Understanding Permeance To Ensure Permanence

“Leaky homes will cost $11.3 billion to fix,” screamed the headline in the New Zealand Herald. The alarming article went on to quote New Zealand Building and Construction Minister Maurice Williamson, who had commissioned a PricewaterhouseCoopers report on the country’s building regulations.

"This leaves thousands of New Zealanders in a terrible position - they may not be able to borrow the money to repair their homes, or to sell them, so their single most important asset is decaying in front of their eyes," Mr Williamson said.

Could such a dire situation arise in Australia? Are our own practices and regulations sufficient for our building sector and climate extremes? Indeed, there are so many serious considerations surrounding moisture control in our building sector, Building Knowledge asked Jesse Clarke, CSR Building Scientist, for his clarifying thoughts. 

To best design buildings with adequate moisture control within building assemblies the industry has a simple set of rules:

  • Exclude rain
  • Control rain water absorption
  • Control air flow
  • Keep water vapour out
  • Allow the system to dry if water or water vapour get in

To help these functions succeed, Clarke said the correct choice of materials, equipment and systems is essential. The building enclosure should be forgiving so if it does get wet in any manner, it can dry to either the interior or to the exterior, or both.

This raises several key questions:

Will exterior cladding layers be permeable or non-permeable? Will permeable or non-permeable pliable building underlays be required? Will an air barrier be required? What material will form the air barrier? What will the thermal resistance of the building envelope be?

“Decisions relating to ‘keeping the building up’ from the perspective of wind loading and foundations should take priority; and decisions relating to ‘keeping the rainwater out’ should also reflect the local conditions. Zones that see a great deal of rain, like the tropical north, will likely need a more robust approach to rain control than other zones,” Mr Clarke said.

“Once structural design decisions and rain water control decisions are made, more specific decisions related to wall materials, roof design and foundation design can be made. These decisions also need to reflect local conditions.”

Clarke emphasised that while insulation use helps reduce the risk of condensation on the cold surfaces of materials, it won’t resolve all issues.

“This is where building performance knowledge is required to ensure vapour control membranes and air barriers are correctly specified and used. What we need to do is look at implementing proposed changes to insulation standards and appropriate building membrane use,” he said.

“Not all membranes work in all climates to allow drying to take place due to our mixed climates across Australia. Membranes that work in Tasmania could cause problems if used in the same way in Darwin and vice versa.

“In very cold climates generally you’ll find water vapour travelling in the same direction throughout the year, from inside to outside, whereas in temperate regions it may vary depending on whether it’s summer or winter, and if a building has air conditioning.

“If you use the wrong membrane in the wrong location you could trap moisture in the structure by restricting drying through vapour diffusion and cause timber rot or steel corrosion.

“If the construction assembly naturally wants to dry to the outside environment caused by the specific nature of the internal and external temperature and humidity conditions, this may be restricted by the use of a vapour barrier on the outside of the studwork and the wall will take much longer to dry,” Mr Clarke said.

Generally speaking, water vapour will migrate to the cooler, less humid side of the wall. Therefore, highly vapour permeable components should be used between the moisture susceptible structure and the cooler, less humid side of the wall.

To give an idea of this, Figure 3 indicates the direction in which vapour diffusion is likely to occur in a cool climate; that is, outwards towards the cavity where the water vapour is carried away by the airflow in the cavity.

In tropical climates in a house with air conditioning (Figure 2) the direction of vapour diffusion is likely to be inwards to the plasterboard layer where the moisture is evaporated into the indoor air. Cold climates usually dry to the cavity and tropical climates dry to the internal air; high vapour resistant products or coatings should not interrupt this process.

Temperate or mixed climates is where it gets more confusing, the vapour migration might change direction in winter and summer, possibly resulting in vapour permeable materials being required on both sides of the framing system.

Broadly speaking, all materials are vapour permeable to some extent. Some are more vapour permeable than others and will promote much faster drying. Vapour diffusion, measured in Pascals (Pa), is driven by differences in temperature and humidity on either side of the material, creating the driving force which makes vapour diffusion occur. Permeance of a product is measured in µg/m².s.Pa which is the volume of water that can be transported per square meter of product area in one second, per unit of pressure.

To illustrate this, the relative permeance of some common materials is listed below:

Density Typical Vapour Permeance Typical Vapour Resistance
kg/m³ µg/MN.s MN.s/g
Brickwork Common (110mm) 1700 0.18 5.5
Glasswool (90mm) 12 - 115 2.22 0.45
Plasterboard (10mm) 800 1.67 0.6
Rendering (10mm) 1600 1 1
Timber stud (90mm) 500 85.47 0.0117
Oriented Strand Board (10mm) 553 - 616 0.14 – 0.15 6.7 – 7.2
Sheathing Plywood (10mm) 500 0.1 – 0.66 1.5 – 10.0
Aluminium foil membranes 2680 0.001 1000
Enviroseal ProctorWrap 4.0 – 4.55 0.22 – 0.25
Metals and metal cladding 7830 0.0001 10000
Paint (Emulsion) 2 0.5
Paint (Gloss) 0.06 15
Paint (Vapour resistant) 0.04 25

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