Foil sarking: reflective membrane selection guide

TL;DR

Foil sarking is the subset of pliable building membranes that carries a metallised foil face on one or both sides. The foil adds radiant heat resistance on top of the standard sarking jobs (water shedding, wind, vapour control). It only delivers that radiant performance when installed with an adjacent reflective airspace of at least 20 mm, per AS/NZS 4200.2:2017.

If you can’t deliver the airspace, foil sarking gives you a vapour barrier and not much else. In cool climates (BCA climate zones 6-8) that vapour-barrier property can trap moisture against the framing and cause condensation; a vapour-permeable membrane is usually the right call there instead.

The volume Australian brands are Sisalation (foil + kraft), Anticon (foil + glasswool), and Bradford Enviroseal (vapour-permeable foil), with several private-label versions sold through hardware chains.

For the category overview see sarking; for thermal-only reflective products without a substrate membrane see reflective foil insulation.

What “foil sarking” actually is

Foil sarking is a composite product:

Layer	Role
Metallised foil (one or both faces)	Reflects radiant heat across an adjacent airspace
Substrate membrane (kraft paper, woven polymer, glasswool batt, or polymer film)	Water shedding, wind barrier, structural integrity
Optional scrim reinforcement	Tear resistance during install

The foil is what makes it “foil” sarking. Without an airspace next to the foil, the metallised layer adds no thermal benefit. It’s just a vapour barrier face.

When the foil actually delivers R-value

AS/NZS 4200.2:2017 sets the installation rule: a reflective membrane only contributes to thermal R-value when installed with a reflective airspace of nominally 20 mm or more on the reflective face (verified 2026-05-16, Standards Australia store, AS/NZS 4200.2:2017).

Typical applications where the airspace works:

Under metal roof sheeting on top-hat battens: the air gap between the foil face and the underside of the sheet delivers the reflective benefit. Anticon (foil + glasswool) is designed for this.
Behind metal cladding on top-hat battens or vertical battens: same principle for walls.
Between brick veneer and the timber stud face (older detailing): the cavity itself is the airspace.

Applications where the airspace does not work:

Direct contact with tile underlay or batten: foil pinned hard against another surface gives no airspace. No thermal R-value contribution.
Sandwiched between two layers of bulk insulation: again, no airspace.
Foil-faced sarking under direct-fix cladding with no cavity: vapour barrier only.

The cool-climate condensation trap

Foil sarking is typically a Class 1 or Class 2 vapour barrier under AS/NZS 4200.1:2017. In hot or temperate climates (BCA zones 1-5) that’s helpful: the membrane prevents external moisture migration into the wall or roof.

In cool climates (zones 6-8), where the inside of the house is warmer than the outside for much of the year, indoor moisture migrates outward through the wall and roof and condenses on the first cold impermeable surface it hits. If that surface is the back face of a Class 1 foil sarking, the moisture pools there and wets the framing.

The NCC 2022 (Volume Two, Part H2) and the ABCB Condensation Handbook both flag this risk. In zones 6, 7 and 8 (cool to alpine), specify a vapour-permeable membrane (Class 3 or Class 4 permeance) on the external side of the wall and roof instead. Bradford Enviroseal and ProClima Solitex Mento are common picks.

Major Australian brands

Brand	Composition	Vapour class	Typical use
Sisalation 450/470/750	Foil + kraft + scrim	Class 1 (barrier)	Roof underlay (tile or metal), wall sarking warm/temperate zones
Anticon 60/80/130	Foil + glasswool batt + scrim	Class 1 (barrier)	Under metal roof with top-hat battens (provides air gap)
Bradford Enviroseal ProctorWrap	Foil-faced vapour-permeable	Class 3 or 4 (permeable)	Wall and roof in cool climates (zones 6-8)
Kingspan AIR-Cell Permeable	Foil + bubble core + permeable face	Class 3 (permeable)	Roof and wall, contributes some R-value via cell core

Brand sheets vary by product code; always confirm the specific product’s vapour class and BAL rating on the supplier datasheet before specifying.

Specification call-outs for builders

On the drawings or in the spec, foil sarking selection needs:

Vapour permeance class (Class 1, 2, 3, 4 per AS/NZS 4200.1).
Water-control class (W1 high, W2 medium, W3 low per AS/NZS 4200.1).
BAL rating for bushfire-prone sites under AS 3959:2018.
Reflective face requirement: which side faces the airspace.
Adjacent airspace dimension: ≥20 mm where the reflective contribution is claimed.

If any of these is missing on the drawings, ask the designer rather than substituting on site.

Installation essentials per AS/NZS 4200.2

The standard’s install requirements that matter most on a residential site:

Laps: minimum 150 mm side and end laps, taped or tucked under the framing rule.
Sag in roof applications: deflection between battens should not exceed 50 mm under the weight of the membrane plus expected snow or debris load.
Fixings: cap nails or staples through battens; do not staple through the field of the membrane where water can find the perforation.
Penetrations: every pipe, vent, conduit, or down-light cutout must be taped or boot-flashed with manufacturer-approved tape.
Reflective face orientation: the foil side must face the airspace it relies on for the R-value claim. Getting this backwards is a common site error and wipes out the design R-value.

What to ask the supplier

When ordering, get a written confirmation of:

Product code and roll size (typical 1.35 m x 30 m for residential).
AS/NZS 4200.1 classification: vapour permeance class, water-control class, fire spread index, mass per unit area.
BAL rating if the site is in a bushfire-prone area.
AS/NZS 4859.1 R-value table with airspace dimension assumptions, where the product is marketed as contributing R-value.
Manufacturer’s install guide for tapes and accessories. The same brand’s tape is usually a warranty condition.

A supplier who can’t email the datasheet within an hour usually doesn’t have a current AS/NZS 4200 compliance certificate. That’s a red flag on a Class 1a residential build.

Common builder issues

Foil pressed flat against the underside of roof sheeting: no airspace, no thermal benefit. Top-hat battens or counter-battens are the fix.
Class 1 foil sarking specified in zone 7 or 8: condensation risk. Push back to the designer with a vapour-permeable alternative.
Tears at penetrations not taped: water shedding compromised. Tape every cut and overlap per the manufacturer’s install guide (and AS/NZS 4200.2 generally requires 150 mm laps).
Mixing foil sarking with bulk insulation either side: the foil between two bulk layers contributes nothing. Either move it to a position with an airspace or replace it with a vapour control layer that does the right job.
Stapling holes left untaped: each unsealed perforation is a moisture path. Tape or use cap nails per the install spec.

References

AS/NZS 4200.1:2017, Pliable building membranes and underlays, Part 1: Materials (verified 2026-05-16)
AS/NZS 4200.2:2017, Pliable building membranes and underlays, Part 2: Installation (verified 2026-05-16)
ABCB Condensation Handbook 2019 (verified 2026-05-16)