While the management of moisture in the building envelope is certainly not a new topic, the recent introduction of Condensation Management within NCC 2019 Vol 1 (Commercial) and Vol 2 (Residential) is finally starting to push a greater consideration of durability and mould growth.
Located under Health and Amenity, FP6.1 Condensation and Water Vapour Management in Vol 1 and P2.4.7 in Vol 2, there duel Performance Requirement is to identify the “Risks associated with water vapour and condensation must be managed to minimise their impact on the health of occupants”.
In the drive towards better buildings envelopes, we think it’s a great introduction that can readily highlight the variety of interstitial condensation or moisture-related issues in wall designs and product specifications.
In order to avoid the occurrence of moisture-related issues, a quick and easy way to assess the amount of water vapour likely to be generated within the building envelope is now required in colder Australian Climates of Building Classes 1, 2 and 4.
However, today’s options are not great given the complexity of numerical models, while a great deal of specialism is required resulting in cost hurdles to design teams. Therefore, to assist the market in this transition to better building envelopes, we have added a new feature that automatically undertakes ISO 13788:2012 interstitial condensation calculations.
Give the grand title ‘‘Hygrothermal performance of building components and building elements. Internal surface temperature to avoid critical surface humidity and interstitial condensation. Calculation methods’, ISO 13788:2012 (Or the Glaser Method or Dew Point Calculations) sets forth calculation methods to assess the risk of surface condensation, mould growth and interstitial condensation, which determines three criteria for assessment.
- 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.
- Any interstitial condensation, which might occur in winter, should evaporate during the following summer, preventing an accumulation of moisture year on year.
- The risk of degradation of materials should be assessed in terms of the maximum level of condensate, which might occur.
So the good news, is that you can now automatically identify potential condensation issues in your wall system on Speckel. But, this comes with some major caveats as there are some critical limitations to the this method that need to be understood by all users.
First up, while recognised as an established International Standard and methodology, it is a simplified procedure, based on average monthly temperatures, vapour pressure and steady-state conduction of heat to determine if critical condensation points are reached within one year.
As a result, it ignores some major design challenges, such as moisture and heat accumulation in materials and built-in moisture. Furthermore, it does not consider the dependence of material properties on humidity and temperature, the capillary transport of liquid water and the rising damp and therefore misses the potential risk attributed to the aspect of moisture storage.
To be a Verification Method under NCC 2019, in order to meet the specific Performance Requirement stated above, a calculation methodology needs to be adopted that assesses the effects of
- indoor and outdoor temperature and humidity conditions; and
- heating and cooling setpoints; and
- rain absorption; and
- wind pressure; and
- solar radiation; and
- material hygrothermal properties,
determines that moisture will not accumulate—
- interior to the primary water control layer within a building envelope; or
- on the interior surface of the water control layer
Therefore, ISO 13788:2012 cannot be used and justified as an approach to modelling the accurate representation of moisture in wall systems for NCC 2019 requirements as its accuracy and reliability can be questioned on many fronts. Indeed, neglecting moisture transfer in the liquid phase may even overestimate the risk of interstitial condensation in many cases.
However, to give the widest possible audience the opportunity to gauge the potential risks of excessive moisture, we believe it is better used as a simple check to encourage a greater adoption of better moisture control strategies rather than ignoring its simplified merits. Furthermore, where a moisture potential is highlighted, a user may find greater value in undertaking a formal Verification Method in the form of a hygrothermal modelling, to consider different climatic site and conditions to realistically evaluate the potential moisture levels in building components.
So give it a try and provide your feedback. Is this a useful addition to Speckel or a distraction from the real-world complexities of moisture management?