WUFI Part 1 – What is WUFI and when is it appropriate to use it?

Wufi is a dynamic hygrothermal simulation tool, that predicts the way heat and moisture move through a construction. It can be used to assess the risk of moisture accumulation within any part of a wall, floor or roof build-up, and to inform design decisions that reduce these risks.

In the last five or ten years, the use of tools like Wufi has increased and with improvements in computing power, the software, and the relevant research it has become widely accepted.

Until the introduction of dynamic hygrothermal simulation, the only calculation tool for assessing moisture in construction was the ‘Glaser Method’, as codified in BS EN 13788. This method is still widely used, and is commonly referred to as an “interstitial condensation check”. If you request such a check from the manufacturer of an insulation that is being specified for example, this is the method they will almost certainly use; it is about as easy as a U-value calculation, with the right software. Unfortunately, it is often not appropriate to use this simple method. BS EN 13788 gives a list of factors and situations that preclude its use. These include:

Where there is moisture ingress from the ground or from precipitation, Airflow from the interior of the building into the structure (i.e. imperfect airtightness), variation of material properties with moisture content, and liquid moisture transfer by capillary suction and surface diffusion.

One or more of these limitations apply to many common building materials. For example, brick and other masonry products store significant amounts of both heat and moisture, and liquid water can move through them by capillary suction; their thermal conductivity and vapour permeability can also vary with moisture content. This means that most constructions that incorporate masonry cannot be meaningfully assessed for moisture risk using the Glaser method. For highly ventilated brick rain screen systems, the method may give a reliable result, but in our work we have found this is very dependent on the level of ventilation achieved in the cavity.

Retrofitting of existing buildings, many of which have solid masonry walls, is high on the national agenda at the moment, and this is a particular example of where the Glaser method is unlikely to be reliable.

Where the Glaser method cannot be applied, it is necessary to use a more sophisticated tool such as Wufi that is able to take all the above effects into account.

Next time you request an interstitial condensation calculation, check what method has been used, and consider whether it is valid. If you need Wufi calculations, get in touch to find out how we can help.


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