COP v3.0:roof-ventilation; wall-underlay

10.6 Wall Underlay 

Permeable underlays are required under profiled metal wall cladding on residential buildings. Under direct-fixed metal cladding they should also be absorbent, but behind a drained cavity they may be either absorbent or non-absorbent
Under the NZBC wall cavities must be drained, not ventilated, i.e., they should be closed at the top to limit ingress of air in the cavity from entering the ceiling space.
Wall underlays must have a minimum side lap of 150 mm, and an end lap of 75 mm.  Wall underlay on a drained cavity should be on the dry (inside) face of the cavity, and be rigid enough to restrain wall insulation from contacting the cladding, or have secondary strapping to achieve such
The COP does not require rigid air barriers to be used with profiled metal wall cladding in very high and extreme wind zones.


10.6A Properties of Permeable Wall Underlays

Classification W2W4W3
Grade Heavy weightAbsorbentNon-absorbent
Type KraftSyntheticSynthetic
Application Any wall cladding ** See note
Absorbencyg/m²≥ 100≥ 100n/a
Water Vapour ResistanceMN s/g≤ 7≤ 7≤ 7
Water resistancemm head≥ 20≥ 20≥ 20
Tensile Strength MDKN/m≥ 9≥ 2≥ 2
Tensile Strength CDKN/m≥ 4.5≥ 1≥ 1
Edge Tear Resistance MDN≥ 40≥ 100≥ 100
Edge Tear Resistance CDN≥ 35≥ 55≥ 55
Based on Table B1 of NZS 2295 Amendment 1:2017

Run length:

  • Kraft based underlays shall have shrinkage less than 0.5% and maximum run length of 10m.
  • Synthetic underlays may have any length of run.

** Note - non absorbent underlays can only be used under an absorbent direct fixed cladding or non-absorbent cladding on a drained cavity.


Any underlay is regarded as Fire retardant if it has a FI (Flammability Index) of 5 or less when tested to AS/NZS 1530 Part 2.

pH between 5.5 and 8.0.