Walk through a roof space and you may see a shiny foil layer under the roof — reflective insulation, working in a completely different way to the batts on the ceiling. Reflective foil reflects radiant heat rather than resisting conducted heat, which makes it especially useful against summer roof heat but limited as a standalone solution. Understanding what foil does, and what it does not, helps you see where it fits. This guide explains how reflective foil works, why it needs an air gap, where it is used, and how it complements bulk insulation.
What Reflective Foil Is
Reflective foil insulation is a thin, shiny material — typically aluminium foil bonded to a backing such as paper or a woven membrane for strength. Unlike bulk insulation, which is thick and full of trapped air, foil is thin and works entirely through its reflective surface. In a roof, it is most often seen as sarking: the foil layer running under the roof tiles or sheeting. Its role is to deal with radiant heat, the part of heat transfer that bulk insulation is least focused on, which is why the two are different tools for different parts of the same job.
How It Works — Radiant Heat
Heat moves in three ways: conduction (through materials), convection (via moving air), and radiation (radiating across a space). Reflective foil targets the third. A hot roof radiates heat downward, and a reflective surface facing that radiation bounces most of it back rather than absorbing and re-emitting it onward. This is the same principle as a foil sunshade or a thermos. Because it reflects rather than absorbs, foil stays effective at blocking radiant heat as long as its surface remains shiny and faces a gap. It does little, however, against conducted heat — which is where bulk insulation comes in. See our how insulation works guide.
Why the Air Gap Matters
The single most important thing to understand about reflective foil is that it needs an adjacent air gap to work. Foil reflects radiant heat across a gap; if it is pressed directly against another material, heat conducts straight through the contact and the reflective benefit is largely lost. This is why foil sarking is installed with the shiny side facing into the roof cavity air space, and why a squashed or dust-covered foil loses much of its effect. Dust settling on a horizontal foil surface over time also reduces its reflectivity. Correct installation — shiny side to the air gap, kept clean — is essential for foil to deliver.
Where It Is Used
The most common use of reflective foil in Melbourne homes is as roof sarking — the foil layer beneath the roof covering — where it reflects radiant heat from the hot roof before it enters the cavity, and also provides a secondary barrier against wind-driven rain and dust. It can also be used above the ceiling alongside bulk insulation. In new builds and re-roofing, reflective sarking is easy to include as part of the roof. Retrofitting foil into an existing, finished roof is more constrained, depending on access and how the roof is built — which is why bulk insulation, laid on the ceiling, is usually the more practical retrofit for an existing home.
The Limits of Foil
Reflective foil has real limits that mean it is not a complete insulation solution on its own. It only addresses radiant heat, so it does little against the conducted heat loss that dominates in winter. It depends entirely on an air gap and a clean, shiny surface, both of which can be compromised. And its performance is not counted in the bulk R-value, so a ceiling relying on foil alone would be poorly insulated by modern standards. Foil is a useful tool for a specific job — reflecting radiant roof heat, mainly in summer — not a substitute for adequate bulk insulation on the ceiling. See our bulk vs reflective guide.
Foil With Bulk Insulation
The best result comes from using foil and bulk insulation together, each doing what it does best: reflective foil under the roof reflecting radiant summer heat, and bulk insulation on the ceiling resisting conducted heat in both seasons. This combination tackles all the modes of heat transfer — radiation reflected at the roof, conduction and convection resisted at the ceiling. For most existing Melbourne homes, the priority and the practical retrofit is bringing the bulk ceiling insulation up to R5.0–R6.0; where reflective sarking is already present or can be added (during re-roofing, for instance), it is a valuable complement, especially for summer comfort. FreshDuct can advise on the right combination for your home — call 0431 918 137.