Does Foil Insulation Actually Work? Foil vs Fibreglass vs PIR

Does foil insulation actually work?

Yes — but not in the way most people assume, and not as a replacement for the insulation you already have.

Foil insulation works on a different physical principle from mineral wool or PIR board. It doesn’t trap air; it reflects radiant heat. That makes it genuinely effective at the job it is designed for, and largely ineffective at the job people sometimes buy it for. The honest answer is that a foil membrane is an excellent supporting layer and a poor substitute for bulk insulation.

Used properly — on the warm side, with an air gap, alongside a primary insulant — foil earns its place in a build-up. Used as a straight swap for 100mm of loft roll, it will disappoint.

How reflective foil insulation works

Heat moves through a building in three ways. Conduction is heat travelling through solid material. Convection is heat carried by moving air. Radiation is heat travelling as infrared energy across a space, needing no medium at all — the way you feel warmth from a fire across a room.

Bulk insulation like mineral wool and PIR tackles conduction and convection. It works by holding millions of tiny pockets of still air, and still air is a poor conductor. The thicker the layer, the more resistance to heat flow.

Foil tackles radiation. A polished aluminium surface has very low emissivity, meaning it radiates very little heat itself, and high reflectivity, meaning it bounces incoming radiant heat back where it came from. Our aluminium foil membrane offers up to 97% radiant heat resistance. That is why it belongs facing the warm side of the structure — it reflects the room’s radiant heat back into the room.

This also explains why foil needs an air gap. Radiant heat only travels across a space. If you press the foil hard against a plasterboard or an insulation board, heat crosses the contact by conduction and the reflective property does nothing. Leave a cavity — even 25mm — and the reflective mechanism engages.

Foil vs fibreglass vs PIR

The three are not really competing. In a well-built wall or roof you will often find all three principles at work: PIR or mineral wool doing the bulk thermal work, a foil membrane on the warm side handling vapour control and radiant reflection, and an air gap between them making the foil effective.

When foil is the right choice (and when it isn’t)

Foil is the right choice when you are short of depth and need every millimetre to count; when you need vapour control and want thermal benefit from the same layer; when you are insulating under a floor with underfloor heating and want to push heat upwards; when you are working on a timber-frame, garden room or modular build where weight and handling matter; and when summer overheating is as much of a concern as winter heat loss — reflective layers are particularly good at rejecting solar gain through a roof.

Foil is the wrong choice when you are trying to hit a demanding U-value on its own; when you cannot provide an air gap on at least one face; when the space allows plenty of depth and cheap bulk insulation would do the job better per pound; and when the product will be compressed or covered on both faces with no cavity.

Be sceptical of any claim that a few millimetres of foil equals 100mm of mineral wool in all conditions. In the right build-up a foil membrane makes a real contribution, and the manufacturer’s tested figures for that specific build-up are what you should design to.

The air-gap rule: why installation makes or breaks performance

This is the single detail that separates foil insulation that works from foil insulation that doesn’t.

The reflective face needs an air space adjacent to it. In a wall, that usually means battening out on the room side of the foil before boarding, which conveniently also creates a service void so cables and socket boxes never puncture the membrane. In a roof, it means maintaining a gap between the foil and the plasterboard beneath, or between the foil and the insulation above.

A 25mm gap captures most of the available benefit. Larger gaps add a little more but with diminishing returns, and a very large gap starts to allow convection currents that work against you.

Some laminated foil products are designed to be installed tight against insulation and still perform, because their construction accounts for it — always check the product notes rather than assuming.

Orientation matters too. The reflective aluminium surface faces the warm side, towards the room. Fitted the other way round it reflects heat away from where you want it.

Using foil membrane as a vapour barrier and insulator

This dual function is where foil earns its keep on most UK jobs. One layer, one fixing operation, two benefits.

As a vapour control layer, taped foil membrane forms a continuous barrier that stops humid internal air entering the structure — preventing the interstitial condensation that damages insulation and rots timber. As a reflective layer it returns radiant heat to the room. And because it is a sealed, taped layer, it also contributes to airtightness, which matters for Part L compliance.

In practical terms that means one product does the work of a polythene VCL plus a separate reflective layer, with better tear resistance than thin polythene and easier detailing with reinforced foil tape. The membrane is compatible with PIR boards, mineral wool and EPS, so it drops into whatever primary insulation your build-up already uses.

What to pair foil with for building-regs U-values

For regulated elements you need a build-up that hits the target U-value, and in practice that means bulk insulation as the primary layer with foil supporting it.

Typical pairings that work well:

• Timber-frame wall — mineral wool between studs, foil membrane on the warm face, battened service void, plasterboard
• Room in roof — PIR between and under rafters, foil membrane on the warm face, service void, plasterboard
• Cold loft — mineral wool loft roll at joist level, foil VCL above ceiling plasterboard
• Underfloor heating — floor insulation below, foil membrane above, screed over
• Garden room — mineral wool or PIR in the frame, foil membrane inside, breathable membrane outside

Our premium foil membrane outperforms standard foil products, achieving a 0.35 U-value with an 89mm stud wall. In favourable build-ups a foil layer can reduce the insulation thickness required by around 5–10mm, and up to 10–20mm in the best cases, though this varies with construction type and target U-value. Where depth is genuinely tight, that saving can be the difference between a build-up that fits and one that doesn’t.

Frequently asked questions

Does foil insulation really work?

Yes, for what it is designed to do — reflecting radiant heat and controlling vapour. It works best with an air gap on the reflective face and alongside bulk insulation rather than instead of it.

Is foil or fibreglass insulation better?

Neither is better in isolation; they do different jobs. Fibreglass provides bulk thermal resistance cost-effectively. Foil reflects radiant heat and provides vapour control in a few millimetres. Most good build-ups use both.

Does foil insulation need an air gap?

Usually yes — the reflective surface works best with an air space, typically 25mm or more. Some laminated foils can be installed tight to insulation, so check the product notes.

Can foil insulation replace loft roll?

No. For loft insulation, bulk material like mineral wool loft roll does the primary thermal work. Foil membrane complements it, most usefully as a taped vapour control layer above the ceiling.

Which side of foil insulation faces out?

The reflective aluminium surface faces the warm side of the structure — towards the heated room, not towards the cold side.

Products mentioned in this guide

• 50m² Aluminium Foil VCL 1m × 50m
• Aluminium Bubble Foil Insulation Roll 0.6m × 7.5m
• Reinforced Aluminium Foil Tape
• Insulation category (Knauf Earthwool, DriTherm)

Related guides

• What Is a Vapour Control Layer (VCL)?
• Vapour Barrier vs Breathable Membrane