Do walls actually breathe?

This Issue This is a part of the Managing moisture feature

By - , Build 133

It is common to hear that walls need to breathe to control moisture and that filling stud cavities with insulation might hinder the passage of air and moisture inside the wall. Is this right?

Figure 1: Range of possible airflows inside walls.
Figure 2: Measured and calculated ventilation rates inside insulated cavities in the BRANZ test building. The units are litres of air/sec/m of wall.
Figure 3: Ventilation rates measured in the space between cladding and wall underlay.

IN THE EARLY DAYS of New Zealand house building, it was common to drill holes through top and bottom plates to allow air movement.

Now the concern is that air flowing through these holes could transport moisture from the subfloor into the roof space and maybe the holes would compromise the thermal insulation performance.

This raises the question: what role does airflow in wall cavities play in the overall moisture balance of the wall and does it affect insulation performance?

Ventilated wall cavities aid drying

The widespread use of ventilated wall cavities in the wake of the leaky building crisis reintroduced ventilation drying behind claddings to deal with water leakage through claddings.

BRANZ helped develop the science behind how cavities work, with field studies in ventilation and drying rates using tracer gas methods. It established an engineering basis for cavity design, and this has incidentally provided the tools to delve deeper into walls.

It is now possible to find out if there are circumstances where moisture from living spaces could accumulate in the insulated spaces. Currently, there is little evidence of this problem in New Zealand, but new insulation and underlay materials are available with a wider range of air and vapour tightness characteristics, and BRANZ considers it prudent to re-evaluate this particular risk.

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Airflow paths within walls

The range of possible airflows inside of walls is shown in Figure 1. All but flow path J are small cracks between materials nailed or screwed together and are an incidental consequence of the way in which buildings are put together. Importantly, though, these ventilation paths play a big part in managing water leaks through claddings. Those marked in blue are less well understood but are certain to play a part in moisture entering and leaving the insulated cavity.

Figure 1: Range of possible airflows inside walls.

By measuring the airtightness of the insulated cavities in the BRANZ weathertightness test house, it has been possible to understand the flow paths shown in blue (paths A to G) and estimate the ventilation rate – the total amount of air flowing through the volume – in the insulated space. A series of day-averaged ventilation rates were calculated using the airtightness results and compared with ventilation rates measured directly with tracer gases.

The reasonable agreement between measured and calculated ventilation (see Figure 2) gives us the confidence to see if there are circumstances where indoor moisture might accumulate as condensation in the insulation or elsewhere in the wall.

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Cavities greatly improve ventilation

For comparison, ventilation rates measured behind claddings (with and without cavities) are shown in Figure 3 as bars representing several hundred day average measurements in the BRANZ experimental building. These ventilation rates are a result of airflows through paths H, I, J and K, shown in red.

It might be surprising that there is such a large difference – up to 1,000 to 1 – between ventilation rates in cavities with engineered ventilation openings, for example, brick veneer cavities, and in the spaces behind monolithic or large sheet claddings with no engineered provision for ventilation.

That, and the difference in drainage capability, explains the different tolerances to rainwater leakage of walls. However, it raises the question of the influence air movement has on the ability of walls to deal with damp air from inside the building. It’s early days, but the following is clear from ventilation measurements in the insulated cavities:

  • From an energy perspective, the airflow in the insulated cavity represents, at most, a few percent of the heat flow through a wall with R=2 m2C/W insulation and is therefore not that significant with current minimum insulation requirements.
  • From a moisture perspective, moisture carried by ventilation can be at least as significant as moisture diffused through wall linings. This means that moisture carried by the air has to be understood and included in any moisture-related prediction.

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What about vapour and air barriers?

Internationally, there is considerable discussion around how, and where, vapour and air barriers should be used to control moisture entering walls in airflow or by vapour diffusion.

One thing is clear, however. There are downsides to the unnecessary application of moisture barriers – they can clamp down on useful drying paths in walls. It will be interesting to see if further research finds an argument to refine recommendations for using vapour control layers in New Zealand house construction.

The current recommendation is to leave them out except in rooms with a high internal moisture load like a spa or swimming pool, or where the building is located in an alpine area.

Figure 2: Measured and calculated ventilation rates inside insulated cavities in the BRANZ test building. The units are litres of air/sec/m of wall.
Figure 3: Ventilation rates measured in the space between cladding and wall underlay.

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

Figure 1: Range of possible airflows inside walls.
Figure 2: Measured and calculated ventilation rates inside insulated cavities in the BRANZ test building. The units are litres of air/sec/m of wall.
Figure 3: Ventilation rates measured in the space between cladding and wall underlay.

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