Understanding Insulation Foil

This guide covers what each type actually is, where it belongs, and how to read the performance figures so you are comparing like with like.

How Reflective Insulation Works

Heat moves three ways: conduction through solid material, convection through moving air, and radiation across space.

Conventional insulation targets the first two. It traps still air in a fibre or foam structure so heat has to fight its way through.

Foil targets the third. Aluminium has very low emissivity, meaning it radiates very little heat away from its surface, and reflects most of what arrives. Emissivity figures for the products in this category typically sit between 0.03 and 0.16, against roughly 0.9 for a plasterboard or timber surface.

Bubble Foil Insulation

• What it is: a polyethylene air bubble membrane laminated with reflective foil on one or both faces, typically 3mm to 4mm thick. The bubbles trap air, the foil faces create low emissivity surfaces facing adjacent cavities.
• What it does well: it is the cheapest reflective product per square metre, arrives in a manageable roll, cuts with scissors and needs no PPE. Fully taped it also gives you an airtight vapour control layer, which in a draughty outbuilding is often worth more than the thermal figure.
• Where it belongs: garages, sheds, workshops, conservatory roofs, agricultural buildings, campervans and motorhomes, behind radiators, and as a supplementary layer over or under something else. Browse the full range of bubble foil insulation.
• Where it does not: any element you need to sign off against Approved Document L. Core thermal resistance on a bubble product is a fraction of a m²K/W. Manufacturers publish considerably higher installed figures, and those figures are real, but they include the air cavities either side. On its own, in a habitable room, this is not a Part L solution and no reputable supplier should tell you otherwise.

One practical warning: roll widths vary between 1.05m and 1.5m across the market at similar headline prices. Check the coverage in m², not the price of the roll.

Multifoil Insulation

What it is: multiple layers of reflective foil interleaved with wadding, foam or film, welded into a quilt. Thicknesses in the UK market run from roughly 30mm to 40mm, with anywhere from seven to nineteen layers depending on the product. Browse our full multifoil range.

Unlike bubble foil, multifoil works against all three heat transfer mechanisms at once. The outer reflective layers deal with radiation, the fibrous core resists conduction and convection. Most are a vapour control layer in their own right once taped and sealed.

There are two sub-types, and confusing them causes real problems.

The better products in this group compress to a few millimetres under tile battens, which means you can drape them over rafters and batten straight on top rather than adding counter battens. Check the certificate before assuming yours does. Some need counter battening, and some specify a minimum rafter depth when draped.

Orientation matters on these. Where a product has a distinct reflective face it will be certified to be installed one way round only.

Foil Backed Insulation

What it is: rigid or semi rigid insulation, usually PIR, phenolic or mineral wool, with a foil facing bonded to one or both faces. See the full foil backed range.

Here is the honest version, and it is worth saying plainly because the category name invites the wrong assumption: the foil is not the insulant. Effectively all of the thermal performance comes from the core. Take the same board with a paper or glass tissue facing and the lambda value barely moves.

The facing earns its place three other ways:

• Vapour control. It acts as a vapour control layer.
• Protection. It protects the core in handling and on site.
• Low emissivity. It provides a low emissivity surface to any adjacent unventilated cavity.

That third point has real value and it is properly recognised. Under BR 443, the conventions document behind UK U-value calculations, where a foil faced product sits next to an unventilated airspace of at least 25mm the airspace itself can be credited with additional thermal resistance in the calculation. That is a genuine gain, it just belongs in the cavity line of the calculation rather than in the board's R-value.

If you are shopping foil backed boards, compare them the way you would compare any rigid board: lambda value, thickness, compressive strength, fire classification. The facing is a specification detail, not the headline.

The R-Value Problem

This is where the category gets people into trouble. Three different numbers are all called R-value in this market, and they can differ by a factor of three for the same product.

None of these are dishonest in themselves. The problem is quoting the third and calling it the first.

The BBA's conventions for reflective foil insulation are explicit about how this has to be handled in a real calculation, and the rules are stricter than the marketing suggests:

• Compression counts as zero. Where the product is compressed between battens and rafters or studs, its thickness is taken as zero unless the compressed thickness and matching R-value have been robustly determined under representative installed conditions. Compression is not a marginal effect. Published compressed R-values for some quilts drop to under a tenth of the uncompressed figure.
• Sag is assumed. For a product installed above or below rafters, a 30/70 split of the product's opening is assumed to account for sag under gravity. In a wall, a 50/50 split.
• These are requirements, not guidance. The conventions are mandatory for calculations under the BBA and TIMSA U-value competency scheme.

One manufacturer in this market states openly that it quotes worst case R-values and declines to add air gap resistance into its headline figures, on the basis that doing so can mislead. That is the standard the whole category should be held to.

This also explains why some building control officers push back on multifoil. The certificates are legitimate, but they carry installation conditions that are frequently not met on site. Meet the conditions and the numbers hold. That is the whole argument.

For context, the dispute goes back to the mid 1990s, when multifoil arrived in the UK and manufacturers argued that in situ testing reflected real world performance better than the hot box testing conventional insulation went through. It was settled by Government Circular 06/2009, which established that U-values must be calculated using standardised European Norms or tests forming part of a relevant European Technical Assessment. The category is legitimate and regulation compliant. It just has to be calculated properly.

The Four Things That Decide Whether It Works

Almost every underperforming foil installation fails on one of these.

What Foil Genuinely Does Better

Set the disputed thermal claims aside and there is a solid case for these products.

• Space. This is the real reason most people buy multifoil. Thirty to forty millimetres doing work that would otherwise eat into head height. In a loft conversion with a tight ridge, that is often the difference between a habitable room and not.
• No fibres, no itch, no PPE. Foil quilts cut with a knife, do not shed irritant fibres and do not need a mask. If you are doing the work yourself over several weekends, that matters more than the spec sheet suggests.
• Recycled content. Recycled content in the leading multifoils runs from around 40% to over 80% depending on the product.
• Combined function. Insulation plus VCL, or insulation plus breather membrane, in a single pass. Fewer trades, fewer layers, fewer opportunities to get the sequencing wrong.
• Dimensional stability. No slumping or settling over time, unlike loft roll in an awkward space.

Claims to Be Sceptical Of

• "R-value up to X." Ask what is included. If it is a system figure, ask for the build up.
• "Reflects 95% of radiant heat." These percentages appear across the market with no consistent test behind them. Emissivity is the measurable property, and it is on the certificate.
• "Meets Part L." No insulation product meets Part L. Constructions do, and only after a calculation.
• "Non-combustible." Foil products are not. Reaction to fire classifications in this category typically sit between Class D and Class F to BS EN 13501-1. Note that combustible insulation is restricted in the external walls of buildings above 11 metres, which rules most of these products out of that application entirely.
• Certificate numbers copied between products. It happens more than it should, particularly between a flagship product and its lighter variant. If the certificate number on two different products is identical, check it.

Which One Do You Need

If you are not sure which side of the vapour closed and breathable line your job falls on, that is the question to ask before you buy anything. Everything else can be worked around. That one cannot.

What We Stock

Grouped by the three categories above rather than by brand, since the category is the decision that matters.