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Exterior Cladding System Design for Weathertight Medium Density Housing

The NZ Building Code sets out objectives, functional requirements, and performance requirements for all buildings; how these are met is up to an individual building owner and their designer.

The traditional weathertightness model

The traditional model for the weathertightness design of wall claddings (apart from masonry veneer) used a simple approach based on two lines of defence (or layers): a 1st line of defence on the outside (the wall cladding) with a 2nd line of defence behind (building paper).

In this approach, the idea is to keep water out and to have a back-up layer in case it does get in. If ‘keeping water out’ is effective, then the back-up layer will rarely be used. Claddings were traditionally designed to shed water effectively, with underlays and flashing systems providing drainage of occasional leaked moisture back outside.

The 4Ds approach

The 4Ds approach is a more comprehensive model of weathertightness that goes beyond the simple two lines of defence concept by identifying the key qualities required for a wall to be weathertight.

The concept, borrowed from Canadian research, is known as the 4Ds of weathertightness design (also as mentioned in the External Moisture document published by DBH in 2006):

  • Deflection: Shed water by a cladding system, including deflecting devices such as eaves and ‘weathering’ deflectors
  • Drainage: A back-up system to direct water that may bypass the cladding back to the outside
  • Drying: Remove remaining moisture by ventilation or diffusion
  • Durability: Provide materials with appropriate durability

Marshall Innovations' solutions

Flexible Wall underlay is designed and intended to act as a secondary line of defence, and it is important to select the correct underlay for the building, cladding type, or application.

Marshall Innovations offers two varieties of flexible, non-woven synthetic wall underlay:

  • TEKTON: A ‘non-absorbent’ underlay suitable for when installed behind a cavity, with ‘absorbent’ cladding types. TEKTON is a breathable, spun-bonded polypropylene, with excellent water holdout ability, which is also vapour permeable and allows moisture to escape.
  • HYDRA Wall: An ‘absorbent’ underlay suitable for use in a direct-fix cladding system, with using ‘non-absorbent’ cladding types. HYDRA is also a flexible, non-woven and breathable underlay which assists/controls and releases/diverts moisture in the event of water-penetration or moisture condensation build-up within the cladding system.

Both TEKTON and HYDRA Flexible Wall Underlays are supplied with their own BRANZ appraisals and guarantee of meeting or exceeding NZBC requirements as set out under B2 Durability, and these products are readily available through all leading merchant channels throughout NZ.

Alternatively, a rigid air barrier is sometimes selected for use in Kāinga Ora projects for providing this barrier to air-movement and, as a secondary barrier, to water ingress — as this can contribute to bracing or fire-resistance.

  • SUPER-STICK flashing tape and TRADE SEALS are high-performance products also supplied by Marshall Innovations, which are designed and tested for use with rigid air barrier — SUPER STICK for sealing around window and door openings, and for addressing sheet joins.
  • TRADE SEALS are a BRANZ-appraised one-piece self-adhesive collar for effectively sealing against water and air penetration around electrical and plumbing services — as set out in the Kāinga Ora Product Performance Requirements for rigid air barrier substrates.
  • HYDRA is an effective tri-laminate, synthetic roof underlay for medium density housing — which provides a secondary, protective, weather-resistant layer — preventing external moisture from entering the roof space/structure and absorbing internal moisture.

If you are interested to learn more about pre-cladding Weathertightness or a complete system approach for your next project, please visit www.mwnz.com to locate your nearest Marshall Innovations contact.

View more information on Marshall Innovations, including contact details.
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July 2021 EBOSSNOW Product News

News from Marshall Innovations

Exterior Cladding System Design for Weathertight Medium Density Housing
Tekton High Performance Flexible Wall Underlay and Air Barrier
One System, One Specification, One Supplier
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