News From Eight

New technical guidance: Assessing condensation and moisture in buildings (BS5250:A1 2016)

14/09/2016

It has been recognised by the recent amendment to BS5250:2011 ‘Code of practice for control of condensation in buildings’ that the simplified Glaser method of condensation prediction is not sufficient under many circumstances and more sophisticated modelling tools should be used instead.

The Glaser method only gives part of the picture, regardless of how simple or complex the construction.  Eight Associates uses the WUFI software in its hygrothermal analysis to provide more accurate assessments of the risk of condensation and advice on how this risk can be corrected at the design stage.

Condensation control of all aspects of the building envelope is important to protect the building fabric and the health of its occupants.  BS5250 has recommended that architects assess the risk of surface condensation, mould growth and interstitial condensation using the criteria set out in BS EN ISO 13788:2012, in particular:

1) The design of the structure and the heating system should ensure that, over the coldest month, the average relative humidity at internal surfaces does not exceed 80% – the limit for mould growth.

2) Any interstitial condensation, which might occur in winter, should evaporate during the following summer, preventing an accumulation of moisture year on year.

3) The risk of degradation of materials should be assessed in terms of the maximum level of condensate, which might occur.

However, BS5250 has now highlighted the need for greater accuracy of measurement:

‘Designers should be aware that BS EN ISO 13788 considers only the risks arising from the diffusion of water vapour through the building fabric; it does not take account of the much greater risk of condensation occurring as a result of air leakage, which transports water vapour through gaps, joints and cracks in the building fabric.’

The Glaser method is a standard static interstitial moisture calculation, developed in 1958, used to adhere to BS EN ISO 13788. To date the Glaser methodology has been widely used to assess the amount of water vapour likely to be generated within the building and the resultant increase in internal vapour pressure above that of external air. The simplified calculation used is based on average monthly temperatures, vapour pressure and steady state conduction of heat to determine if critical condensation points are reached within one year.

The Glaser method identifies vapour diffusion; how easily water vapour can pass through the fabric of the building. This method assumes however, vapour moves in one direction only, from inside to outside, and omits driving rain completely from its calculations. Absorption and porosity are not measured, which also means the potential risk attributed to moisture storage is omitted.

The limitations of this method, which offers a simplified average, have become apparent with the advent of more sophisticated analysis and assessment methods. Another standard, BS EN 15026:2007 (‘Hygrothermal performance of building components and building elements. Assessment of moisture transfer by numerical simulation.’) increasingly is being seen as the way forward.

According to the BS5250 amendment, BS EN ISO 13788 is not appropriate for solid masonry walls with internal insulation. Whilst BS 5250 stops short of definitively stating when it is necessary to assess a wall with the BS EN 15026 methodology, it highlights the factors that point to a more complex analysis being required, which include:

  • Absorbent masonry
  • The condition of the masonry including the existing moisture content
  • South or south west facing orientation
  • High exposure to driving rain.

BS EN 15026 relies on more sophisticated hygrothermal assessment, which takes into account heat, vapour and moisture transfer through the elements of a building, and can be used to provide an accurate measure of temperature, relative humidity and water content within the elements of a building over a specific time period.

Hygrothermal analysis considers different climatic conditions to realistically evaluate the potential moisture levels in building components, using sophisticated computer modelling to simulate the interactions between building envelopes, building services and the use of buildings. This has the key benefit of identifying weaknesses at the design stage, which can then be corrected to optimise the building fabric design and longevity.

Eight Associates uses WUFI® software, which is fully compatible with BS EN 15026. WUFI®, an acronym for Wärme Und Feuchte Instationär (‘heat and moisture transiency’), provides detailed heat and moisture calculations and is used to dynamically predict moisture movement and storage as well as condensation. It delivers exceptional insight, with the architect able to see a prediction over a given period of years, worst-case scenarios and the drying out of the fabric build up.

An additional benefit of using the WUFI® software is that external weather including driving rain and solar radiation is predicted in a cycle and the specific internal environment that the building will be exposed to can also be selected. This means that high performance vapour control and vapour permeable membranes can be accurately positioned to ensure a healthy building fabric, for roofs or walls.

So, whilst the Glaser method is passable for Building Regulations, WUFI® is the only tool that can accurately tell you how a construction will perform in terms of moisture and heat transfer.

Comparison at a glance: Glaser versus WUFI methods

Glaser WUFI Pro
 

One-dimensional calculation

 

 

✓*

 

Moisture transport by vapour diffusion

 

 

 

Moisture transport by liquid transport (surface diffusion and capillary flow)

 

 

 

Storage of moisture within components

 

 

 

Material properties affected by moisture content

 

 

 

Variable internal and external conditions in a month

 

 

 

Solar radiation

 

 

 

Driving rain

 

 

 

Air movement from the building into the component through gaps

 

 

*A two-dimensional calculation is also available with WUFI 2D.

(Click for enlarged view of comparison table: risk-of-condensation-assessment-glaser-vs-wufi-methods)

Conclusion

The Glaser method should be the basic minimum health check for any construction. However, it is important to recognise that this can only give you part of the picture, regardless of how simple or complex the construction.

Eight Associates uses the WUFI® software to give its clients greater insight at the early design stage, in terms of accuracy and scenario-modelling, of the risk of condensation and how to address this risk if necessary. We’ve had projects where a relatively basic construction was fine when assessed under the Glaser method, while the WUFI® analysis identified potential risks, which were subsequently reduced.

Please get in touch to discuss how this hygrothermal assessment can benefit your project, to identify weaknesses early in the design process and optimise the building fabric’s design and lifespan. We’d be delighted to talk you through our case studies, or to discuss your requirements.