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📋 Part of our Curtain Wall Guide

Related: Glass Curtain Wall Systems | How to Reduce U-Factor

The curtain wall thermal performance of a building envelope is determined by the interaction of the framing system, glazing, spacers, pressure plates, thermal breaks, joints and surrounding construction.

It is therefore not accurate to reduce curtain-wall thermal performance to one aluminum profile or one glazing U-value.

For Germany, the calculation framework is also different from simply applying window terminology. ISO 12631:2017 specifically addresses the calculation of thermal transmittance of curtain walling, while DIN EN ISO 10077-2:2025-06 provides the numerical method for calculating thermal transmittance of frame profiles and their junction with glazing or opaque panels.

The practical objective is:

reduce heat transfer → control thermal bridges → maintain appropriate interior surface temperatures → minimise condensation risk.

 

Quick Summary

The key point: a curtain wall should be evaluated as a complete thermal system, not by selecting a thermal break and a glass U-value independently.

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What Determines Curtain Wall Thermal Performance?

A curtain wall has several interacting heat-transfer paths.

The main ones are:

1. Through the aluminum framing

Aluminum is highly conductive, so the frame geometry and thermal separation have a major influence on heat flow.

2. Through the glazing

The glass and IGU cavity form another major heat-transfer path.

3. Through the glazing edge

Spacer, sealant and frame-glass interfaces can create a different thermal path from the centre of the glazing.

4. Through the system interfaces

Pressure plates, fasteners, anchors and connections can create local thermal bridges that are not captured by looking only at the nominal mullion profile.

5. At the building interface

The connection between curtain wall, slab edge, opaque wall, roof or adjacent construction can create additional thermal-bridge effects.

This is why a brochure value for the aluminum profile does not represent the thermal performance of the installed façade.

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Thermal Breaks: What They Actually Do

A thermal break is a low-conductivity component separating parts of a metal profile to reduce heat flow through the framing system.

It does not eliminate heat transfer.

It reduces the conductive path.

For Germany, DIN EN 14024:2024-03 covers the mechanical performance, requirements, proof and testing of metal profiles incorporating thermal barriers. It is relevant to the structural/mechanical assessment of thermally separated profiles, not a substitute for thermal-transmittance calculation.

This distinction matters because a supplier may describe:

“PA66 GF25 thermal break”

without demonstrating the thermal performance of the actual curtain-wall section.

The more useful procurement requirement is:

Provide the thermal calculation for the proposed curtain-wall configuration.

The material of the barrier is only one part of that calculation.

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Curtain Wall Glazing Thermal Performance

Glazing is usually a major part of a curtain wall because transparent areas can represent a large proportion of the façade.

Important variables include:

• double or triple IGU
• glass thickness
• Low-E coating
• solar-control coating
• cavity width
• gas fill
• spacer
• laminated or toughened glass where required
• visible light transmission
• solar factor

Low-E coatings reduce radiative heat transfer.

Gas filling can change the heat-transfer characteristics of the IGU cavity.

The glazing U-value is determined using the applicable glazing calculation method. The current German standard DIN EN 673:2025-01 specifies the calculation method for determining the U-value of building glass. It replaces DIN EN 673:2011.

But the glazing U-value is still not the curtain-wall U-value.

That distinction should remain explicit throughout the article.

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Ug, Uf and Curtain-Wall U-Value: Do Not Mix Them

This is one of the most important technical corrections to the original draft.

Ug

Ug refers to the thermal transmittance of the glazing under the relevant calculation conditions.

Uf

Uf refers to the thermal transmittance of the frame/profile configuration.

Curtain-wall thermal transmittance

A curtain wall is a larger assembly and requires a curtain-wall calculation methodology.

ISO 12631:2017 specifically covers the calculation of thermal transmittance of curtain walling.

Therefore, do not write:

“Uw is simply Ug + Uf + edge.”

That is not a technically appropriate general formula.

For a German B2B article, the safer wording is:

The thermal performance of a curtain wall is calculated from the combined thermal characteristics of its frame, glazing, joints and relevant components according to the applicable curtain-wall calculation methodology.

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Germany: What U-Value Should a Curtain Wall Have?

There is no single universal curtain-wall U-value that should be applied to every German project.

The applicable requirement depends on:

• building type
• new construction vs renovation
• heating regime
• regulatory pathway
• project design
• calculation methodology

The current GEG reference-building provisions for non-residential buildings include U = 1.4 W/(m²K) for curtain walling under the listed reference-building conditions.

For certain alterations to existing non-residential buildings, the current GEG table lists Uc = 1.5 W/(m²K) for post-and-beam curtain wall construction corresponding to DIN EN ISO 12631, with 1.9 W/(m²K) under the lower design-temperature condition in that table.

These figures are regulatory/reference conditions, not a universal product rating.

A manufacturer should therefore not say:

“Our curtain wall is 1.8, therefore it complies with Germany.”

The project requirements and applicable calculation route need to be checked first.

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Condensation: What Does It Actually Mean?

Condensation occurs when a surface temperature falls below the dew-point temperature of the surrounding air.

The basic relationship is:

Indoor air temperature + indoor relative humidity → dew point

The building component then needs to maintain an adequate surface temperature under the relevant boundary conditions.

This means condensation is fundamentally a surface-temperature and humidity problem, not simply a “thermal break problem.”

For a curtain wall, the cold location could be:

• mullion
• transom
• glass edge
• spacer
• pressure plate
• frame corner
• anchor region
• perimeter junction

That location can provide a useful diagnostic clue, but it does not automatically prove which component is responsible.

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Why Condensation Appears at a Mullion

A mullion can become a local cold surface when the frame assembly has relatively high thermal transmittance or a local thermal bridge.

Possible contributors include:

• insufficient thermal separation
• poor frame geometry
• pressure-plate configuration
• metal continuity
• anchors or fixings
• connection details
• inadequate interior surface temperature
• high indoor humidity
• very low outdoor temperature

Therefore:

Frame condensation does not automatically mean “the thermal break is missing.”

The correct response is to assess the surface temperature and the relevant thermal paths.

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Glass-Edge Condensation

Condensation at the glass edge can have different causes from condensation at the centre of the mullion.

Potential contributors include:

• spacer thermal performance
• edge seal configuration
• glass-frame geometry
• frame temperature
• edge thermal bridge
• indoor humidity
• exterior temperature

This is why the statement:

“Edge condensation = weak spacer”

is too simplistic.

The spacer is one possible contributor, but the whole glazing/frame boundary condition needs to be considered.

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Condensation Across the Glass

Condensation on the interior surface of the glass can occur when the interior glass surface falls below the indoor dew point.

Possible factors include:

• glazing U-value
• low exterior temperature
• indoor humidity
• indoor air movement
• glass surface temperature
• thermal bridging around the frame

Low-E glazing can improve thermal performance, but it does not guarantee that condensation can never occur.

A building with unusually high interior humidity can still experience surface condensation even with high-performance glazing.

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The Role of Warm-Edge Spacers

Warm-edge spacers are designed to reduce heat transfer at the edge of an IGU compared with conventional highly conductive spacer configurations.

Their value is therefore not simply “making the window more efficient.”

They can influence:

• edge U-value
• interior glass-edge temperature
• condensation risk
• thermal comfort near the glazing

But again, spacer selection must be evaluated together with the curtain-wall frame and glazing configuration.

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Thermal Bridging at Curtain-Wall Interfaces

A common mistake is to calculate the curtain-wall centre section and assume the result represents the entire installed condition.

Interfaces can behave differently.

Examples include:

• slab-edge anchors
• brackets
• fasteners
• mullion reinforcement
• perimeter connections
• curtain wall / opaque wall junctions
• window interfaces
• parapet connections

These details may introduce local thermal bridges that are not represented by a simple profile U-value.

The current DIN EN ISO 10077-2:2025-06 calculation method itself notes that certain three-dimensional heat-transfer effects and thermal bridges to the building structure require separate consideration rather than being automatically represented by the 2D frame calculation.

This is particularly important for large commercial façades.

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A Practical Thermal Specification

For a German curtain-wall project, the specification should look more like this:

This is more useful than specifying:

“PA66 GF25 + Low-E + argon = U 1.8.”

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Curtain Wall Thermal Performance: What to Request From a Manufacturer

For procurement, request:

Frame information

• mullion and transom sections
• profile geometry
• thermal barrier arrangement
• pressure-plate configuration
• gasket arrangement

Glazing information

• glass build-up
• Ug
• Low-E coating
• gas fill
• spacer
• glass thickness

Thermal calculations

• curtain-wall U-value
• frame thermal calculation
• relevant thermal bridge assumptions
• calculation boundaries
• configuration used in the calculation

Condensation assessment

Where the project requires it:

• interior surface temperature
• dew-point assessment
• critical junction temperatures
• condensation-risk evaluation

Regulatory documentation

For Germany:

• applicable GEG target
• calculation method
• product/system documentation
• project-specific evidence

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Thermal Performance vs Condensation

These concepts are related but not identical.

This distinction helps diagnose problems correctly.

A system can have a reasonable overall U-value and still have a local condensation problem at a junction.

Conversely, improving the U-value does not automatically resolve every condensation problem if indoor humidity or a local thermal bridge remains high.

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Decision Guide: Diagnosing Curtain-Wall Thermal Problems

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How to Reduce Curtain Wall Heat Loss

There is no single “thermal break upgrade” that solves the whole system.

A coordinated approach is more effective:

1. Improve frame thermal separation

Use an appropriately engineered thermally separated framing system.

2. Optimise glazing

Select the glass build according to:

• Ug
• solar control
• daylight
• acoustic needs
• project energy targets

3. Improve the glass edge

Specify an appropriate spacer and edge configuration.

4. Control thermal bridges

Review:

• anchors
• brackets
• reinforcement
• slab-edge connections
• perimeter junctions

5. Evaluate the actual assembly

Use the relevant calculation methodology for the complete curtain-wall configuration.

6. Check surface temperatures

Where condensation is a concern, evaluate the critical surface temperatures rather than relying only on the U-value.

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Why “Lower U-Value” Is Not the Whole Story

A lower U-value generally indicates lower steady-state heat transfer under the defined calculation conditions.

But a curtain wall project also needs to consider:

• solar gains
• daylight
• condensation
• thermal comfort
• thermal bridges
• air leakage
• building interfaces
• heating and cooling loads

Therefore, chasing a lower U-value without considering the rest of the envelope can produce an over-simplified design decision.

The correct target is appropriate whole-assembly thermal performance for the building and climate.

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Germany Curtain Wall Thermal Specification Matrix

The standards serve different functions and should not be substituted for one another.

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Frequently Asked Questions

How is curtain wall thermal performance calculated?

Curtain-wall thermal performance is calculated using the relevant curtain-wall thermal-transmittance methodology, taking into account the framing, glazing and relevant interfaces. ISO 12631:2017 specifically addresses thermal-transmittance calculation for curtain walling.

What is a thermal break in a curtain wall?

A thermal break is a low-conductivity component that reduces heat transfer through a metal framing profile. It reduces conductive heat flow but does not eliminate it.

Is PA66 GF25 required for every thermally broken curtain wall?

No. PA66 GF25 is a common thermal-barrier material, but the specification should be based on the engineered system and required performance rather than assuming one material grade is universally mandatory.

What is the U-value of a curtain wall?

There is no single universal value. The required value depends on the project and applicable regulatory requirements. The current GEG reference-building table for specified non-residential conditions lists 1.4 W/(m²K) for curtain walling, while other GEG provisions use different values depending on the application.

Is 1.8 W/m²K a good curtain-wall U-value?

It cannot be judged independently of the project. A U-value must be compared with the applicable building requirements, calculation method and complete curtain-wall configuration.

What is the difference between Ug and curtain-wall U-value?

Ug describes the thermal transmittance of the glazing. Curtain-wall thermal transmittance represents the curtain-wall assembly and therefore includes the framing and other relevant components according to the applicable calculation method.

Why does a curtain wall condense?

Condensation occurs when the relevant surface temperature falls below the dew point. Causes may include low outdoor temperature, high indoor humidity, frame or edge thermal bridges, glazing performance and junction details.

Does condensation on the mullion mean the thermal break failed?

Not necessarily. It indicates that the interior surface temperature at that location may be too low under the actual conditions. The cause can be frame geometry, thermal separation, junctions, indoor humidity or other thermal bridges.

Does Low-E glass eliminate condensation?

No. Low-E glazing can improve thermal performance and increase interior glass-surface temperature, but condensation also depends on indoor humidity, outdoor temperature, edge conditions and frame geometry.

What standard is used for curtain-wall frame thermal calculations?

For numerical frame-profile thermal transmittance calculation, the current German edition is DIN EN ISO 10077-2:2025-06. For curtain-wall thermal transmittance as an assembly, ISO 12631:2017 is specifically applicable.

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Final Takeaway

A technically sound curtain wall thermal performance specification should not be reduced to:

Thermal break + Low-E + argon = low U-value.

The correct engineering chain is:

Frame geometry → thermal separation → glazing → spacer → pressure plate → junctions → whole-curtain-wall calculation → surface temperature → condensation risk

For Germany, the terminology and standards also need to remain separated:

ISO 12631:2017 → curtain-wall thermal-transmittance calculation
DIN EN ISO 10077-2:2025-06 → numerical frame thermal-transmittance calculation
DIN EN 673:2025-01 → glass U-value calculation
DIN EN 14024:2024-03 → mechanical performance of metal profiles with thermal barriers
GEG → applicable building-energy requirements.

The most important practical lesson is:

A lower U-value is not the same thing as a condensation-proof curtain wall.

The system must be designed so that its thermal performance, junctions and interior surface temperatures are appropriate for the project conditions.