Reflective foil insulation: sarking, vapour barriers, and where it actually delivers R-value

TL;DR

Reflective foil insulation is a fundamentally different category from bulk batts. The foil (almost always aluminium, bonded to a paper, kraft, polyethylene, or bubble-foam backing) reflects radiant heat rather than blocking conductive heat. The key consequence: a sheet of foil on its own has near-zero R-value. Foil only delivers thermal performance when paired with an adjacent air gap of 20 to 100 mm where the radiant heat exchange happens. The two volume applications: roof and wall sarking (foil-faced building membrane laid under the cladding or roof sheet, creating an air gap to the structural framing or batten) and multi-layer hybrid products (Kingspan AIR-Cell, Anticon, SuperFOIL) that combine bulk fibre or foam with foil facings to add radiant performance to thermal mass. The Australian standards are dual: AS/NZS 4859.1:2018 for the thermal performance claim, and AS/NZS 4200 for the pliable building membrane requirements (vapour control, water resistance). The job-killer is installing foil with no air gap (pressed flat against the framing or cladding): a sandwich without a cavity is just a thin sheet with R0.05; the marketing-claimed R0.5 to R1.5 from foil + air gap evaporates. AS/NZS 4200 also drives vapour-permeable vs vapour-impermeable choice based on climate zone (verified 2026-05-13).

What it is

Reflective foil insulation is a thin layer of aluminium foil bonded to a flexible substrate. The aluminium reflects long-wavelength infrared radiation (radiant heat), which is one of three heat-transfer mechanisms in a building envelope:

Conduction: heat moves through solid materials (bulk insulation reduces this)
Convection: heat moves with air currents (sealing reduces this)
Radiation: heat radiates across air gaps from a hot surface to a cold surface (foil reduces this)

In a residential wall or roof cavity, all three mechanisms operate. Bulk insulation (glasswool, polyester, rockwool, rigid foam) addresses conduction. Reflective foil addresses radiation across air gaps. The two mechanisms are complementary, not interchangeable.

The air-gap rule

A reflective foil sheet on its own delivers approximately R0.05: essentially zero thermal resistance. The reflectivity only converts to R-value when there’s an air gap adjacent to the foil surface. The radiant heat exchange across the gap is what the foil reduces.

Foil + air gap	Approximate R-value contribution
Foil with no air gap (pressed flat to substrate)	R0.05 (effectively zero)
Foil + 20 mm still air gap on one side	R0.5 to R0.7
Foil + 40 mm still air gap on one side	R0.7 to R1.0
Foil + 100 mm still air gap on one side	R1.0 to R1.5 (diminishing returns above 100 mm)
Foil + air gaps on both sides	Roughly double single-sided values

The “Total R-value contribution” of a foil membrane in a wall or roof depends on the actual installed air-gap geometry. Marketing claims of “R1.5 foil” are based on specific air-gap test configurations that may or may not match the field install.

Where reflective foil dominates

Roof sarking: laid down the rafters or trusses, under the roof sheet (metal or tile), with battens above creating an air gap between the foil and the cladding. The standard residential application. Foil-faced kraft paper (Sisalation) is the classic budget product; foil-faced bubble (Anticon) adds bulk-insulation properties.

Wall sarking (cavity side or external face of frame): a foil-faced membrane stretched over the wall framing before cladding goes on. The cavity behind the cladding (typical 20 to 30 mm batten space) is the air gap.

Underfloor (suspended floor systems): foil sarking stretched between joists under the subfloor, creating the air gap below the floor. Less common in Australia; ground vapour barriers are a separate product class.

Composite multi-layer products: Anticon (bulk fibre + foil), Kingspan AIR-Cell (closed-cell foam + foil), SuperFOIL (multi-layer foil + bulk). These deliver R-value through both mechanisms simultaneously and are sized to fit the cavity geometry.

Australian products

Product	Type	Where used
Sisalation (Fletcher Insulation)	Heavy-duty kraft paper + aluminium foil	Roof sarking, wall sarking; the heritage Australian product
Anticon (Fletcher Building, Bradford-related)	Foil-faced glasswool blanket	Roof sarking with built-in bulk insulation; common on residential
ThermaSeal	Foil-faced bubble	Roof sarking, wall sarking
Kingspan AIR-Cell Insulbreak	Closed-cell polyethylene foam + foil facings	Roof sarking system; high R per thickness
Kingspan AIR-Cell Permishield	Permeable variant for vapour-permeable applications	Cool-climate applications
SuperFOIL	Multi-layer foil + thin bulk core	Wall and roof; specifier-led
Permaflux (Fletcher Insulation)	Foil + paper sarking	Volume residential

AS/NZS 4200: the pliable building membrane standard

Reflective foil sarking does two jobs: thermal reflectivity and vapour control / water resistance. The second job is covered by AS/NZS 4200 (Pliable building membranes and underlays).

AS/NZS 4200.1:2017 classifies membranes by:

Property	Classification
Water-vapour permeance	Class 1 (low permeance, vapour barrier), Class 2 (moderate), Class 3 (high permeance, vapour-permeable)
Water-vapour transmission	Sd-value (equivalent diffusion thickness)
Water resistance	W1 (highly resistant to liquid water), W2 (resistant), W3 (limited)

For NCC 2022 condensation compliance, the vapour permeance classification matters:

Climate zones 6 to 8 (cool / cold): vapour-permeable membrane (Class 3) typically required externally to allow the wall to dry to outside in winter
Climate zones 1 to 3 (warm humid): vapour-impermeable membrane (Class 1) typically used to prevent humid outside air entering the wall

Generic foil sarking is Class 1 (vapour barrier) which works for warm-humid climates. For cool-climate applications, a vapour-permeable foil product (with perforations or vapour-permeable backing layer) is needed.

Where reflective foil is the right call

Roof sarking under metal or tile roofing: standard residential application; reflective foil reduces radiant heat from the hot roof sheet in summer
Wall sarking behind cladding with batten cavity: when there’s a guaranteed air gap, foil sarking adds R-value cheaply
Where vapour control matters: foil sarking simultaneously addresses thermal and vapour functions
Subfloor reflective barrier where bulk batt isn’t possible: e.g. tight subfloor heights, retrofit
Composite hybrid products in cavity-constrained walls: AIR-Cell or Anticon adds R-value where bulk batts won’t fit

Where reflective foil is wrong (or oversold)

Pressed flat against substrate with no air gap: zero R-value from the foil; you’re paying for marketing
Inside a cavity already filled with bulk insulation: the bulk insulation has eliminated the air gap, so the foil contributes nothing
Below an exposed roof structure in tropical climate without venting: condensation forms behind the foil on the cool side, soaking the underlying framing
As the primary thermal envelope: foil alone cannot reach the Total R-value targets in cool climates (R2.8 wall, R5.1 ceiling per NCC zone 5); needs bulk insulation alongside
Behind cladding with no air gap (direct-fix cladding): same problem as no air gap above; the foil delivers nothing

Common defects and on-site issues

Foil pressed flat against framing or cladding: install error that eliminates the air-gap mechanism. Battens or air-gap spacers are required.
Tears at corners and overlaps: foil sarking has limited tear resistance; rough handling tears it. Patch with foil-faced tape; replace if extensive.
Wrong vapour permeance for climate zone: AS/NZS 4200 Class 1 vapour barrier installed in zone 7 wall where Class 3 vapour-permeable is needed. Condensation builds inside the wall; framing rots within years.
Marketing R-value claims taken at face value: “R1.5 foil” sold as a complete insulation system. Buyer thinks they have an R1.5 wall; actual installed Total R-value is materially lower without bulk insulation alongside.
Foil contact with electrical cables: aluminium foil is conductive; contact with cabling can create short-circuit or shock risk if wiring is damaged. Maintain clearance.
Lap not sealed: overlap edges between sarking sheets must be sealed with foil tape per AS/NZS 4200.2:2017 for the membrane’s water and air resistance to count.
Halogen downlight in contact with foil: 50 mm clearance from light fixture per glasswool-batts rule.

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

Product	Per square metre
Plain foil sarking (Sisalation 412/424)	$2-4
Heavy-duty foil sarking	$4-7
Anticon foil-faced glasswool (R1.5 to R2.5 ranges)	$8-14
Kingspan AIR-Cell Insulbreak	$14-22
AS/NZS 4200 vapour-permeable foil sarking	$5-9
Foil tape (50 mm roll, 50 m)	$18-28

The premium for AS/NZS 4200-compliant vapour-permeable products over standard foil sarking is roughly 30 to 70%. For NCC condensation compliance in cool climates, this premium is mandatory.

Standards and references

Standards Australia, AS/NZS 4859.1:2018 Thermal insulation materials for buildings. https://store.standards.org.au (verified 2026-05-13).
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).
Australian Building Codes Board, NCC 2022 Volume Two Part H6 Energy efficiency. https://ncc.abcb.gov.au/editions/ncc-2022/adopted/volume-two/h-class-1-and-10-buildings/part-h6-energy-efficiency (verified 2026-05-13).