Sarking: pliable building membrane for roofs, walls, and underfloor

TL;DR

Sarking is the pliable building membrane laid under roof sheeting, tiles, or external wall cladding. It does three jobs simultaneously: water resistance (catches and sheds any water that breaches the primary cladding before it reaches the structural framing), vapour control (allows or blocks moisture migration through the wall or roof to balance the building’s drying capacity), and wind / air barrier (reduces uncontrolled air movement through the envelope). The primary standard is AS/NZS 4200.1:2017 (materials) and AS/NZS 4200.2:2017 (installation). Sarking is a different functional category from bulk insulation (glasswool, polyester) and from thermal reflective foil as a stand-alone product; however, many sarking products also carry a reflective foil face and contribute to thermal R-value via an adjacent air gap (see materials/foil-reflective for that thermal angle). The two specification calls: vapour permeance classification (Class 1 vapour barrier, Class 2 moderate, Class 3 vapour-permeable) and water-resistance classification (W1 high resistance, W2 resistant, W3 limited). For bushfire-prone sites under AS 3959:2018, the sarking must also meet the relevant BAL rating. The volume Australian products are Sisalation (foil + kraft, vapour barrier), Anticon (foil + glasswool, vapour barrier with insulation), and Permishield / AIR-Cell Permeable (vapour-permeable for cool-climate condensation control).

What it is

Sarking is a flexible membrane stretched across the roof framing or wall framing before the external cladding is fitted. It sits between the structural framing and the primary weatherproof skin (metal sheet, tile, weatherboard, fibre cement cladding) and forms a secondary weather defence behind the primary cladding.

The three functions:

Function	What it does	Why it matters
Water resistance	Catches water that breaches cladding (driven rain, condensation, ice melt) and sheds it to the eaves or out the wall base	Prevents water reaching framing, insulation, internal lining
Vapour control	Modifies moisture migration through the wall or roof cavity, balancing drying capacity	Reduces condensation risk in the cavity; critical in cool climates
Wind / air barrier	Limits air movement through cavity from external wind	Maintains insulation effectiveness; reduces draught

A bare wall or roof without sarking exposes the framing and insulation to direct weather penetration. AS/NZS 4200 sets requirements for materials that perform these functions.

Sarking product categories

Sarking is split by chemistry and functional emphasis. The volume Australian products:

Product type	Base material	Vapour permeance	Where used
Foil + kraft paper (Sisalation, Permaflux)	Aluminium foil bonded to kraft paper	Class 1 (vapour barrier)	Roof sarking, warm-climate walls
Foil + bulk insulation (Anticon, Foilboard)	Foil-faced glasswool or foam	Class 1 (vapour barrier)	Roof sarking with built-in thermal
Foil + closed-cell foam (Kingspan AIR-Cell Insulbreak)	Foil bonded to closed-cell PE foam	Class 1 (vapour barrier)	Wall and roof; high R per thickness
Vapour-permeable (Kingspan Permishield, ProcLima Wraptite-SA)	Microporous polymer with foil face perforated for vapour	Class 3 (high permeance)	Cool-climate walls (zones 6-8)
Heavy-duty sarking (Bradford Enviro Wrap, Tyvek HomeWrap)	Spunbonded polyolefin	Class 2 to 3 (moderate to high)	Walls behind weatherboard, fibre cement
Plain foil (no insulation backing)	Aluminium foil + paper	Class 1	Budget roof sarking

AS/NZS 4200 classifications

AS/NZS 4200.1:2017 classifies pliable building membranes by water resistance and vapour permeance.

Water resistance (W classification)

Class	What it means
W1	Highly resistant to liquid water
W2	Resistant to liquid water
W3	Limited liquid water resistance

Most sarking products under cladding are W1; roof sarking under metal sheet is W2 or W1.

Vapour permeance (Class 1 / 2 / 3)

Class	Sd-value range (m)	Where used
Class 1 (vapour barrier, low permeance)	Sd > 1.0 m	Warm humid climates (zones 1-3): prevents external humidity entering wall
Class 2 (moderate permeance)	Sd 0.05 to 1.0 m	Temperate climates (zones 4-5): mixed performance
Class 3 (vapour-permeable, high permeance)	Sd < 0.05 m	Cool / cold climates (zones 6-8): allows wall to dry to outside in winter

The right classification depends on the building’s climate zone and the internal condition. NCC 2022 requires a condensation assessment for buildings in climate zones 6 to 8 to confirm the sarking choice supports drying capacity.

NCC and BAL compliance

NCC 2022 Volume Two Part H2 (Damp and weatherproofing) and Part H6 (Energy efficiency) reference AS/NZS 4200 for sarking. The key NCC requirements:

Requirement	Source
Roof sarking under metal cladding	NCC Volume Two clause H2D6, with AS/NZS 4200 compliance
Wall sarking behind cladding	NCC Volume Two H2 (water-shedding requirement); not always mandatory but standard practice
Condensation management in cool climates (zone 6-8)	NCC Volume Two H6 / Specification 37 condensation provisions; requires vapour-permeable membrane

For bushfire-prone properties under AS 3959:2018, the sarking must also meet the relevant BAL rating:

BAL rating	Sarking requirement
BAL-12.5	Standard sarking acceptable; check product BAL rating on data sheet
BAL-19	BAL-rated sarking required at gable, eaves, ridge details
BAL-29	BAL-29-rated sarking throughout; specific products certified
BAL-40	BAL-40 rated; limited product range available
BAL-FZ	Non-combustible sarking only

Most major manufacturers publish a separate BAL-rated product line (e.g. Bradford Bushfire Wrap, Anticon BAL-29).

Where sarking is mandatory

Application	Mandatory?
Under metal roof sheet	Yes (condensation control, water-shedding)
Under roof tiles	Yes (AS 2050 + NCC require sarking under tiles in most climates)
Behind external wall cladding (weatherboard, fibre cement)	Practical / industry standard; verify NCC for specific climate
Behind brick veneer	Generally yes (water-shedding into cavity, vapour control)
Behind direct-fix cladding (no batten cavity)	Yes; sarking serves as the water-shedding plane
Internal walls	No (no external moisture exposure)
Floor under concrete slab	Different product class (vapour barrier under slab)

Install requirements (AS/NZS 4200.2)

Standard installation rules per AS/NZS 4200.2:2017:

Rule	Why
Lap minimum 100 mm horizontally, 50 mm at vertical seams	Provides water-shedding integrity at joints
Lap sealed with foil tape or product-specific adhesive	Prevents air and water leakage at laps
Reflective face oriented to the air gap	Reflective performance only works toward air
Continuous around penetrations (windows, doors, services)	Maintains envelope integrity
Counter-flashed at openings	Sheds water around openings
Stretched taut, no sags	Sagging traps water; allows wind to fill the cavity
Anchored per manufacturer’s spec (typically galvanised staples or cap nails)	Maintains tension and prevents tear at fixing points
BAL-rated joints in bushfire-prone properties	Per AS 3959 detailing

Where sarking goes wrong

No sarking installed: roof or wall without sarking exposes the framing to direct weather penetration. Common on older houses; common in renovation where the new cladding is fitted over old framing without re-sarking.
Wrong vapour permeance for climate zone: Class 1 vapour barrier in zone 7 traps internal moisture in the wall cavity. Use Class 3 vapour-permeable.
Reflective face installed away from air gap: foil face pressed against the cladding or substrate. The reflective component delivers zero R-value.
Lap not sealed: laps open to wind and water; envelope integrity compromised.
Penetrations not flashed correctly: window and door openings need both sarking and flashing tape to maintain the water-shedding plane.
Tears and punctures during install: chippies walking on the sarking during install, or fixings driven through without care. Patch with foil tape.
Wrong BAL rating for property: standard sarking installed on BAL-29 site. Compliance defect.
Sarking pinched against insulation or cladding: the air gap function disappears; reflective performance lost.

Common Australian products

Brand / product	Type	Where used
Sisalation 412 (Fletcher Insulation)	Heavy foil + kraft, Class 1	Roof sarking, walls; the heritage volume product
Sisalation 424 (heavy duty)	Heavy foil + kraft, Class 1, W1	Wall sarking, BAL-12.5
Anticon (Fletcher / Bradford-related)	Foil + glasswool blanket, Class 1 + R0.7 to R2.5	Roof sarking with built-in thermal
Foilboard	Foil-faced bulk	Roof / wall combined sarking and insulation
Kingspan AIR-Cell Insulbreak	Foil + closed-cell foam, Class 1	Roof, wall premium
Kingspan AIR-Cell Permishield	Vapour-permeable foil-faced cellular	Cool-climate walls (Class 3)
ProClima Wraptite-SA	Self-adhered vapour-permeable	Premium passive-house and tightly air-sealed builds
Bradford Enviro Wrap	Spunbonded polyolefin, Class 2	Walls behind weatherboard
Bradford Bushfire Wrap	Sarking rated to BAL ratings	Bushfire-prone properties
Tyvek HomeWrap (DuPont)	Spunbonded polyolefin, Class 3	Premium walls, vapour-permeable

Pricing (2026 indicative, ex-GST, supply only)

Product	Per square metre
Sisalation 412 (basic foil + kraft)	$2-4
Sisalation 424 (heavy duty)	$4-7
Anticon (R1.5 to R2.5 grades)	$8-14
Kingspan AIR-Cell Insulbreak	$14-22
AIR-Cell Permishield (vapour-permeable)	$16-26
Bradford Bushfire Wrap (BAL-rated)	$6-12
ProClima Wraptite-SA	$20-32
Tyvek HomeWrap	$8-14
Foil-faced tape (50 mm roll, 50 m)	$18-28

The premium for vapour-permeable products (Class 3) over standard foil sarking (Class 1) is typically 30 to 70%. For cool-climate condensation compliance, this premium is mandatory.

Standards and references

Standards Australia, AS/NZS 4200.1:2017 Pliable building membranes and underlays, Part 1: Materials. https://store.standards.org.au (verified 2026-05-13).
Standards Australia, AS/NZS 4200.2:2017 Pliable building membranes and underlays, Part 2: Installation requirements. https://store.standards.org.au (verified 2026-05-13).
Standards Australia, AS 3959:2018 Construction of buildings in bushfire-prone areas. https://store.standards.org.au/product/as-3959-2018 (verified 2026-05-13).
Australian Building Codes Board, NCC 2022 Volume Two Part H2 and Part H6. https://ncc.abcb.gov.au/editions/ncc-2022/adopted/volume-two (verified 2026-05-13).