Short answer
Foil insulation uses reflective surfaces facing air spaces to cut radiant heat transfer. Certified multifoil quilts are a mainstream thin insulation for roof slopes and loft rooms; thin bubble foils are radiant barriers for outbuildings. On an open loft floor, mineral wool usually does the same job for less.
- Three products share the name
- Multifoil is certified primary insulation
- Air spaces are part of the system
- Best in roof slopes and loft rooms
- Wool wins on open loft floors
The short answer
“Foil insulation” covers three quite different products. Single-layer reflective foil (often a foil face on bubble wrap) is a radiant barrier with modest insulation value. Multifoil quilts stack many reflective films with wadding and are a serious rafter-level insulation in their own right. Foil-faced boards such as PIR use foil as a facing on a rigid core. When people search for foil insulation they usually mean the first two — and the difference between them is the difference between a marketing product and a building material.
Where are you insulating?
Roof slope and loft rooms
The best case for multifoil. Fitted under or between rafters with an air gap either side it delivers a warm roof without losing headroom to thick boards. Foil roof insulation.
Open loft floor
Usually the wrong case. Mineral wool to 270mm costs a fraction and performs as well. Foil earns its place on a loft floor only when storage boarding is going over it. Foil for lofts.
Walls
Multifoil on stud walls or as part of an internal wall system where depth is tight. Never in a cavity.
Floors and outbuildings
Between joists in a suspended floor, or lining a garage, shed or campervan where a thin, easy-to-fix product suits. These are DIY-friendly jobs.
How it works
Heat crosses a roof three ways: conduction through solid material, convection through moving air, and radiation across gaps. Mineral wool and PIR slow conduction by holding still air in a thick layer. Foil attacks radiation: a low-emissivity surface facing an air gap reflects most radiant heat back where it came from. A multifoil quilt does both — its wadding layers hold air, and each reflective film facing a gap cuts radiant transfer. That is why the air spaces in the manufacturer's certified build-up matter: the reflective contribution needs a space to act across, and it is a separate quantity from the resistance of the quilt's own core.
What the declared performance means
Multifoil manufacturers publish performance in one of two ways: a certified thermal resistance for the quilt including its air gaps, or a “PIR equivalent” claim. Ask for the third-party certificate (BBA, LABC or similar) and check what build-up it assumes. The figure only applies when the product is installed exactly as certified — taped joints, air gaps on both faces, continuous layer. Building control will want to see the certificate and the installation match.
Where it works well
- Thin: 30–45mm quilt, ideal where headroom or depth is limited
- Fast to fit: staple, overlap and tape; no cutting boards to fit between rafters
- Built-in vapour control layer on the warm side
- Lightweight, so no structural concern on old roofs
- Good in loft conversions, dormers and garages
Reasons not to choose it
- Open loft floors: mineral wool does the same job for far less
- Performance collapses without the specified air gaps
- Cost per m² is higher than wool or basic PIR
- Cannot be compressed or boarded directly over
- Not a cavity wall product and not a substitute for a damp-proof course
Answer six questions. We tell you whether multifoil, PIR or plain mineral wool is the sensible starting point — before asking who you are.
Foil against the alternatives
| Multifoil | PIR board | Mineral wool | |
|---|---|---|---|
| Thickness for a roof slope | 30–45mm + gaps | 120–150mm | 200mm+ |
| Install complexity | Low | Medium — cutting and sealing | Low |
| Cost band | Mid | Mid to high | Low |
| Best for | Loft rooms, low headroom | Warm roofs, floors, walls | Loft floors |
| Moisture risk | Low if gaps kept | Needs careful sealing | Needs ventilated loft |
Full head-to-heads: Multifoil vs PIR and Multifoil vs mineral wool.
Installed cost
Professionally fitted multifoil to a roof slope runs roughly £45–£80 per m² including battens and taping, or £2,000–£6,500 for a typical semi-detached roof. Material alone for a DIY job is £8–£15 per m² for single-layer reflective foil and £15–£30 per m² for a certified multifoil. Full cost breakdown.
Building regulations and installation
Whether Building Regulations approval is required depends on the nature and scope of the work. Some insulation work within existing roof spaces may not itself require an application, while renovation of a thermal element, structural alterations, conversions or other controlled work can trigger requirements. Transitional arrangements and the date and scope of a project can affect which edition of the guidance applies. Check property-specific projects against current Building Regulations guidance and, where appropriate, local building control. Where approval applies, your installer or building control will need the product certificate, the target U-value and the ventilation strategy. Three rules that every good foil installation follows:
- Air gap of at least 20–25mm on both faces of the quilt, usually made with counter-battens.
- Joints overlapped and taped with the manufacturer's tape; the layer is continuous, including around dormers and at the eaves.
- Ventilation of the roof void maintained, or a breathable membrane and a designed cold-roof ventilation path.
Funding
Foil is a product, not a funded measure. If your home may qualify for funded roof or room-in-roof insulation, the installer chooses the material. See current insulation grants.
Everything else you need to know about foil
A home assessment checks roof, loft, ventilation, rafter depth and existing insulation, then a specified build-up and fixed quote.
Questions homeowners ask
Does foil insulation actually work?
Which way should foil insulation face?
Does foil insulation need an air gap?
Does foil insulation cause condensation?
Can foil replace mineral wool in a loft?
What U-value does foil insulation achieve?
- SuperFOIL — product range — checked 2 September 2026
- Energy Saving Trust — Roof and loft insulation — checked 2 September 2026
- GOV.UK — Room in roof insulation best practice — checked 2 September 2026
- GOV.UK — Approved Document L, Volume 1: Dwellings (current edition, as amended) — checked 2 September 2026
- BSI — BS 5250:2021 Management of moisture in buildings — checked 2 September 2026