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Before specifying an Internal Wall Insulation (IWI) system, it is essential to have a clear understanding of the moisture risks for the specific building. This should form part of the whole building approach to retrofit, as laid out in PAS 2035: 2023 (Retrofitting dwellings for improved energy efficiency. Specification and guidance). It should also be carried out in line with BS 5250: 2021 (Management of moisture in buildings. Code of practice)
There are various avenues by which water (liquid or vapour) may interact with a building’s structure and cause problems, for example:
The whole-building approach to understanding moisture risk can be broken down into four key principles.
a) Understand the context of the building and the building project and ensure compatibility of the design with this context.
b) Ensure coherence in approach and detailing.
c) Build-in extra capacity in the design and construction phase for uncertainties in the build process and future challenges.
d) Ensure that caution is taken in the use, maintenance and after-care phase where there are ongoing requirements of care and uncertainty of outcomes.
Particular care is needed if a property is determined to be at increased risk from wind driven rain. This is because this moisture may be absorbed into the masonry and can cause problems if it becomes trapped there.
A map of exposure to wind driven rain is provided in Approved Document C. This separates the country into four exposure zones from Zone 1 (sheltered) to Zone 4 (very severe). BS 5250: 2021 highlights that in critical cases, more in-depth analysis can be carried out following the assessment methodology in in BS 8104: 1992 (Code of practice for assessing exposure of walls to wind-driven rain). This allows for localised factors for the building.
General points to note include:
It’s also worth noting that local factors can affect the level of exposure that a given building or even wall is subject to. Even if a property is located in Exposure Zone 4, local topography and shelter can reduce the exposure of particular walls or the entire property significantly. Use of render, cladding, or a barrier cream can also assist in reducing the impact of wind-driven rain. Equally, even in exposure Zone 1 or 2, the same parameters can shift the actual exposure level up the scale.
There are two commonly used approaches for assessing condensation risk in solid walls:
• the Glaser approach (contained in 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); and
• the WUFI approach (contained in BS ISO EN 15026: 2022 (Systems and software engineering. Systems and software assurance)).
Both are potentially useful tools, but have limitations which mean you cannot expect a simple yes/no answer covering all scenarios.
The Glaser approach indicates how much condensate is deposited, how much evaporates and any balance which might accumulate year on year. The method uses monthly mean external conditions to calculate the amount of condensate deposited and/or evaporated in each of 12 months, following the first predicted occurrence of condensation.
The method is a useful assessment tool, suitable for comparing different constructions and assessing the effects of design changes.
However, it has limitations:
a) it does not allow for airflow within and through components, solar and longwave radiation, wind speed and direction, and precipitation;
b) it makes no allowance for the moisture in materials or rainwater absorbed during construction; and
c) it is a one-dimensional, steady-state model which cannot deal with interfaces and junctions.
Consequently, while it is useful for comparing the performance of different structures, it does not always provide an accurate prediction of moisture conditions within the structure under in-service conditions, particularly where wind-driven rain is significant.
The WUFI approach specifies a system of equations for calculating one-dimensional, non-steady heat and moisture flows through a structure made up of a number of different materials, each with complex transport properties.
Heat storage in dry building materials and also absorbed water are accounted for, as well as heat transport by moisture-dependent thermal conduction, latent heat transfer by vapour diffusion, moisture storage by vapour sorption and capillary forces, moisture transport by vapour diffusion, and moisture transport by liquid transport (surface diffusion and capillary flow).
The following climatic variables are used as inputs: internal and external temperature, internal and external humidity, solar and longwave radiation, precipitation (normal and driving rain), and wind speed and direction.
Whilst the WUFI approach has some benefits over the Glaser approach, it also has some limitations which project teams need to be aware of:
a) it does not allow for airflow through components, which can be the dominant mechanism for moisture movement in some structures;
b) it is a one-dimensional model which cannot deal with interfaces and junctions, where many moisture problems commonly arise;
c) there is currently no background guidance for the use of BS EN 15026: 2022 and its correct use requires expertise, as well as an understanding of the principles underlying moisture movement in buildings and the specific context and condition of the building, or building element being assessed;
d) it requires, and the document refers to, the use of good weather data (which might not be readily available); and
e) it requires information about the properties of materials, which is not necessarily available or accurate for UK construction materials, either new or historic.
In those cases where there is uncertainty about the properties of the materials present and initial modelling suggests a marginal risk of moisture problems, the effect of varying the properties of the materials in the model should be investigated. In critical cases, the measured properties of the actual materials present should be obtained.
