Short answer
Foil's advantages are thickness, speed and built-in vapour control; its disadvantages are dependence on air spaces, higher cost per m² than wool, and being wrong for open loft floors and cavities. The balance flips by application.
- Best in roof slopes and loft rooms
- Worst on open loft floors
- Air spaces are the risk
- Cost per m² above wool
The overall balance
Where it works well
- Thin: 30–45mm quilt does the job of 120mm+ of board in a certified roof build-up
- Fast and clean to fit: staple, overlap, tape — no cutting boards between uneven rafters
- Built-in vapour control on the warm side
- Light, flexible, no dust; easy around dormers and odd angles
- Reduces summer radiant gain into loft rooms
- Certified products with BBA or LABC assessments exist
Reasons not to choose it
- Performance collapses without 20–25mm air gaps on both faces
- Higher cost per m² than mineral wool and basic PIR
- Cannot be compressed, boarded over or squashed by storage
- Vapour-tight: in the wrong position it causes condensation
- Marketing 'equivalence' claims are often stretched beyond the certificate
- Wrong product for open loft floors and cavity walls
Roof slopes and loft rooms
For: headroom, speed, continuity around dormers, certified performance. Against: the gaps must be formed with battens and protected for the life of the roof, and the ventilation path must be designed. Verdict: the strongest case for foil. Roof guide.
Open loft floors
For: very little. Against: cost, a vapour-resistant layer over the ceiling, and mineral wool does the same job for a fraction. Verdict: choose wool unless boarding forces a thin layer. Loft guide.
Under loft boarding
For: adds performance without adding height, keeps wool from being crushed. Against: still needs an air gap to the boards. Verdict: sensible as a supplement. Boarding guide.
Walls
For: thin lining for stud walls and garage conversions. Against: never in a cavity; solid walls usually need a designed system. Verdict: niche. Wall guide.
Floors
For: easy to drape between joists in a suspended timber floor. Against: underfloor ventilation must be kept; performance relies on the gap. Verdict: reasonable, wool or board often better value. Floor guide.
Sheds, garages and garden rooms
For: cheap, quick, big comfort improvement. Against: not a substitute for proper insulation in a room used all year. Verdict: the best DIY use of thin foil.
The check tells you whether foil is a sensible starting point for your property before any survey.
Questions homeowners ask
Is foil insulation worth it?
What is the biggest disadvantage?
Is foil better than fibreglass?
- Energy Saving Trust — Roof and loft insulation — checked 2 September 2026
- SuperFOIL — product range — checked 2 September 2026
- BBA — Agrément certificates search — checked 2 September 2026