A reflective liner is a thin sheet, usually a layer of bubble wrap or foam sandwiched between two skins of metallised film, that blocks heat carried by radiation rather than heat carried by conduction. Drape it over a pallet or slip it inside a parcel and it works the way a car windscreen shade works: it reflects radiant heat, sunlight or a hot loading dock, away from the surface underneath, instead of slowing heat that is already moving through direct contact.
That is a narrow job, and it is worth being precise about it, because reflective liners are marketed far more broadly than the physics supports. They are genuinely useful and genuinely cheap. They are not a substitute for expanded polystyrene, foam, or any other insulation that slows conductive heat flow, and confusing the two is the single most common mistake in how they get specified.
Radiant heat versus conducted heat
Heat moves into a shipment three ways: conduction through direct contact, convection through moving air, and radiation through electromagnetic energy, mostly infrared and sunlight. A foam wall works against conduction, its trapped air or gas slows heat moving through the material itself. A metallised film works against radiation only, its reflective surface bounces radiant energy back rather than absorbing it and letting it conduct through.
Once a shipment sits inside a closed truck, a warehouse, or a dark aircraft hold, radiant heat mostly disappears as a factor, because there is no direct sun or hot surface radiating at it. A reflective liner keeps doing some work against a warm surface nearby, but conduction and convection take over as the dominant heat paths, and a liner alone does very little against either.
The marketing overclaim
The most common overclaim is a reflective liner sold or specified as if it insulates the way foam does, sometimes with a claimed hold time that assumes the foil is doing conductive work it physically cannot do. A thin foil bubble layer has almost none of the thermal mass or trapped-gas thickness that gives EPS or polyurethane its holding time, so a shipment relying on foil alone in a hot truck or a cold warehouse drifts out of band far faster than the marketing implies.
The honest test is simple: strip radiant heat out of the picture, put the load in a dark, enclosed space, and a reflective liner on its own does close to nothing. Any claim built on a sunlit-tarmac test alone describes the one scenario where the liner actually helps, not the shipment's whole route from origin to destination. A buyer comparing two shippers on a stated hold time should ask which test produced that number before trusting it.
Correct use: a cheap supplement, not a replacement
Used correctly, a reflective liner is a cheap add-on wrapped around or over an already-insulated load: a foil layer over a pallet of insulated shippers waiting on a hot tarmac, or a metallised bubble sleeve inside a parcel that already has a foam liner doing the conductive work. In that role it buys real, if modest, extra margin during the exposed legs of a trip, loading docks, tarmac staging, the back of an uncooled van in direct sun, at a cost close to nothing per shipment.
It earns its keep because it is light, thin, and cheap enough that adding it costs almost nothing against the box's total price. That is also its ceiling: a supplement layered onto a properly insulated system, not a system on its own. A reflective liner never shows up as the sole line item on a qualified shipper's bill of materials; where it does, the shipper has not been tested against the conditions it will actually face.
Pallet covers and parcel liners
The two common formats are a pallet-scale thermal cover, a large foil-faced blanket or shrink-wrap-style sheet draped over a stacked pallet for tarmac and yard staging, and a parcel-scale liner, a foil bubble sheet cut to fit inside a box alongside or instead of a foam layer. Food distributors and freight forwarders use pallet covers for short radiant exposure during transfer and staging; e-commerce and pharmacy parcel shippers use liners as a low-cost layer inside passive packaging that still needs a real coolant charge and a real insulation layer to hold its band.
Neither format replaces the insulation and coolant that do the actual holding. Both are legitimate, low-cost tools when a shipper understands exactly which kind of heat they stop, and a liability when specified as if they stop all three. Packaging engineers and procurement teams comparing quotes should ask a supplier directly which heat path a liner is rated against, since a stated performance figure with no ambient condition attached is not a specification, it is a guess dressed up as one.