Reflective insulation
Reflective insulation is low-emittance foil. Only contributes to R-value if installed next to a still airspace 20-25 mm+. Without airspace it conducts and adds nothing.
Ask Chalkline about this →Reflective insulation is a low-emittance foil-faced product (emittance e ≤ 0.05) that reduces heat transfer by reflecting infrared radiation back toward its source rather than absorbing or conducting it. Unlike bulk insulation (which traps still air in fibres to resist conduction), reflective insulation works only against the radiant component of heat transfer, and only when installed adjacent to a still airspace of at least 20-25 mm. Without that airspace, the foil acts purely as a conductor (aluminium has high conductivity), the radiant benefit collapses, and the R-value falls to near zero. Verified per AS/NZS 4859.1:2018 (2026-05-16).
How reflective insulation works:
| Heat-transfer mode | Reflective response |
|---|---|
| Radiation (infrared waves from sun, hot surfaces) | Low-emittance foil reflects ~95% back |
| Conduction (direct contact) | High; aluminium foil conducts well; the foil contributes nothing if in direct contact with a hotter or colder surface |
| Convection (air movement) | Low contribution; depends on whether the airspace is still |
The key insight: the foil’s job is to reduce radiant heat transfer across an airspace. If you eliminate the airspace, the foil’s purpose is defeated.
The airspace requirement (critical):
| Airspace dimension | Effect on R-value contribution |
|---|---|
| < 5 mm | Negligible R-value contribution |
| 5-15 mm | Limited R-value (~R0.5-1.0) |
| 20-25 mm | Significant R-value (~R1.5-2.0 across the airspace) |
| 40 mm+ | Optimal radiant blocking; ~R2.0-2.5 |
| Compressed (foil in contact with material) | R-value falls to zero; pure conduction |
The R-value is the cumulative effect of the airspace + foil combination, not the foil alone. NCC 2022 energy efficiency calculations require the system R-value including the airspace, not just the foil component.
Typical applications:
| Application | Description |
|---|---|
| Roof sarking under tiles | Foil sheet draped over rafters/battens, creates airspace above with the tile cavity; reflects heat back into the cavity, reduces transmission to the ceiling |
| Roof sarking under metal roofing | Same role as tile sarking; reflects radiant from hot metal |
| Wall sarking behind cladding | Behind the cladding face, creates an airspace at the cladding cavity |
| Foil-faced batts | Bulk batts with reflective foil facing toward the airspace |
| Foil-faced foam boards | Often used for under-slab insulation, where the foil faces the (intentional) airspace beneath the slab |
Reflective vs bulk insulation:
| Property | Reflective | Bulk |
|---|---|---|
| Mechanism | Radiates back | Traps still air |
| R-value (alone) | Near zero | Direct R-value from thickness |
| Requires airspace? | Yes, mandatory | No, works directly |
| Compression | Catastrophic R-value loss | Some R-value loss; less catastrophic |
| Moisture vulnerability | High (dirt/moisture coats the foil, increases emittance) | Some (especially fibreglass, cellulose) |
| Best for | Radiant-heat-dominated (roofs in summer) | Conductive-heat-dominated (cold winter walls) |
| NCC compliance | Contributes to system R-value with airspace | Direct R-value reading |
Common defects:
- Foil compressed against cladding or roof underside: airspace lost; R-value falls to zero. Most common defect.
- Dirty foil in a roof space: dust and condensate cover the foil; emittance increases over years; R-value declines.
- Foil torn or perforated: Although tiny holes don’t affect radiant reflection much, they allow convection across the airspace, reducing thermal performance.
- Foil installed without an airspace “to save space”: worse than no insulation in some cases (because no useful R-value from a thermal break either).
- Anti-glare matt finish foil specified: matt black foil has e > 0.05, doesn’t meet reflective insulation criteria.
Vapour permeance interaction:
Some reflective insulation products are also classed as vapour membranes under AS/NZS 4200.1:2017. The vapour permeance class (1-4) determines whether the foil can be installed on the “warm in winter” side without trapping condensation:
| Climate zone | Foil placement |
|---|---|
| Cold climate (zones 6-8) | Vapour-impermeable foil on warm-in-winter side: OK in some cases |
| Mixed climate (zones 4-5) | Avoid vapour-impermeable foil on warm side; risk of summer condensation |
| Hot-humid climate (zone 1-3) | Vapour-impermeable foil works on cold side (cooled side) |
A reflective product’s permeance class matters for both thermal and condensation control. Specify by climate zone.
Builder takeaway:
- Always pair reflective with an adequate airspace (20 mm+).
- Don’t expect a reflective product to do bulk-insulation work.
- In hot-summer climates (zone 1-3), roof sarking foil is high-value.
- In cold-winter climates (zone 6-8), bulk insulation does most of the work; reflective is supplementary.
Also known as: foil insulation; radiant barrier; emissive barrier; low-e barrier; sarking (loose usage; sarking technically refers to the under-roof or under-cladding placement).
Category: Materials.
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Last updated: 2026-05-16. Verified: 2026-05-16. Quarterly review for currency.