Reflective insulation is a material that sends back the heat it receives through radiation instead of absorbing it. Unlike mineral wool or polystyrene, which slow down the heat passing through their thickness, a reflective surface refuses it from the start.
It's the only difference that matters, and everything else follows from it: what these products are used for, where they're installed, why they need an air space, and why two products that look identical can perform completely differently.
How it works
Heat reaches a wall or a roof through three paths: through the solid material, along with circulating air, and through radiation — as a wave, without heating the air along the way.
In summer, radiation is the main way heat enters through a roof. The tiles or metal sheeting heat up in the sun, and heat comes down from there like from a radiator. Mineral wool absorbs it, holds it in its mass, and releases it a few hours later, indoors — which is why an attic is hottest in the evening, not at noon.
A reflective surface doesn't absorb it. It sends it back before it can enter. In winter, the same surface works in reverse: the heat from the room is reflected back into the room instead of being absorbed into the structure.
The condition most people get wrong
A reflective film glued directly to another material doesn't reflect anything. It conducts.
To work, it needs a layer of still air next to the reflective surface. Without that gap, the product can be excellent and the result zero. This is the most common installation mistake, and it explains most cases of "I put up the film and nothing changed."
Real aluminum or metallized film
This is where the difference lies that nobody sees on the shelf, because both shine the same way.
Aluminum foil is real metal, laminated, tens of microns thick. Metallized film is a layer of vaporized metal on plastic, a fraction of a micron thick.
The difference is measured by a single value — emissivity, meaning the proportion of heat the surface lets through instead of reflecting back. The lower it is, the better:
| Surface | Emissivity | What it means |
|---|---|---|
| Pure aluminum (Isoflect Roof Pro, Gold) | 0,05 | Almost all the heat is reflected back |
| Aluminized layer (Isoflect Roof Eco) | 0,18 | More than three times as much heat passes through |
Values from Isoflect technical data sheets, measured according to prEN 15976, before and after the standard's artificial aging test.
How to tell them apart when buying
Not by how it looks and not by the word "reflective" on the packaging. Ask for the emissivity value in the technical data sheet.
A manufacturer that works with pure aluminum will state it, because it's their selling point. If the value is missing from the data sheet or the seller can't tell you, you already have your answer.
What reflective insulation does not do
It does not replace thickness. Mass handles the heat that passes through the solid material, and that's where mineral wool or fiber insulation remains necessary.
A correct build-up uses both: fiber insulation for the heat passing through the material, a reflective surface for the heat coming through radiation, and sealed joints for the heat that leaves with the air. Anyone selling reflective insulation as a replacement for conventional insulation is selling it wrong.
Where it's used
- Under the roof covering, over the roof sheathing — against summer heat and condensation
- Indoors, between and under the rafters — in attics, where there are no inches to spare
- On walls and ceilings, under the drywall board
- In the floor, under the screed and under heating systems
- On unheated attics, as a shield against heat waves
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