This is the most common question we get about foil insulation, and the honest answer is the one many suppliers avoid: without an air gap, a reflective surface does almost nothing as insulation. Here is why, and what to do about it.
How a radiant barrier works
Heat moves in three ways. Conduction passes through materials that touch. Convection moves with air. Radiation travels as infrared through empty space, from any warm surface to any cooler one facing it. A low-emissivity aluminium surface is poor at radiating and good at reflecting, so when it faces an air space it blocks most of the radiant transfer across that space.
The key phrase is faces an air space. Radiation needs a gap to cross. If the foil is pressed against plasterboard, timber, concrete or another insulation layer, there is no radiation to reflect; heat simply conducts through the foil and into whatever it touches. Aluminium is an excellent conductor, so the foil then does slightly worse than nothing.
What the core adds
Foil-bubble and foil-foam products have a polyethylene core that provides a small amount of resistance to conduction. It is real but modest, comparable to a thin sheet of foam. Suppliers who quote a large R-value for a thin foil-bubble sheet are including the air gaps of a specific test assembly in that number. Read the test report: the gap is part of the result.
Designing the assembly
- Leave a gap of at least 20 mm between the foil face and the next surface. More is not better beyond about 40 mm; less quickly loses effect.
- Face the foil towards the heat source in hot climates (under a roof, foil up towards the roof deck) and towards the heated space in cold climates, or use a double-faced product.
- Keep the foil clean. Dust on the surface raises emissivity over time. In roof spaces the downward-facing foil stays cleaner than the upward-facing one.
- Seal the edges so warm air does not convect around the barrier and bypass it.
- Combine with bulk insulation where a real thermal resistance is required. Reflective insulation complements mineral wool or foam; it does not replace them.
Which way should the foil face?
Towards the side the heat is coming from, with the air gap on that side. Under a roof in a hot climate, the foil faces up towards the deck with the gap above it, so the radiant heat coming down from the sun-heated deck meets the reflective surface first. In a cold climate the logic reverses: the warm side is the room, so the foil faces the heated space to keep radiant heat from leaving. Where the direction of heat flow changes with the season, a double-faced product with foil on both sides and a gap on each side does both jobs. Whatever the orientation, a foil face pressed against a solid surface is facing the wrong way, because there is no air space for it to work across.
Where reflective insulation earns its place
Wherever there is a cavity to install it in and radiation is a large share of the heat crossing that cavity. Roof spaces in hot climates are the classic case: the underside of a sun-heated deck radiates strongly downward, and a foil layer with a gap below it intercepts most of that. Wall cavities, floor voids over crawl spaces and the inside of metal-clad buildings are similar. It earns less in a cold, well-sealed building where most heat leaves by conduction through the envelope, and nothing at all where it is sandwiched with no gap.
When there truly is no room for a gap
Use the foil for what it does without a gap: a vapour barrier, a dust and moisture skin, a reflective finish, or a lightweight cushioning liner in a bag. Choose the construction for those properties and do not count on it for thermal resistance.
We supply all three constructions by the roll and will tell you plainly which one suits your assembly — the closed-cell option is Foil-Foam Insulation, XPE 4.5 mm, Grey, which carries its own separation and so does not depend on a site-built cavity. Describe where it will be installed and we will recommend a construction and send its test data.