Reflective airspace
A reflective airspace is the 20 mm air gap (AS/NZS 4200.2) a foil face needs to deliver R-value; pin it flat against a surface and it works only as a vapour barrier.
Ask Chalkline about this →A reflective airspace is the air gap (a minimum of 20 mm under AS/NZS 4200.2:2017) that sits next to a reflective foil face and lets it work as insulation. A foil only resists radiant heat across an adjacent airspace; the gap is where the radiant heat transfer it blocks would otherwise happen. No airspace, no radiant R-value.
This is the most-missed detail with foil sarking and reflective insulation. Pin the foil hard against another surface, the cladding, the frame, or a layer of bulk insulation, and it stops working as insulation entirely; it then acts only as a vapour barrier. That is exactly what happens under direct-fix cladding with no cavity: the foil can still do a vapour-control job, but the R-value the spec claimed for it is gone.
If a wall or roof build-up relies on a reflective foil for part of its R-value, make sure the construction actually leaves the 20 mm airspace on the reflective face, and check whether you have one or two reflective faces (an airspace each side does more). Where you cannot deliver the gap, choose the membrane for its vapour and weather job and get the thermal performance from bulk insulation instead. See bulk vs reflective insulation.
Also known as: Reflective air gap, adjacent airspace, reflective cavity.
Category: Insulation / Building science.
Related
See also
References
- AS/NZS 4200.2:2017 Pliable building membranes and underlays, Installation, Standards Australia (verified 2026-05-16)
- AS/NZS 4859.1 Thermal insulation materials for buildings, Standards Australia (verified 2026-05-16)
Last updated: 2026-05-30. Verified: 2026-05-16. Quarterly review for currency.