1.1 Building Underlays and Flashing Tapes - Timber Frame

1.1.1.Flexible Building Underlay

Fitting the wall underlay correctly is essential, as the underlay helps form a second line of defence to protect the framing from moisture. The wall underlay is either a flexible wall underlay (paper-based or synthetic) and be fixed in place over timber frame and steel frame construction. It is the responsibility of the Architect/Designer to specify the type of material to be used. Building Underlay must be cut and folded around the inside of framing at openings.

It is the builder’s responsibility for installing the appropriate building underlay, flashing tapes, and intermediate underlay support according to E2/AS1.

Check correct material has been used according to E2/AS1. Before installing the Render substrate (polystyrene etc) the builder must repair all damage to flexible underlay / Building Underlay, or alternatively replace the materials required.

Intermediate support of flexible underlay refer to E2/AS1 Paragraph 9.1.8.5.

Fitting insulation into the wall framing may cause the flexible underlay to bulge and block the cavity. If studs are spaced at more than 450 mm centres, fit one of the following to prevent bulging of the flexible underlay.

  • Polypropylene tape to provide maximum 300 mm spacings
  • Extra battens to provide maximum 300 mm spacings

Builder responsibility: Strapping tape to be applied across the exterior of the building underlay to ensure that the internal insulation does not close or restrict the cavity.

Building  Underlays must be returned around the jamb, sill and window head framing and flashing tapes installed must be installed at all openings. For more information regarding this please contact WANZ (Window Association of New Zealand) for details on Window Installation and details on their weathertight window installation system (wanz-wis).

1.1.2 Rigid Building Underlay

Plywood or fibre cement sheet complying with NZBC Acceptable Solution E2/AS1, Table 23, or rigid sheathing covered by a valid BRANZ Appraisal for use as a rigid air barrier system. All external walls of buildings must have barriers to airflow in the form of interior linings with all joints stopped for wind zones up to and including a Very High, and rigid underlays for buildings in Extra High wind zone and specifically designed buildings up to 2.5kPa design differential ULS wind pressure. Unlined gables and walls must incorporate a rigid sheathing or an air barrier which meets the requirements of NZBC Acceptable Solution E2/AS1, Table 23. For attached garages, wall underlays must be selected in accordance with NZBC Acceptable Solution E2/AS1, Paragraph 9.1.3.4.

1.1.3.Flashing Tapes

Flexible flashing tapes must be fixed in place over the flexible underlay according to the flashing tape manufacturers specifications.

E2/AS1 : reference paragraph 4.3.11 – Flexible flashing tape

Flexible flashing tapes must comply with parts 3.2 and 4 of ICBO Acceptance criteria AC148. These tapes must be compatible with any adjacent building underlay or roof underlay.