Sarking and building wrap are the thin silver or white membranes you see stretched across a house frame and laid under the roof before the cladding and roofing go on. They are two of the least understood components in an Australian home, and two of the most consequential, because once the walls are closed up nobody sees them again for the life of the building.
Homeowners routinely conflate these membranes with insulation. Both go on at roughly the same stage, both get described loosely as helping with heat, and reflective sarking looks like the foil facing on some insulation products. But they do different jobs. Insulation manages conducted heat and is measured in R-values. Sarking and wrap manage radiant heat, water, air movement and vapour, and their performance depends almost entirely on how they were installed rather than on the product specification alone.
This guide explains what sarking and building wrap actually do, how they differ from insulation, what the National Construction Code and the relevant Australian Standards require, where installation typically goes wrong, and what a homeowner can reasonably check on site before the building reaches lockup.
What sarking and building wrap are
Both products belong to a family of materials the industry calls pliable building membranes. A pliable building membrane is a flexible sheet material, usually supplied in rolls, that is fixed to the structure to control the movement of water, air, vapour or radiant heat. The membrane is not structural and it is not a bulk insulator. It is a thin functional layer that changes how the building envelope behaves.
In Australian usage, sarking is the membrane laid over the rafters or top chords and under the battens. Building wrap, wall wrap or house wrap is the membrane fixed to the outside face of the wall frame, behind the cladding. Some manufacturers supply a product rated for both, but the performance requirements differ enough that each should be specified separately.
Membranes are classified by two properties. The first is reflectivity. A reflective membrane has a low emittance metallic face, usually aluminium foil on a reinforcing scrim, which reflects radiant heat when it faces an air space of at least 20 millimetres. A non-reflective membrane works purely as a water and air control layer.
The second is vapour permeability. A vapour barrier resists the passage of water vapour. A vapour permeable membrane allows water vapour to pass through while still shedding liquid water. This single property is the difference between a wall that dries out and a wall that traps moisture inside the frame, and it is the property most often chosen by habit rather than by climate zone.
What sarking does
Roof sarking performs four separate functions, and a given project may rely on some or all of them.
Radiant heat barrier
In an Australian summer the roof is the single largest source of unwanted heat gain in a house. A reflective sarking facing the air space beneath the tiles or sheeting reflects a large proportion of the radiant heat that would otherwise be re-radiated down into the roof space. This is why reflective sarking earns a modest R-value contribution in the roof system calculation, but only when the required air gap is preserved. Lay the same product flat against the underside of the battens with no air space and the reflective benefit is largely lost.
Secondary moisture protection
No roof covering is watertight in every condition. Wind driven rain gets pushed up under tile laps, capillary action draws water sideways, and a cracked tile or lifted sheet will let water in. Sarking is the second line of defence: it catches that water and directs it down the slope and out over the fascia into the gutter, provided it has been dressed correctly into the gutter line.
Condensation control in the roof space
Warm moist air rising from the living space meets the cold underside of a metal roof on a winter night and condenses. Where the sarking is vapour permeable and the roof space is correctly ventilated, that moisture escapes. Where an impermeable membrane is used in the wrong climate zone, condensation collects under the sheeting and drips onto the ceiling insulation, which loses performance and eventually stains the ceiling.
Dust and ember resistance
Sarking closes off the small gaps around tiles and sheeting through which windblown dust and, more critically, burning embers enter a roof space. In designated bushfire prone areas, sarking with the required flammability index forms part of the roof ember protection strategy at higher bushfire attack levels, and that specification is not interchangeable with a standard product. For the assessment that sets the rating, refer to the BUILTLY guide on bushfire zones and BAL ratings in Australia.
Reflective and non-reflective sarking
Reflective sarking is the default on most Australian projects because the radiant heat benefit is meaningful across almost every climate zone. Non-reflective sarking is specified where the reflective surface would serve no purpose, for example where there is no air gap available, or where a highly vapour permeable product is needed and the available permeable membranes are non-reflective.
What building wrap does
Wall wrap sits on the outside face of the frame, over the bracing and studs, and behind the cladding. In a modern Australian wall it is the layer that makes the difference between a wall that manages water and one that merely hopes the cladding never leaks.
Weather barrier
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Air barrier
Uncontrolled air leakage through the wall carries heat out in winter, hot air in during summer, and moisture in both directions. A taped and sealed wall wrap reduces that leakage, which is why air tightness has become a growing focus as energy requirements have tightened. It also improves the bulk insulation in the stud cavity, which performs poorly when air moves freely through and around it.
Vapour permeability
This is the property that most affects long term durability. Air inside a house carries water vapour generated by cooking, showering, drying clothes and breathing. That vapour migrates through the internal lining into the wall cavity. If the outside of the wall is sealed with an impermeable membrane in a cool climate, the vapour reaches the cold outer face, condenses to liquid and has nowhere to go. Over successive winters the frame stays wet long enough for mould and decay to establish.
A vapour permeable wrap allows that vapour to escape outward while still shedding liquid water from the cladding side. In much of southern Australia this is now the required approach. In hot humid northern zones the vapour drive can run the other way for much of the year, so the correct specification differs, and it should be set by the designer against the climate zone rather than assumed.
Where the wrap sits in the wall
From the inside out, a framed wall runs lining, then the stud cavity holding bulk insulation, then the frame, then the wall wrap on the outside face of the studs, then a cavity where the cladding system requires one, then the cladding. Insulation sits inside the frame, not behind the wrap. For how that insulation layer is specified, refer to the BUILTLY guides on insulation in Australia and the best insulation for the Australian climate by region.
Sarking vs building wrap vs insulation
The clearest way to separate these three is by the problem each one solves. They are not competing products and none substitutes for another. A house needs all three, in different places, doing different jobs.
| Question | Sarking | Building wrap | Insulation |
|---|---|---|---|
| Problem it solves | Radiant heat, water past the roof covering, embers and dust | Water past the cladding, air leakage, trapped vapour | Conducted heat flow through the envelope |
| Where it goes | Over rafters or trusses, under battens and roofing | Outside face of the wall frame, behind cladding | Inside the stud cavity, ceiling and under floors |
| How performance is measured | Emittance, vapour permeance, flammability index | Water barrier class, vapour permeance, air permeance | R-value |
| Does it carry a useful R-value alone | Only a small contribution, and only facing an air gap | Only a small contribution, and only facing an air gap | Yes, this is its entire purpose |
| Main failure mode | Not dressed into the gutter, wrong permeability, torn | Lapped upside down, torn, not flashed into openings | Compressed, gapped or omitted at edges |
One sentence captures the relationship. Insulation slows heat passing through solid materials, and it only works if it stays dry and uncompressed. Sarking and wrap are what keep it dry, keep air from washing through it, and reflect the radiant heat insulation is not designed to handle. That is why an owner who paid for a high performing insulation package can still end up with a house that is hot in summer and damp in winter: the insulation was specified correctly, the membranes around it were not.
Australian standards and NCC requirements
Pliable building membranes in Australia are governed by a combination of the National Construction Code and referenced Australian Standards. The following are the references that matter most on a residential project. They are noted here as plain text so you can raise them with your builder or designer by name.
The product standard
AS 4200.1 sets out the material requirements for pliable building membranes, including the classification system for water barrier performance, vapour permeance and flammability index. A compliant product states its classification on the roll and in the data sheet. If a product cannot show one, it should not be on the building.
The installation standard
AS 4200.2 covers installation: lap directions, minimum lap widths, support, fixings and the treatment of penetrations. Almost every membrane defect on Australian sites is a departure from this standard rather than a product failure, which is why it is worth naming in correspondence when a defect is disputed.
Condensation management in the NCC
Recent editions of the National Construction Code introduced significantly strengthened condensation management provisions for Class 1 buildings. In broad terms, these require a vapour permeable membrane on the outside of the external wall frame in the cooler climate zones, set minimum ventilation for roof spaces in specified circumstances, and restrict the use of impermeable membranes where they would trap moisture in the frame. These provisions sit alongside the energy efficiency requirements rather than replacing them. For how the energy side of the code interacts with the building envelope, refer to the BUILTLY guide on NCC 2022 7 star energy requirements in Australia.
Bushfire and wind provisions
AS 3959 governs construction in bushfire prone areas and sets the sarking requirements that apply at each bushfire attack level, including the flammability index the membrane must meet. Wind classification under AS 4055 also affects whether sarking is required under a given roof covering and how it must be supported and fixed. Both are determined at design stage and both should be visible on the approved drawings rather than left to a site decision.
Common installation defects
Membranes are cheap, quick to install and installed under time pressure by trades whose main task that day is something else. That combination produces a predictable set of defects.
Laps run the wrong way
Membranes must be lapped in shingle fashion, with each upper sheet overlapping the sheet below on the outside face, so water running down the membrane stays on the outside. Reverse the lap on a single course and every litre of water reaching that point is directed straight into the lap and onto the frame. This is the single most common and most damaging membrane defect, and it is invisible from three metres away.
Insufficient lap width
Laps that are too narrow, or that fall between framing members with nothing holding them closed, let water and air track through. Laps should land on a stud, rafter or batten where the standard requires it, and minimum lap widths increase with lower roof pitches and higher exposure.
Tears, punctures and flapping sections
Wrap gets torn by scaffolding, by wind before the cladding goes on, and by trades pushing services through it. Small tears can be taped with the manufacturer's approved product. Large tears and sections that have flogged loose in the wind for weeks should be replaced. Tape used on a dusty or damp membrane will not hold, so the repair itself needs to be done properly.
Prolonged UV exposure
Every membrane has a stated maximum exposure period before cladding must be installed, commonly measured in weeks rather than months. A wrap left exposed through a summer on a stalled site can become brittle and lose much of its water resistance while still looking intact, so the exposure limit is a fair question to put in writing.
Wrong product for the climate zone
Using an impermeable membrane where the code requires a vapour permeable one is the defect with the longest fuse. Nothing appears wrong at handover. Moisture accumulates in the frame over successive winters until mould, staining or decay emerges, usually well outside any warranty window and often attributed by the builder to the way the occupants live in the house.
No integration with flashings and openings
Wrap cut flush at an opening rather than dressed into the reveal, sill trays omitted, and head flashings layered wrongly against the wrap all create a direct path into the frame at the points where water concentrates. Window surrounds are where most wall water ingress claims originate.
Sarking not dressed into the gutter
Sarking must terminate over the fascia so that any water it collects discharges into the gutter. Where it stops short, the water it has faithfully carried down the roof is delivered straight into the eaves cavity and the top of the wall. Where consequences of this kind become structural, the issue moves into the territory covered by the BUILTLY guide on structural defects in new builds in Australia.
What to check during construction
There is a short window between frame completion and cladding installation when all of this is visible. Once that window closes it does not reopen. Two site visits, correctly timed, are worth more than any number of questions asked after handover.
At frame stage, before wrap goes on
- Ask which membrane products are specified for the roof and the walls, and request the technical data sheets
- Confirm the wall wrap is vapour permeable if your climate zone requires it, and that this matches the energy assessment
- Confirm the sarking meets the flammability index required for your bushfire attack level if your site is in a designated area
- Check that the products named on site match the products named in the specification and contract documents
After wrap goes on, before cladding
- Walk each elevation and photograph it end to end, including corners and the base of the wall
- Check the laps run downhill in shingle fashion on every course, on every elevation
- Look for tears, loose flapping sections and holes around service penetrations
- Check the wrap is dressed into window and door openings rather than cut flush at the frame edge
- Confirm sill trays and head flashings are installed and correctly layered with the wrap
- Check the product branding is still legible, which tells you the membrane has not degraded and, on directional products, that it is facing the right way
- Ask how long the wrap has been exposed and compare that to the manufacturer's stated exposure limit
At roof stage
- Confirm sarking has been installed under the battens, not omitted because the roof is metal
- Check the sarking is dressed over the fascia and discharges into the gutter line
- Confirm the required air gap is preserved beneath a reflective product rather than the membrane being drawn tight against the roofing
- Photograph the roof plane from a safe vantage point before the tiles or sheets are laid
How to raise an issue
Raise membrane concerns at a scheduled site meeting and put them in writing the same day with photographs attached. Reference the specific installation requirement rather than describing the problem in general terms. A note saying the laps on the northern elevation appear reversed, and asking for confirmation against AS 4200.2, will get a substantive answer. A note saying the wrap looks wrong will not.
BUILTLY is building a record of completed Australian homes that credits the builder, designers and trades behind each one.
BUILTLY and pre-lockup evidence
Membranes are the clearest example of construction quality that becomes permanently invisible. Once the cladding is fixed, no inspection or building report will tell you whether the laps ran the right way. The only evidence that will ever exist is the record made before lockup.
BUILTLY is building a record of completed Australian homes that credits the builder, designers and trades behind each one.
For an owner selecting a builder, asking to see how a previous build was recorded through the frame and lockup stages is a practical test of attention to detail that costs nothing and reveals a great deal. For the broader construction context around these checks, refer to the BUILTLY complete guide to building a house in Australia.
Frequently asked questions
Is sarking the same thing as insulation?
No. Sarking is a thin pliable membrane fixed under the roof covering. It reflects radiant heat, sheds any water that gets past the tiles or sheeting, and resists dust and ember entry. Insulation is a bulk material such as glasswool, polyester or rigid board that slows conducted heat flow and carries an R-value. Reflective sarking contributes a small amount to the total R-value of a roof or wall system when it faces an air gap, but it is not a substitute for bulk insulation and will not meet NCC energy requirements on its own.
Is sarking compulsory on a new home in Australia?
It depends on the roof type, the wind region, the bushfire attack level and the state. Sarking is generally required under tiled roofs in most wind and rain exposure situations, and it is required on homes in designated bushfire prone areas at higher BAL ratings because it forms part of the ember protection strategy. Some metal roof profiles at higher pitches in low exposure areas may not require it. Your builder or designer should be able to point to the specific requirement that applies to your roof, and it should appear on the drawings and in the specification.
What is a vapour permeable wall wrap and do I need one?
A vapour permeable wall wrap resists liquid water from the outside while allowing water vapour to pass through from the inside, so moisture that reaches the wall cavity can dry outward instead of condensing inside the frame. Under the condensation management provisions introduced in recent editions of the NCC, a vapour permeable membrane is required on the outside of the frame across much of southern Australia. In warmer northern climate zones the correct product may be different, so the choice should be made against the climate zone rather than habit.
Can sarking or wall wrap be installed after the walls are lined?
No. Both products sit on the outside face of the frame and under the roof covering, which means they must be installed before the cladding and roofing go on. Once the building is at lockup, the membranes are permanently concealed. That is why photographs taken during the frame and lockup stages are the only practical record of how the membranes were installed and lapped.
What happens if the wall wrap is torn during construction?
Small tears can be repaired with the manufacturer's approved tape provided the repair is made before the cladding is fixed. Large tears, sections left flapping loose, or areas where the wrap has been left exposed to sun for months beyond the stated UV exposure limit should be replaced rather than patched. A torn or degraded wrap loses its weather barrier function at exactly the points where water is most likely to get in, and the failure will not show up until staining or mould appears inside the building years later.
How can I tell whether the membranes were installed correctly?
Visit the site at frame stage and again before the cladding is fixed, photograph each elevation, and check that the wrap laps run downhill in shingle fashion, that the product branding is still legible and facing the correct way, and that the membrane is dressed into window and door openings rather than cut flush. Ask your builder for the product data sheets and keep them with your build records. If you cannot get to site, ask for dated photographs of each elevation before the cladding goes on.