Short answer
Often, yes. Reflective insulation derives part of its thermal performance from adjacent air spaces, but the required arrangement depends on the exact product and certified construction. Many UK multifoil systems specify approximately 20–25mm cavities; the manufacturer's current certified build-up should control the installation.
- Air spaces and core resistance are separate
- Typically ≈20–25mm per face
- Still and sealed, not ventilated
- Formed with battens
- Certificate governs
Why a reflective surface needs a gap
Radiant heat travels across open space. A low-emissivity foil facing that space emits and absorbs far less infrared than plaster, timber or wool would. Take the space away by pressing the foil against another surface and there is nothing to reflect across; the only heat path left is conduction through a film a fraction of a millimetre thick, which resists almost nothing.
How big
| Product | Gap | Which faces |
|---|---|---|
| Multifoil quilt | Typically around 20 – 25mm, per certificate | Usually both — the roof side and the room side |
| Single reflective foil / bubble | 20mm+ | The reflective face |
| Reflective VCL | 20mm+ service void | The reflective face |
Bigger is not better beyond about 25–40mm: convection starts inside a wide void and the benefit flattens. Smaller is much worse: below 15mm the gap contributes little and installation tolerances close it altogether.
Still air, not ventilated air
The gap that gives thermal benefit is a still air space, sealed at its edges. A ventilated space (the cold-roof ventilation path above a membrane, for example) is a different thing with a different job — it removes moisture. A good roof has both, in the right places: ventilated void on the cold side, still gaps either side of the multifoil. Condensation guide.
How installers form the gap
- Rafter-side gap: the quilt is fixed to the rafter faces, so the rafter depth above it forms the gap to the membrane or covering. If the rafters are shallow or the roof has PIR between them, a counter-batten is added first.
- Room-side gap: 25mm × 38mm or 25mm × 50mm battens are fixed through the quilt into the rafters, plasterboard goes on the battens. This also creates a service void for cables.
- The quilt is tensioned, not sagging into either gap.
- Edges are taped to adjacent surfaces so the gaps are still.
See how air-gap and ventilation questions are assessed in a typical rafter project →
What happens without it
- The certified system resistance no longer applies; only the quilt's own core resistance remains, which for thin products is a fraction of the system figure.
- Building control can refuse sign-off because the build-up does not match the certificate.
- A vapour-resistant layer ends up hard against a cold surface — a condensation risk.
Rafter depth and headroom are measured on the survey; if the gaps cannot be formed we say so and quote the alternative.
Questions homeowners ask
Does foil insulation need an air gap?
What size air gap for multifoil?
Can the air gap be ventilated?
Can foil touch the tiles or membrane?
- SuperFOIL — product range — checked 2 September 2026
- YBS Insulation — SuperQuilt technical information — checked 2 September 2026
- TLX Insulation — TLX Silver technical information — checked 2 September 2026
- BBA — Agrément certificates search — checked 2 September 2026
- BSI — BS 5250:2021 Management of moisture in buildings — checked 2 September 2026