Research tackles condensation

This Issue This is a part of the Weathertightness feature

By - , Build 138

BRANZ’s new vapour control in walls project will define the condensation limit for typical New Zealand walls and clear up confusion about the role of vapour barriers and vapour retarders.

Figure 2: Cavity ventilation has been shown to have an effect on drying rates.
Figure 1: Airflows in walls. All have the potential to transport moisture.

TYPICALLY, NEW ZEALAND walls do not require a vapour barrier (see Build 99 Getting clear on vapour barriers and underlays), and our houses are not required to meet a particular level of airtightness.

Some areas of the world, however, have one or both of these requirements, in part to prevent moisture damage from condensation within walls. In such cases, the building envelope contains layers that specifically control transport processes – a vapour barrier for diffusive and an air barrier for convective processes.

To date, New Zealand has seen few cases of condensation damage in typical walls, arguably validating its approach to vapour and air control.

Project to identify limits

However, we know from previous work at BRANZ (see Build 127 Changing the air indoors) that houses are being built more airtight and with greater levels of insulation than before.

This may mean that typical New Zealand wall construction is edging closer to a point where condensation issues may be created. However, it is currently unclear just what combinations of building detail and indoor and outdoor climate may tip these walls into a damaging condensing regime. The vapour control in walls project aims to define these limits.

Additionally, the project will provide specific guidance in cases where a wall contains multiple layers of insulation, for example, glass wool in the stud space and a polystyrene sheathing on the outside of the framing. These configurations potentially present a risk of condensation accumulation within the wall unless careful thought is given to their design.

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Expands on WAVE research

This work has not previously been done in New Zealand because:

  • airflow processes in lightweight timber-framed walls have not been fully understood
  • the ability to model these airflows and the effect on the wall performance has been limited.

Recent work in BRANZ’s Weathertightness, Air quality and Ventilation Engineering (WAVE) project has measured airflows in the stud space of walls, and although these are likely to have minimal effect on a wall’s thermal performance, they could transport significant amounts of moisture out of the wall (see Figure 1).

The vapour control in walls project will use and expand on the measurements from the WAVE project to understand their role in moisture management.

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Modelling issues

There is an industry-wide increase in the use of hygrothermal software packages such as Fraunhofer’s WUFI to design wall systems. While this is to be encouraged, it is important to understand the limits of the software’s capability and that careful attention is given to the settings and boundary conditions used in any simulation.

For example, a one-dimensional model with no capability for modelling airflow is perfectly adequate for an airtight mass wall but may not be the best choice for lightweight timber-framed construction that is not necessarily airtight. In general, 2D analysis is preferable for New Zealand walls because it allows the moisture-buffering effect of the timber framing to be included.

Previously, BRANZ, as a collaborative partner with Germany’s Fraunhofer Institute, developed the capability to include cavity ventilation into WUFI models (see Figure 2). In this project, we will further develop the software to account for multiple airflow processes within the wall.

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International modelling standards

There are a few international standards that recognise the use of numerical modelling for designing against moisture problems, among them ASHRAE Standard 160 and BS EN 15026. Both of these will continue to be developed as moisture modelling techniques mature, but at the moment, there is still an onus on the designer to ascertain adequate boundary conditions, and airflow processes are notably absent from the standards.

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Aim to help designers’ selections

As well as the computer modelling work, a number of tests on timber-framed and steel-framed walls will also be performed to experimentally assess the condensation risk in walls with multiple layers of insulation and to verify any findings from the modelling work.

A successful outcome will allow designers to use or reference a robust method when they are selecting a vapour control, air control or insulation layer for their walls and prevent any potential condensation damage.

Whether any changes to current construction practice are necessary will be determined as the project progresses, but even if they are not, we will know how far current practices can be pushed for airtightness, insulation levels and even climate change.

The first results from this project are expected to be available in the second half of 2014, with the project finishing in 2016.

Figure 1: Airflows in walls. All have the potential to transport moisture.
Figure 2: Cavity ventilation has been shown to have an effect on drying rates.

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Articles are correct at the time of publication but may have since become outdated.

Figure 2: Cavity ventilation has been shown to have an effect on drying rates.
Figure 1: Airflows in walls. All have the potential to transport moisture.

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