Thermal conductivity of insulation layers: How water compromises ventilated façades

What is thermal conductivity?
Thermal conductivity (λ, lambda) is the ability of a material to conduct heat per unit of thickness and surface area: the lower the λ-value, the higher the thermal performance.

It is a key parameter in building-envelope design, as it determines the amount of thermal energy that passes through the structure, directly affecting the building's energy efficiency.
In construction, many designers focus on the thermal resistance of the individual layers and on the thermal transmittance (U-value) of the build-up, often overlooking the fact that the λ-value is measured in the laboratory under controlled conditions, whereas on site – both during installation and throughout the building's service life – materials are exposed to:

• rainwater ingress

• interstitial condensation

• post-installation residual moisture

• exposure to UV rays

• installation errors

• ageing of materials
  

These variables can significantly undermine the thermal efficiency of the building envelope over time.

As with any component of the building envelope, thermal conductivity is similarly a key parameter in the overall energy performance of ventilated façades. In this case, the insulation layer is protected by cladding but is still exposed to moisture and water ingress, which alter its performance.

Ventilated façades and insulation: Real conditions, real performance

Ventilated façades were developed to improve the durability and efficiency of the vertical envelope. They protect the insulation from direct exposure to weathering and help disperse moisture.
A typical ventilated façade build-up consists of:

• load-bearing structure

• airtight layer

• thermal insulation layer

• wind-tight layer

• ventilated air cavity

• external cladding

where:

• solar radiation heating the external cladding, combined with differences in atmospheric pressure, activates the chimney effect, which drives airflow through the cavity, speeding up the dispersal of moisture and excess heat

• the cladding provides mechanical and climatic protection for the insulation layer

• the insulation layer may find itself in conditions different to those assumed in the technical data sheets

The outer finishing layer protects both the build-up and the insulation layer from heavy rain and direct solar radiation, but does not fully isolate it from phenomena such as residual moisture, interstitial condensation or water ingress, all of which can create accidental thermal bridges not accounted for at the design stage. This is why thermal conductivity changes over time. When the insulation layer gets wet, λ increases and thermal performance deteriorates.

The analyses performed by Rothoblaas show that the presence of water can significantly affect the thermal conductivity of insulating materials.
Even in a well-designed ventilated façade, water ingress or interstitial condensation can lead to a major drop in thermal performance, with λ-values deviating well beyond those declared under ideal conditions.

That's why designing on the basis of technical data sheets alone is not enough. To achieve durable and predictable results over time, it is essential to assess the real behaviour of materials both on site and throughout their entire service life.

Comparison of the main insulating materials in dry and wet conditions

Ventilated façade with mineral wool (without membrane)

In dry conditions (5°C, RH 40%), mineral wool has an λ-value of around 0.033 W/(m·K). Assuming a critical scenario with relative humidity at 100%, thermal conductivity can rise to 0.0499 W/(m·K). In this situation, the performance of the insulation layer is reduced by roughly 33%. Values may vary depending on climate and installation conditions.

Ventilated façade with EPS (without membrane)

For EPS, assuming relative humidity at 100%, insulation layer performance may be reduced by up to 45%. Again, this is a simulation intended to illustrate the potential impact of moisture on this material.

Mineral wool and EPS with breathable membrane

The role of the breathable membrane in a ventilated façade is threefold: it prevents the infiltration of water/liquids, allows vapour to escape and helps block wind ingress.
A membrane classified W1 in accordance with EN 13859-1/2 offers high resistance to water ingress.

In the comparative analyses conducted by Rothoblaas, the effectiveness of breathable membranes is clear: both mineral wool and EPS, when protected by a properly installed membrane, maintain their thermal conductivity unaltered. This means that although the membrane has no direct insulating function, it preserves the designed performance over time by preventing water ingress and improving the reliability of the entire façade system.

Thermal conductivity table for insulating materials – Rothoblaas

Damage during construction or due to accidental water ingress: What happens if the membrane is missing or inadequate

Numerous analyses conducted on the degradation of building-envelope performance reveal a common cause: unmanaged water ingress or the absence of an effective, breathable, waterproof membrane.

During construction, if the membrane is not W1-certified, is incorrectly installed or outright missing, the insulation layer will behave differently from what was assumed at the design stage.

The consequences are real and measurable:

• increased thermal conductivity,

• reduced energy performance,

• risk of damage due to accidental water ingress,

• formation of mould or mechanical deterioration of the insulating material.

These incidents are not merely theoretical: they actually occur. There are real cases that clearly show how a lack of effective protection can compromise the durability of the building envelope.

Case study: The Banja Luka Prosecutor's Office – when cladding isn't enough

Between 2019 (the year of the last declared renovation) and 2025, the façade of the District Prosecutor's Office building in Banja Luka, Bosnia and Herzegovina, suffered rapid and severe deterioration. The metal cladding panels began to deform and repeatedly detach, especially under strong winds. The insulating material, consequently fully exposed to the elements, is visibly damaged and without any form of protection.

In this case, two critical factors contributed to the deterioration of the insulation layer:

• the direct exposure of the insulation layer due to the panels being detached;

• a design error, namely the use of a non-breathable barrier (BARRIER-type membrane), which prompted the formation of interstitial condensation with no possibility of drying out.

The building now stands as a visible example of non-durable design. The lack of effective control over durability, ventilation and insulation protection has compromised a recently refurbished building envelope, bringing it to a critical state in less than six years.

Breathable membranes and reference standards

In addition to the W classification under EN 13859-1/2, the choice of membrane must also be based on performance parameters, such as reaction to fire (e.g. A2s1, d0 classification), UV resistance and long-term dimensional and thermal stability.

Designing in compliance with standards is not only mandatory – it is essential for the durability and safety of the building envelope.

Which breathable membranes should you choose? Recommended Rothoblaas products

The ideal products for ensuring stable thermal performance are:

• TRASPIR ALU FIRE A2 430, highly breathable reflective membrane: ideal for intense solar exposure, non-combustible with high-level waterproofing.

• TRASPIR EVO UV ADHESIVE, self-adhesive breathable monolithic membrane resistant to UV rays: extra protection for complex construction sites, guaranteeing 10 weeks of temporary protection.

• TRASPIR EVO 300, highly breathable monolithic membrane that guarantees high waterproofing and an excellent reaction to fire.

• TRASPIR EVO UV 210, highly breathable monolithic membrane resistant to UV rays: ideal for ventilated façades with open joints up to 50 mm and up to 40% open surface, with B-s1,d0 reaction to fire and W1 waterproofing.

Rothoblaas W1 breathable membranes are designed to offer high resistance to rainwater while ensuring controlled transmission of water vapour. Combined with proper ventilation, they reduce the risk of interstitial condensation, keeping the insulation layer dry even under critical climate conditions.

Choosing and installing a Rothoblaas membrane means adopting a durable design approach that safeguards envelope performance over time and contributes to the building's safety and energy efficiency.

Designing long-lasting thermal performance: What to really consider

The declared thermal conductivity is only a starting point. Without proper control of moisture, installation and protection, the real performance of the insulation layer may differ significantly from the expected values.

That's why an effective ventilated façade design must account not only for the declared λ- value, but also for:

• the material's sensitivity to moisture

• the quality of the breathable membrane and its correct installation

• continuity of insulation at critical points (joints, window and door frames, fastenings)

• reaction to fire, especially in tall buildings or those with open façades

• possibility for long-term maintenance and predictability of performance

Designing for durability means anticipating how materials will behave under real conditions rather than relying solely on ideal values from technical data sheets.

Protect the long-term thermal performance of your building.
Choose Rothoblaas W1-certified breathable membranes and request a free technical consultation for your next project.

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