Building Wraps, Class 4, Water Control, Condensation, Reflective, What the?

Hopefully this is educational so you can use as a reference to determine what is best for your situation.

With the increasing building regulations and changes in the Building Code, we thought it was timely for a discussion on what it all means.  Different people/different terminology, they get called water control layers, pliable building membranes, wraps, sarking, weather resistant barriers WRB.

If we start with the NCC or Building Code 2022;

  • Building Code – Volume 1 2022
    • F3D3 Sarking – refers you to AS 4200.1 & AS 4200.2.
    • F8D3 External wall construction – pliable building membrane, talks about the AS, location, permeability, potentially a drained cavity, material open or closed to vapour & reflective wraps must have an air gap.
  • Building Code – Volume 2 2022
    • H1D7 & H2D6 Roof and wall cladding – refers you to the Housing Provisions
    • H1D9 Condensation management – refers you to the Housing Provisions
  • Housing Provisions 2022
    • 7.5.2 Timber wall cladding (3) (c) compliance with AS 4200.1
    • 10.8.1 same as F8D3 above

Terminology;

  • Air control membrane: A membrane installed to limit air transfer between each side of the membrane.
  • Primary insulation layer: The most interior insulation layer of a wall or roof construction.
  • Pliable building membrane: (from the BCA) A water barrier as classified by AS 4200.1.  (from AS 4200.1) A material that is able to be folded back on itself without causing structural damage to the product that effects its material properties.
  • Thermal control membrane: a membrane with a surface emissivity (reflective insulation) and/or material R value (bulk insulation) intended to reduce heat transfer.
  • Sarking-type material: A material such as a reflective insulation or other flexible membrane of a type normally used for a purpose such as waterproofing, vapour management or thermal reflectance.
  • Vapour control membrane: a pliable building membrane designed to either allow or restrict the transfer of water vapour across the membrane.
  • Vapour permeance: The degree that water vapour is able to diffuse through a material, measured in μg/N.s and tested in accordance with the ASTM-E96.
  • Water control membrane: a membrane intended to collect and discharge any water that penetrate a building envelope or cladding.

AS 4200.1 Pliable building membranes and underlays;

  • Classifies membranes in several categories to ensure they are “fit for purpose” or suitable for the intended application.
  • Emmitance classification, (or how reflective the product is)
    • IR Reflective has an emittance of ≤ 0.05
    • IR Semi-reflective has an emittance of > 0.05 to ≤ 0.15
    • IR Non-reflective has an emittance of > 0.15
  • Vapour control classification, (how easy water vapour can pass through), permeance measured in μg/N.s which is micrograms of water passing through a material per Newton second of force applied
    • Class 1 – Vapour barrier – 0 to <0.0022
    • Class 2 – Vapour barrier – 0.0022 to < 0.1429
    • Class 3 – Vapour permeable – 0.1429 to < 0.1403
    • Class 4 – Vapour permeable – 0.1403 to with no maximum
  • Water control classification, tested in accordance with AS/NZS 4201.4
    • Water barrier (it passed the test)
    • Non-water barrier (it failed the test)
  • Flammability classification, tested in accordance with AS 1530.2
    • Low, a flammability index ≤5
    • High, a flammability index >5
  • Air control classification, (how air tight or resistance to air passing through it a product is) tested in accordance with ISO 5636-5
    • Air barrier – Air resistance of ≥0.1 MNs/m3
    • Non-air barrier – Air resistance of ≤0.1 MNs/m3

So what is this really all that about?  Condensation mainly and how to manage it, see our post on Condensation!

It is important to note, you cannot stop condensation from occurring, you can take steps to reduce the likelihood of it occurring and plan to manage it with drainage & ventilation to assisting in drying out the materials to avoid mould and decay in your home.

Next thing to talk about is “direction of flow”?, please read more here!

So, the climate will determine the location of the wrap, membrane, control layer etc. we want to use these materials to either prevent passage of air or maximise the potential of it occurring, we don’t want to use materials that are not compatible (say a permeable wrap with impermeable insulation or sheathing board).

Remember;

  • Vapour control: usually a product that inhibits vapour movement to stop vapour from the outside.
  • Vapour permeable: to facilitate vapour movement from the inside to help dry the building out.
  • Thermal control: to keep things warm and not reach dew point temperature.
  • Air control: vapour is airborne moisture, hence if we control air movement we can prevent vapour getting into our walls & roofs to reduce the likelihood of interstitial condensation.
  • Controllable ventilation: can assist in removing vapour if used appropriately.
  • Smart or Intelligent membranes: that can restrict or facilitate the flow of vapour.

The Building code talks about these concepts, some things are mandatory, some are only if you install etc.

The proposed 2025 building code considers a requirement for a drained and ventilated cavity if you don’t have a control layer. If it’s your licence or reputation I would suggest planning for control layers and a drained and ventilated cavity.

What products are available and their characteristics, we have produced a list of materials for you to review & consider if correct for your application.

Who is involved in the process of designing, specifying and building?

  • Building Designer
  • Passive House Consultant
  • NatHERS Assessor
  • Blower Door Tester
  • Builder
  • Carpenter

Ask them some questions about any of the above and see what they say, they may have an excellent understanding or may be new to risks associated with where the Building Code is taking us.

If you don’t get the responses you were expecting contact us to help you with your project.

So what would we do or recommend;

  • The answer is “well that depends on”
    • What climate
    • What construction methods & materials
    • How you plan to manage Condensation, the strategy you will implement
    • The budget
    • Ventilation provided
    • Occupant behaviour

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