

For UK projects, this distinction is particularly important because current Approved Document L guidance requires window U-values to represent the whole fabric element, including the glazing and frame. For new dwellings, the 2026 guidance also specifies calculation or measurement methods and requires the actual frame and configuration to be considered.

Procurement rule:

Always compare the same type of U-value using the same window configuration.

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What Should You Request From an Aluminum Window Supplier?

When comparing aluminum double glazed windows, ask for the complete configuration rather than a generic “double glazing” description.

For UK projects, BS EN 14351-1 covers relevant performance characteristics for windows and external pedestrian doorsets. It is currently listed by BSI as a current standard.

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Double Glazing Specification Example

Instead of accepting:

“Aluminum double glazing, Low-E, argon.”

A useful specification should look more like:

This level of detail gives architects and contractors something they can actually compare between suppliers.

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How to Decide Between Double and Triple Glazing

Instead of using a long sequence of steps, use the project decision table below:

The important point is that pane count is only one variable.

A well-designed double-glazed window can outperform a poorly specified triple-glazed system if the latter uses an unsuitable frame, spacer or glazing configuration.

 

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How to Compare Aluminum Double Glazed Window Quotes

When suppliers offer different prices, compare the technical configuration first.

A cheaper quote may simply contain:

• Basic glass
• Air-filled cavity
• Conventional spacer
• Non-thermal frame
• Higher-performance options omitted
• Incomplete documentation

Compare the build-up before comparing the price.

For accurate project quotations, see our guide on what drawings and information to send to an aluminum window manufacturer.

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A Better Way to Specify Double Glazed Aluminum Windows

The correct specification chain is:

This is a more useful approach than simply specifying “double glazing.”

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Frequently Asked Questions

What are aluminum double glazed windows?

Aluminum double glazed windows use two panes of glass separated by a sealed cavity to form an insulated glass unit. The cavity, Low-E coating, gas fill, spacer and aluminum frame all contribute to the final thermal performance.

What is the U-value of double glazed windows?

There is no single U-value for all double glazed windows. The glass Ug and complete-window Uw depend on the glass, Low-E coating, cavity, spacer, frame, window size and configuration. For UK projects, whole-window U-value should be assessed using the applicable regulatory calculation or measurement method.

Is double glazing enough, or do I need triple glazing?

For many UK projects, high-performance double glazing can provide an effective balance of performance, cost and weight. Triple glazing becomes more attractive where the building has a very low-energy target, severe thermal requirements or specific acoustic objectives.

What is the difference between argon and air-filled double glazing?

Argon is commonly used to improve the thermal performance of an IGU compared with an air-filled cavity. The actual improvement depends on cavity width, Low-E coating, glass configuration and other design variables, so compare the documented Ug rather than assuming a fixed percentage improvement.

What does Low-E glass do?

Low-E glass uses a low-emissivity coating to reduce radiative heat transfer. Depending on the coating, it can improve winter thermal performance or provide stronger solar control in summer.

What is a warm-edge spacer?

A warm-edge spacer separates the two panes around the perimeter of the IGU while reducing thermal conduction at the glass edge. It can improve edge temperatures and reduce condensation risk compared with a highly conductive metal spacer.

Why do double glazed windows fog up between the panes?

Condensation or fogging between the panes usually indicates failure of the IGU’s edge seal, allowing moisture into the sealed cavity. When the seal has failed, the insulated glass unit normally needs replacement rather than simple surface cleaning.

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Bottom Line

Aluminum double glazed windows are not defined simply by having two panes of glass.

The important specification is the complete system:

Low-E + cavity + gas fill + spacer + thermal-break frame + whole-window U-value.

For most projects, compare high-performance double and triple glazing based on the building’s actual thermal and acoustic requirements rather than assuming that more glass automatically means a better window.

For UK projects, send Starveil your window schedule, opening dimensions, orientation, glazing requirements and target thermal performance. We can review the IGU build-up, aluminum frame and whole-window specification before quotation.

Request an IGU & Aluminum Window Specification Review →