There are wall wraps, and then there are wall wraps. Different users have different requirements and expectations for what they want their wrap to do.
In most cases we should, and can expect modern nonwoven textile wraps to control:
- Water – Stop rain from getting into the building during construction before cladding installation – Prevent damaging to insulation/internal services and finishes that aren’t designed to be exposed to the elements. Avoiding health and structural related damage
- Air – Once the building is finished air can impact/exacerbate water ingress, moisture build-up from water vapour, temperature ingress, sound ingress and fire/smoke pathways.
- Water Vapour – Condensing water vapour and high humidity can lead to health and structural damage in completed buildings.
- Not to Contribute to fire combustibility – limiting the amount of combustible material in key parts of our builds.
- Some woven glass/aluminium laminates are deemed to be non-combustible by the building code
- vapour permeable airtight wraps and plastic/foil laminate wraps usually are of low flammability rather than being non-combustible.
Sometimes we may be forced to compromise on some of these control layers, but we also need to pay attention to the minimum requirements in the NCC.
Things have become easier since the NCC 2019 which extended the list of materials that may be used wherever a non-combustible material is required or where the requirement to be non-combustible does not apply.
C1.9 Non-combustible building elements
(a) In a building required to be of Type A or B construction, the following building elements and their components must
(i) External walls and common walls, including all components incorporated in them including the facade covering,
framing and insulation.
(ii) The flooring and floor framing of lift pits.
(iii) Non-loadbearing internal walls where they are required to be fire-resisting.
(b) A shaft, being a lift, ventilating, pipe, garbage, or similar shaft that is not for the discharge of hot products of
combustion, that is non-loadbearing, must be of non-combustible construction in-
(i) a building required to be of Type A construction; and
(ii) a building required to be of Type B construction, subject to C2.10, in-
(A) a Class 2, 3 or 9 building; and
(B) a Class 5, 6, 7 or 8 building if the shaft connects more than 2 storeys.
(c) A loadbearing internal wall and a loadbearing fire wall, including those that are part of a loadbearing shaft, must
comply with Specification C1.1.
(d) The requirements of (a) and (b) do not apply to the following:
(iv) Termite management systems.
(v) Glass, including laminated glass.
(vi) Thermal breaks associated with glazing systems.
(vii) Damp-proof courses.
(e) The following materials may be used wherever a non-combustible material is required:
(ii) Perforated gypsum lath with a normal paper finish.
(iii) Fibrous-plaster sheet.
(iv) Fibre-reinforced cement sheeting.
(v) Pre-finished metal sheeting having a combustible surface finish not exceeding 1 mm thickness and where the
Spread-of-Flame Index of the product is not greater than 0.
(vi) Sarking-type materials that do not exceed 1 mm in thickness and have a Flammability Index not greater than
(vii) Bonded laminated materials where-
(A) each lamina, including any core, is non-combustible; and
(B) each adhesive layer does not exceed 1 mm in thickness and the total thickness of the adhesive layers
NCC 2019 Building Code of Australia – Volume One Page 67
In the Latest NCC 2019 some of the key additions that will help enable the construction of walls that can effectively manage water, air, vapour and thermal control are these materials:
- Sarking-type materials that do not exceed 1 mm in thickness and have a Flammability Index not greater than 5.
- Thermal breaks associated to glazing systems.
Prior to the NCC 2019 revisions, there was a conflict between meeting an:
- air tightness target leakage rate,
- water barrier and
- vapour control when only
- non-combustible elements could be used in type A and type B wall systems.
There never has been a wrap product available that meets the NCC requirement for non-combustibility, while at the same time as being an effective air and water barrier, combined with sufficient vapour permeability for use in cool and temperate climates.
Current products on the market today that are deemed non-combustible are made from pure aluminium foil laminated, an adhesive layer and a woven glass cloth. Aluminium Foil when reinforced can be very airtight, but these products are also extremely vapor tight (Incapable of allowing moisture vapour to escape/permeate to outside).
To get around this issue of vapour control, some aluminium foil based products in an attempt to make them vapour permeable have holes punched into them.
This is a hard balance to strike. Making holes in a material compromises its effectiveness as a water and airtight layer. There is a reason as to why its hard to find an umbrella for sale that has holes pre-punched in it.
On the other side of the equation, with aluminium foil type wraps, if the holes are so small and infrequent, these types of products still struggle as a vapour permeable underlay. Like single glazed glass windows, aluminium foil is a non-porous material and it is prone to condensation forming on its surface. Within the space of a year we have seen perforated deemed non-combustible sarking layer where quoted vapour permeance values have swung wildly from the realistic to the implausible and back again to just mildly astonishing.
Well established polyolefin textile air and watertight vapour permeable wraps can provide a far superior vapour permeability performance without compromising other important features.
If following a deemed to satisfy path using NCC 2016 for non-combustibility of wall elements then in most cases this will inevitably require some compromise of water tightness, air tightness or vapour permeability. The only realistic option to date has been to insulate external the non-combustible sarking, us a non-combustible vapour permeable sheathing board, or follow a performance solution / fire engineered pathway.
Moving on… Looking at the requirements of a water barrier under the AS4200.1: 1994 code for sarking you can see that almost all perforated aluminium based materials are unhelpfully classified as:
“Water Barrier – Unclassified.
6.4 Water barrier The water barrier classification shall be neither high or unclassified. The resistance to water penetration shall be determined, as required, by the method described in AS/NZS 4201.4. as follows:
(a) High The material shall only be classified as high if it passes the test,
(b) Unclassified When the material is not classified as high, it shall be unclassified.
In terms of AS4200.1 : 1994 classification, if a datasheet is reporting a product to be “unclassified”, it means it has not passed the water barrier test which is a simple 10cm standing head of water test.
The current version of AS4200.1: 2018 is more helpful to the user and the product will now be defined as:
“Water Barrier – Non-water barrier”
5.3.5 Water control classification
The water control classifications shall be determined as follows;
(a) Water barrier – if the membrane passes the test specified as AS/NZS4201.4.
(b) Non-water barrier – if the membrane fails the test specified in or has not been tested to AS/NZS 4201.4.
How then have things changed for classification of vapor permeance?
In the 1994 version of the code there was no classification of vapour permeance. Products were either a low, medium or high classification as a vapour barrier. Just because a material wasn’t a particularly fantastic vapour barrier does not imply that it is sufficiently vapour permeable.
The 2019 NCC Condensation Management DTS requirements for climate zones 6,7 & 8 now requires a vapour permeable membrane, In the updated 2017 version of A4200.1 vapour barrier and vapour permeable membranes using table 4. See below. (Southern Australia and Alpine areas)
We should only be seriously using class 4 products with a vapour permeance of >1.14µg/N.s Class 3 seems like a bit of a hangover from the old building code to cover the old “perforated” foil wraps with holes punched into them(Perforated sarking).
When considering that a typical paper faced plasterboard has a permeance of around 2.0µg/N.s then it is a good rule of thumb to ensure the external wrap is at least as permeable (>2.0µg/N.s ) as the interior plasterboard lining. In order to control moisture successfully inside a wall system the vapour permeability of materials are increasingly vapour permeable toward the outside of the wall system.
VAPOUR CONTROL MEMBRANE (VCM) CLASSIFICATION
Vapour permeance (see Note)
|ASTM-E96 Method B Wet Cup - 28 C 50%RH
NOTE: Vapour permeance is the inverse of vapour resistance. It shall be calculated as follows:
Vapour permeance μg/N.s = 1/(Vapour resistance MN.s/g)
Almost all building wraps are combustible, and the caloric value (the heat produced by the complete combustion) of wraps is quite small compared to the volume of materials that are non-combustible in a wall system.
From NCC2019 building wraps/membranes that are less than 1mm thickness with a flammability index of no greater than 5 used wherever a non-combustible material is required. This means you can now have your cake and eat it. For applications using NCC 2016, in most cases, your only solution is to discuss with your fire engineer about a performance solution.
Using Vapour Permeable Wraps at the Bottom of wall detailed with air tightness.
Getting an air tight barrier at the bottom of a brick veneer wall system using damp-course as your air barrier, with an overlap from a vapour permeable wrap.If you have any questions or about this contact please contact us.