Understanding condensation risk analysis

Thermal insulation plays a significant role in minimising the potential risk of both surface and interstitial condensation. However, it must be specified correctly and, crucially, installed properly on-site.

When considering condensation risks, it is important to acknowledge additional factors influencing moisture vapour levels in a building, including:

• The level of airtightness achieved
• Ventilation system specification and performance
• The number of occupants and their activities (e.g., frequency of showers, drying washing indoors).

The starting point for understanding condensation risks

A fundamental aspect of condensation risk analysis is determining whether a construction follows a vapour-open or vapour-closed approach.

Traditional and historic building fabrics, often featuring lime mortars and renders, typically rely on a vapour-open approach.

Moisture vapour can pass through the construction materials—usually from inside to outside, though sometimes in the opposite direction depending on conditions. Modern construction methods predominantly employ a vapour-closed approach, aiming to block or significantly reduce moisture ingress into the construction.

By limiting moisture vapour levels within the structure, the risk of condensation forming at cold interfaces is minimised.

"Particularly when modifying existing building fabric, condensation analysis tools should be used early in the design process to ensure appropriate product selection and prevent issues later" explains David Milner, Technical Team Lead at Celotex.

"Waiting until insulation is about to be installed may make necessary specification changes impractical and more costly."

Celotex insulation products are inherently vapour-closed and support this approach. However, special care is required—especially in retrofit or refurbishment projects—when integrating vapour-closed materials into a vapour-open structure, or vice versa. “This is where expert advice on condensation risk analysis from manufacturer technical support teams is invaluable,” adds David.

David Milner explains: "The 2021 version of BS 5250 provides a comprehensive guide to best practices in moisture management. It adopts a whole-building approach, considering various interactions between different aspects of building design and construction, addressing both moisture as liquid water and moisture vapour. The standard also provides guidance on selecting the most suitable condensation risk analysis method to use for different construction types."

For the simplest and lowest-risk construction types, the standard suggests that condensation risk analysis may not be necessary.

How to calculate condensation risk

For many vapour-closed construction types—such as masonry cavity walls and pitched roof constructions—a simplified condensation risk analysis method is sufficient.

This method, known as the Glaser method, is defined in BS EN ISO 13788:2012 Hygrothermal Performance of Building Components and Building Elements. It assesses internal surface temperature conditions to prevent critical surface humidity and interstitial condensation.

While this method has limitations due to the complex nature of moisture behaviour in materials and structures, BS 5250 provides guidance on applying it correctly. Seeking advice from a manufacturer’s technical team familiar with the standard is recommended.

Despite its limitations, the Glaser method is a quicker and less complex calculation compared to more advanced analysis techniques. It can even be incorporated into online U-value calculator tools, allowing specifiers, installers, and homeowners to conduct condensation risk analysis for suitable constructions.

For more complex moisture movement scenarios or constructions facing particular risks, more advanced analysis tools such as WUFI® should be used, with specialist guidance sought where necessary.

The Glaser method of condensation risk analysis and U-value calculations

A continuous, well-installed thermal insulation layer of sufficient thickness helps maintain higher internal surface temperatures, reducing the likelihood of surface condensation. However, as mentioned earlier, managing internal humidity is equally critical.

In vapour-closed constructions, an Air and Vapour Control Layer (AVCL) is specified alongside thermal insulation to restrict moisture vapour ingress. When external temperatures drop, an effective AVCL ensures there is insufficient moisture present for condensation to occur within the structure.

A simplified heat loss calculation, known as the combined method, is used to determine the U-value of most typical building elements. This method evaluates the thermal performance of a construction by analysing its layers, including insulation and any repeating thermal bridges (such as timber studs, joists, or cavity wall ties).

The Glaser method applies the same layer-by-layer approach, calculating temperature and humidity at the interface of each construction layer to assess whether moisture vapour levels are likely to exceed the dew point.

The only additional information required for condensation risk analysis, beyond what is needed for a U-value calculation, includes:

• The building’s location (for external temperature and humidity data).
• The occupancy type (for representative internal temperatures and humidity levels).

These details can be easily supplied via an online tool or by consulting a technical support team.

It is important to note that neither the combined U-value calculation method nor the Glaser method is suitable for assessing thermal performance and condensation risk at linear thermal bridges (e.g., junctions between building elements). More advanced numerical modelling is required for such assessments.

Free online condensation risk analysis tools from Celotex

The Celotex online U-value calculator has long been a valuable tool. Its functionality has now been enhanced to include condensation risk analysis (conducted using the Glaser method) alongside combined-method heat loss calculations.

Users can obtain U-value calculations and, for construction types recommended by BS 5250:2021, condensation risk analyses at their convenience. Technical specifications and installation guidance for Celotex products are also available, allowing for project-specific specification development.

Discover more about our comprehensive calculation tools, including U-value and condensation risk analyses. For construction types not covered by the combined method or Glaser method, contact our technical support centre for expert advice.