Well it’s wet, but what if it’s not leaking?

Moisture issues in existing buildings, some of them only a few years old, are a rising problem.

Black mould on window and surroundSuch concerns come regularly to InsideOut’s attention, often bringing a combination of potential weather tightness issues together with internal moisture management problems.

Sometimes there’s visible mould, sometimes cracks in concrete, or rot in the framing. Often leaks are a part of the story around joinery, at difficult details, or near changes in materials. Keeping precipitation outside the building has unfortunately become a well known issue in our New Zealand construction history. Less commonly though is the prevalence of internal or interstitial condensation reported on.

When affected buildings are to be remediated the debate around air, vapour and water barriers, ventilated or unventilated cavities and types of insulation in wall and roof constructions is a regular feature of design. Should we include them and where should they be positioned in envelope constructions? How should the heating, cooling and ventilation be handled? After all nobody wants to remediate a building to discover several years on that things are not as they had hoped and the building needs additional work.

Indoor environment is a condensation risk

In InsideOut’s experience the hourly temperature and humidity data both inside and outside the building play a much bigger part in the prevalence and risk of condensation than often appreciated. Simplifying this too far, for example to monthly means and standard relative humidity levels, can allow significant issues to remain undetected. Understanding the function of the building and the way in which it will be occupied is vital. Luckily for us, dynamic thermal and hygrothermal modelling can take a detailed look at the cumulative effects of condensation based on an annual hour by hour set of conditions. This allows us to provide advice on:

  • Condensation on glazingDesign of construction details; membranes, insulation, cavities and structure
  • Location and prevalence of condensation
  • Thermal bridging
  • Indoor and outdoor climate profiles
  • Mould risk and health indicators
  • Material longevity and performance
  • Opportunity for drying of condensation
  • ISO 13788 – monthly and hourly


Providing for occupancy

Steam rising from potConstruction choices are not the only approach to improving condensation risk. Extract of internal moisture at source, ventilation and internal temperature also play a part so we factor in the potential indoor environmental control options which can be realistically considered. A lot depends on the function of the building and the way it is occupied. The moisture load arising in an office building is usually small and limited to daytime hours. A home, whether that is a house or an apartment, or a hotel might have much higher vapour production and very variable levels of heat provision. This combined with reduced ventilation or cold indoor temperatures is a recipe for the construction to be put under considerable pressure.

We’re all familiar with condensation, ‘crying windows’, and brightly and darkly coloured mould growth. These affect occupants in a myriad of ways from the daily wipe down of walls and windows to the exacerbation of health issues. Part of the answer is affordable heating and appropriate ventilation together with reduction of the availability of moisture to the indoor environment. All InsideOut’s designs take this into consideration and are occupant aware.

Building envelope design

Spalled concrete due to corroded rebarBuilding materials all respond differently to moisture transport and to the opportunity to dry. Some are impermeable; some tend to act as a sponge; some allow moisture to pass without issue; and some easily suffer performance degradation. The knock on effects depend very much on the material, for example:

  • Wood can hold a remarkable quantity of moisture and above certain limits mould spores can start the process of rotting.
  • If some types of insulation hold moisture the R-value starts to reduce.
  • Ferrous metals can of course rust.
  • Plasterboard  can ‘blow’ or lose its rigidity.
  • Moisture conditions in concrete can cause reinforcement bars to corrode in the worst cases spalling the material itself.

Not only can moisture affect the materials’ performance but in some cases we can get vapour condensing on the material surfaces within a roof or wall. Then it’s important to know what will happen to it. Can it dry? Will it run or drip? Where will it end up if it does? Even better, it’s important to know how to design the structure to avoid it.

The first thing is to know how the construction is likely to behave. If there are already problems in an existing building a good first step is to identify how and where they have arisen. Simulation can help here and allow a better understanding of the issues to overcome. Then, with a proposed option, a second scenario can be tested to appreciate its benefits. Sometimes, where indoor or outdoor conditions are particularly harsh, it is not possible to rely solely on the various building elements to avoid issues of moisture. Then measures around occupation become paramount.

Comfortable, dry, healthy buildings are unfortunately not a given in New Zealand but with care for occupants and an understanding of their needs and abilities, and attention to the choice of materials and design of thermal envelopes the future is looking a lot brighter.

Author: Ruth Williams – Principal, Buildings That Work InsideOut Ltd

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