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22 November 2021

The Building Code System: Is It Ready for Mandatory Product Information? Part Two: A Case Study

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Back in September my blog was focused on the Building Code and its relationship to building products and methods. In it I outlined a framework for establishing what code clauses apply for a product or method. I concluded with the following statement:

What I have described above requires a high degree of judgement and inconsistencies in the drafting of the code make the task more difficult that it first appears. In the next few blogs I will look in more detail some of these difficulties.

This blog is the first of those case studies. This one looks at a common product where there is a range of product information available to compare approaches — flexible wall underlays.

What Code Clauses do you commonly see in product information for flexible wall underlays?

Most compliance information for flexible wall underlays generally includes some or all of the following Code clauses as being applicable:

B1 - Structure

B1.3.1 Buildings, building elements and sitework shall have a low probability of rupturing, becoming unstable, losing equilibrium, or collapsing during construction or alteration and throughout their lives.

B1.3.2 Buildingsbuilding elements and sitework shall have a low probability of causing loss of amenity through undue deformation, vibratory response, degradation, or other physical characteristics throughout their lives, or during construction or alteration when the building is in use.

B1.3.3 Account shall be taken of all physical conditions likely to affect the stability of buildings, building elements and sitework, including:
(a) self-weight,
(e) water and other liquids
(f) earthquake
(h) wind
(q) time dependent effects including creep and shrinkage, 

B1.3.4 Due allowance shall be made for:
(a) the consequences of failure,
(b) the intended use of the building,
(c) effects of uncertainties resulting from construction activities, or the sequence in which construction activities occur,
(d) variation in the properties of materials and the characteristics of the site, and
(e) accuracy limitations inherent in the methods used to predict the stability of buildings.

The inclusion of B1 – Structure code clauses are included presumably on the basis of the loss of amenity that could arise from the failure of a flexible wall underlay. In a later blog I will look at the use of the term amenity in the building code as it is a common term that is applied in a range of code provisions. As I discuss below, others providing compliance information cover the mechanical (i.e. structural) requirements of a flexible wall underlay under E2.3.7.

B2 - Durability

B2.3.1 Building elements must, with only normal maintenance, continue to satisfy the performance requirements of this code for the lesser of the specified intended life of the building, if stated, or:
the life of the building, being not less than 50 years…
15 years ….

B2.3.2 Individual building elements which are components of a building system and are difficult to access or replace must either:(a) all have the same durability, or
(b) be installed in a manner that permits the replacement of building elements of lesser durability without removing building elements that have greater durability and are not specifically designed for removal and replacement.

Again practice varies here sometimes with logic and sometimes not. Some compliance information simply states B2.3.1 (b) using seemingly as a basis for this the example given in the Code clause B2.3.1 (b) (“including the building envelope”). As I discussed in my July Blog covering durability – B2.3.1 (a) is probably more appropriate where the underlay is specified for use under durable cladding systems such as masonry brick. This point is often not clear as to whether the scope of use includes use behind durable systems. It could be argued that underlays with a stated durability compliance of B2.3.1 (b) should not be used behind durable claddings.

C3 – Fire affecting areas beyond the fire source

(C3.4) suspended flexible fabrics and membrane structures used in the construction of buildings must have properties resulting in a low probability of injury or illness to persons not in close proximity to a fire source.

This clause appears on some compliance information and is appropriate where the underlay can be used in areas exposed to view such as warehouses and other unlined buildings.

There have been some that have used C3.4 (a) (“materials used as internal linings must meet the performance criteria set out. … (various Material Group numbers” )) as the relevant Code clause for this situation. Again, arguments can be constructed to support both options.

E2 – External Moisture

E2.3.2 “Roofs and exterior walls must prevent the penetration of water that could cause undue dampness, damage to building elements, or both.”

This clause (E2.3.2) relates to the prime function of a flexible wall underlay, i.e. part of the external moisture management system and is therefore quoted in all compliance information. Some compliance information makes it clear that this is a contribute clause i.e. a wall underlay is not an exterior wall but part of a system that achieves compliance with E2.3.2 but does not do this by itself.

E2.3.5Concealed spaces and cavities in buildings must be constructed in a way that prevents external moisture being accumulated or transferred and causing condensation, fungal growth, or the degradation of building elements.”

Some compliance information includes E2.3.5 is included as a (contribute to) clause on the basis that an underlay forms part of a cavity wall (external side).

E2.3.7Building elements must be constructed in a way that makes due allowance for the following:
(a) the consequences of failure:
(b) the effects of uncertainties resulting from construction or from the sequence in which different aspects of construction occur:
(c) variation in the properties of materials and in the characteristics of the site.

The inclusion of E2.3.7 on compliance information product information is a relatively new addition. As I indicated in my discussion on the inclusion of B1, some argue that the mechanical properties required of flexible wall underlays are more closely related to E2.3.7 and the durability requirements of B2. In support of this argument they point out that the mechanical properties set out in the cited Standard NZS 2295: 2006 “Pliable, permeable, building underlays” is in E2/AS1 and not within the B1 family of acceptable solutions.

As a flexible wall underlay is a building element, the application of the rule associated with deciding if compliance claims are “with” or “contribute to” means that this is a compliance “with”. However, some argue that E2.3.7 (c) “characteristics of the site” cannot apply for a wall underlay and therefore at best E2.3.7 could be contribute to clause. I would argue that since most compliance information for underlays specifies wind zones, is this not a characteristic of a site?

F2 – Hazardous Building Materials

F2.3.7 The quantities of gas, liquid, radiation or solid particles emitted by materials used in the construction of buildings, shall not give rise to harmful concentrations at the surface of the material where the material is exposed, or in the atmosphere of any space.

This clauses appears on all and is not subject to any debate even through the performance clause itself has some issues – a subject for a later blog.

H1 – Energy Efficiency

H1.3.1 the building envelope enclosing spaces where the temperature or humidity (or both) are modified must be constructed to—

(b) limit uncontrollable airflow.

This clause is generally included where the conditions of use of the underlay includes making sure that all laps and junctions with other aspects of the building are fully sealed to prevent air flow.


As this relatively “simple” example illustrates, current compliance information related to building underlays varies. Most of the variations have a logic to them. However, to avoid confusing the market as such information becomes mandatory, the sector should work together to agree a common approach. As I said back in September, part of this should be MBIE reviewing the Code to make sure it is ‘fit for purpose’ in a world of mandatory product information. 

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