Building control: condensation assessment
Reading the signs
14 March 2014
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
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 thethermal performance of the element and/or junction is poor, or the external temperature is lower than that for which the element was designed theheating 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
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||
|Numerical analysis defined in EN 15026||
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
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