Building Wraps and Steel Frames

The use of underlay with steel frames requires consideration of many factors but they can be divided into two groups.

While historically wall claddings have been directly fixed to the frame recently the type of construction has been governed by the NZBC E2/AS1 requirement for cavity construction determined by the risk factor detailed in that document.  Although E2/AS1 is an Acceptable Solution it is only one means of compliance with the NZBC and Alternatives must be considered on reasonable grounds.

Steel framing standards are contained in NASH 3405 which is regarded as an Alternative Solution, however wherever possible it is aligned to E2/AS1 and cavity construction is therefore required for steel frames in a similar manner as for timber. Because steel frames are more conductive than timber they require a thermal break which is generally a non-absorbent, inert high density polystyrene strip 10 -20mm thick. Treated timber can be used as an alternative but it is absorptive and is 18 - 20mm thick. A separation strip is required between metal claddings because of the likelihood of corrosion of steel at the interface and an underlay can provide this dual role.  While not specifically designed as such a timber batten also acts as a thermal break.

There are instances where a thermal break is not required in a cavity, which include brick veneer and EIFS construction.

Historically the choice of an underlay was a simple matter as it was limited to two types of underlay- bituminous and fire-retardant in various weights however this is now no longer the case.  Recently the use of synthetic wall wraps has become commonplace but they have been introduced as a substitution without full understanding of their differences. There are many types of synthetic underlay’s but few have all the properties of kraft underlay’s that have served their purpose well over time. Some are absorbent, some are not. Some are permeable, some are not.

The type of cladding can now determine if a synthetic underlay is suitable or not as claddings have been classified as ‘absorbent’ or ‘non- absorbent’.  NZS 2295 has been updated to include synthetic underlay’s but stops short of general endorsement simply because of the lack of test information and in service performance.

The requirements for underlay’s used in conjunction with metal frames generally follow the same basic rules as for timber – non- absorbent claddings require an absorbent underlay while, absorbent claddings do not.  Kraft underlay’s have been traditionally used with metal claddings because they possess absorbency, permeability as well as waterproof qualities and they have a long history of prior use and therefore remain the preferred underlay.

Different synthetic wraps have performance differences simply because they do not absorb water or water vapour in the same manner. While the science of these differences does not require to be discussed in detail the performance of different products do require to be understood if their correct use is to be specified.

Underlay’s, also known as breather or permeable membranes have minute pores created by the intersection of the fibres. These permit vapour to flow from one side to the other, which enables the equalisation of vapour pressure across the underlay but resist the passage of water.  As underlay’s act as an absorbent layer, a permanent air gap must be maintained on one side of the underlay and ventilation must not be inhibited by insulation or a second layer of underlay or foil underneath it.

Underlay’s should be placed next to the steel stud in all instances.

The following is a Table used to determine the type of Wall underlay required:

N = Non-Absorbent, A = Absorbent, K = Kraft

Direct Fix

Framing Cladding Underlay 1 *
Steel + Thermal break N K
Steel + Thermal break A K

* If the thermal break is not continuous at the base then the position of the underlay should be behind the thermal break.

Cavity

Framing Batten Cladding Underlay 1 Underlay 2 #
Steel A A K -
Steel A N K or A or N K
Steel N A K or A or N -
Steel N N K or A or N K

# The reason why underlay 2 is required is because it is a drainage plane and is also used as a separation.

The following is an extract from the revised NZMRM Code of Practice:

Types of Underlay

An underlay of one of the following types shall be used under metal roof and wall cladding.

As metal cladding can with maintenance last for 50 years, the underlay must also fulfil its function for that time NZS 2295:2006 classifies underlay’s by their use in roofs or walls. Minimum requirements for underlay’s that are suitable for metal roof and wall cladding are:

# For direct fixed metal wall cladding, in addition to meeting W4, the synthetic wrap shall also comply with the water resistance criteria, equal to or greater than 100mm.

Table 4.3.1.

ROOF UNDERLAY(R1,R2) WALL UNDERLAY(W2,W4)
Absorbency g/m2 140 100
Permeability g/m2/day 36 26
Vapour transmission resistance (1/P) ≤ 7MNs/g ≤ 5MNs/g
Shrinkage % < 0.5 < 0.5 *
pH 5.5.-.8.0 5.5.-.8.0
Water resistance mm head > or = 100mm > or = 100mm
Max length m 10 ,/7(FR) ** N/A
Side and end laps mm 150 100

* NZS 2295 allows greater than this amount and requires each roll to be labelled with the material type and application included in the above information.

** Fire retardant

Absorbent type underlay’s must be used under roof cladding to absorb and remove moisture that could form on, or drip from, the cladding as condensation, but it should exert the least possible vapour transmission resistance compatible with it remaining watertight.

The most important rule with metal framing (or metal cladding) is that it must not remain wet.