Building control: condensation assessment

Reading the signs

14 March 2014

Fanoula Ziouzia examines the two main methods used to calculate condensation risk

There are a lot of parameters, such as structural or issues of durability, that play a key role in assessing the performance of a building product, but its thermal performance is also critical. This does not only take into account the thermal conductivity of materials but it is also closely linked to the risk of condensation for a building element. It is therefore common for commercial U-value calculator software packages to include this risk.

In technical terminology, condensation is defined as the formation of moisture on a surface, which is caused by the temperature of that surface falling below the dew point temperature of the air adjacent to it. We are all familiar with the manifestations of condensation on windows, and also the long-term effect such as mould, not only in windows but commonly observed on walls and roofs.

Such condensation can form inside a building, for example on an internal surface or within a construction element (a wall) as a result of one or a combination of the following:

  • the vapour load inside the building is high, i.e. too much moisture is being generated by activities such as cooking, heating or from a shower
  • the thermal performance of the element and/or junction is poor, or the external temperature is lower than that for which the element was designed
  • the heating regime is insufficient, a phenomenon commonly observed in communal heated areas, or unheated corridors.

Effects of condensation

When it persists over a sufficient period, condensation can cause materials to become wet/damp. This can then lead to problems with freeze/thaw during winter, or mould growth, which can damage timbers and may lead to health problems for the building occupants.

Additionally, rain can penetrate into a poorly designed or constructed or damaged external envelope, potentially leading to water pooling, material/electrical damage and mould growth.

It is not surprising, therefore, that a series of British and European Standards have been developed over the years to cover this topic. It is of particular importance in the retrofit arena because there is a growing debate on the effect that additional insulation can have on the existing building elements’ ability to accommodate and dissipate moisture, not only from condensation but also from rain ingress. Two risk assessment methods are currently widely used.

The Glaser method

Currently, all UK countries make reference to BS 5250:2011 Code of practice for control of condensation in buildings (appendix D) and BS EN ISO 13788:2012 Hygrothermal performance of building components and building elements: Internal surface temperature to avoid critical surface humidity and interstitial condensation – calculation methods as methods of meeting the statutory requirements. Additional guidance can also be found in BR 262:2002 Thermal insulation: avoiding risks. BS EN ISO 13788 defines the Glaser method of modelling the risk of interstitial condensation within an element (e.g. wall, roof or floor).

There is a growing debate on the effect that additional insulation can have on the existing building elements' ability to accommodate and dissipate moisture

This method considers that the vapour generated travels (by diffusion) from internal to external and will condense if it finds a cold enough area inside the construction. So the best solution, according to this approach, is to keep as much vapour as possible in the warm side of the construction by the use of a vapour control layer (VCL) while at the same time allowing the small amount of vapour that does penetrate the VCL to escape easily. This can be achieved by having ventilated cavities on the cold side of the thermal insulation line and/or materials with low resistance to vapour diffusion.

For a proposed element a building humidity class is selected that defines the indoor temperature and humidity (e.g. Class 4 for a high-occupancy dwelling). Then, monthly mean external temperatures and humidities for the proposed location are selected – usually for a 1-in-10-year return risk. With the vapour pressure difference established and the thermal and diffusion resistance of each layer in the construction defined, the risk of condensation occurring and/or accumulating/evaporating at each interface is calculated for each of 12 months.

The Glaser method will either predict no condensation; some condensation accumulating in the winter months at some interfaces but dissipating in the following summer months; or persisting accumulation at one or more interfaces, i.e. does not fully dissipate before the next heating season adds more condensate.

The dynamic approach

The dynamic approach to condensation risk analysis has been developed in recent years and is defined, in broad principles, in BS EN 15026:2007 Hygrothermal performance of building components and building elements —assessment of moisture transfer by numerical simulation. This approach could be seen as tackling many of the ‘limitations’ of the Glaser method.

When the right parameters are input into the software, the output provided is very valuable to analyse in depth the risk of condensation and moisture accumulation/dissipation. However, the set of properties required for each material can be extensive and needs to be entered correctly, along with the external conditions and other parameters such as location and orientation. This calls for great care in the interpretation of the final results because they can be determined by one or more of these properties/parameters. Drawing meaningful conclusions can also be very complicated, requiring clear guidance and conventions documents (including materials properties and external conditions).

Some new products developed in recent years follow this new approach to condensation, for example intelligent membranes and renewable materials insulation, and a dynamic approach should be taken for risk analysis to fully appreciate their benefit. It has been shown that these new products can reduce considerably the risk of condensation(in situations where the Glaser method predictions are not accurate enough), although care should be taken to ensure the right inputs for the properties/parameters and external conditions. In addition, the conclusions will be only applicable to the particular situation of the dynamic simulation.

Table 1 itemises some of the pros and cons associated with the two condensation risk assessment methods.



Glaser method defined in BS EN ISO 13788
  • simple and easy to understand
  • long established and widely used in the industry
  • many software packages exist
  • reasonable approximation for most constructions 
  • one-dimensional heat transfer
  • internal and external temperatures are not constant over a month
  • effect of solar and 'clear sky' radiation considered?
  • air movement neglected
  • capillarity ignored
  • effect of vapour phase change
  • material properties and external/internal conditions not constant for each month
Numerical analysis defined in EN 15026
  • can be one or two-dimensional heat transfer
  • includes effects such as capillarity, air movement, solar and 'clear sky' radiation, rain penetration, thermal inertia, and variable material properties
  • can be used to assess risk of condensation in all constructions where 'needed' material properties and external conditions are known
  • only applies to the modelled construction, location, orientation
  • complex
  • requires expensive specialist software
  • results are not pass/fail and require expert interpretation

Annex D of BS 5250 implies that the Glaser method is useful as a tool to compare different structures but does not provide an accurate prediction of moisture conditions within the structure under service conditions. For a more accurate result, the standard suggests the method standardised in BS EN 15026.

The problem of correctly identifying the input parameters of materials becomes more complex when dealing with a retrofit situation. It may not be possible to know the exact material and, therefore, its properties. Moreover, the results of the dynamic method requires careful examination by an experienced individual, such as a building physics professional to ensure that the results and their limitations are fully understood and presented correctly.

Because there is more focus on the thermal performance of renovated homes, a closer look and examination of the robustness of the condensation risk analysis, and moisture issues in general, is becoming increasingly important. There is a clear need for conventions, particularly in the dynamic approach, to ensure a level playing field across the industry and also to educate professionals on issues of best practice in construction of thermally significant alterations.

Fanoula Zizouzia is Head of Business Development and Administration at the British Board of Agrément

Further information

Related competencies include: T013, T051