Materials compared
SuperQuilt vs mineral wool: which do you need?
The comparison everyone asks for, with an answer most people do not expect: they are not really competitors. One is a cheap, thick, extremely effective floor insulator. The other solves a problem the first one physically cannot.
Last reviewed by the Loft Insulation London survey team.
They work by different mechanisms
This is the thing that makes the comparison confusing, and it is worth getting straight before looking at any numbers.
Mineral wool — glass or rock fibre spun into a thick, low-density mat — works by trapping still air. Air is a poor conductor of heat, and the fibrous structure stops it circulating and carrying heat across by convection. The mechanism scales with thickness: more depth, more trapped air, better performance. It is cheap, it is inert, it is easy to fit correctly, and where you have depth to spare it is very hard to beat on cost per unit of performance.
Multifoil — SuperQuilt and similar products — is a layered composite with reflective aluminium faces. Its dominant mechanism is reflection of radiant heat, which does not depend on thickness at all. A 40mm quilt reflects radiant heat about as well as a 400mm one would. What it needs instead is an air gap on each side to reflect into, which is why the installed build-up is always thicker than the product.
| Mineral wool | SuperQuilt multifoil | |
|---|---|---|
| Typical installed depth | 270mm (two cross-laid layers) | ~40mm product, plus battened air gaps |
| Main mechanism | Trapped still air (conduction and convection) | Reflection of radiant heat |
| Best position | Cold loft floor, between and over joists | Rafter level, sloping ceilings, rooms in roof |
| Cost per m² | Low | Noticeably higher |
| Installation sensitivity | Forgiving — main risks are compression and blocked eaves | Unforgiving — needs correct air gaps and sealed laps |
| Costs you headroom | Yes, if used on a slope | Minimal |
| Fire classification | Non-combustible (A1 typical) | A/A+ reaction to fire |
Which situation are you in?
A cold loft you do not live in
Insulation goes at ceiling level, the roof space above stays cold and ventilated, and depth is free because nothing up there needs headroom. This is the overwhelming majority of London homes and the answer is mineral wool, cross-laid to 270mm. Using multifoil here would cost you considerably more for no practical gain — you would be paying a premium to solve a depth problem you do not have.
A room in the roof
Now the insulation has to follow the slope, fixed to or under 100mm rafters, and every millimetre of build-up comes directly out of a room that is probably already low. Achieving a good U-value with wool means deepening the rafters and losing that depth from the room. This is precisely the case multifoil exists for.
Both, which is very common
A Victorian terrace with a converted attic bedroom typically has both conditions in the same roof: cold void either side of the room, sloping ceiling over it. The correct specification uses wool in the voids and multifoil on the slope. Anyone insisting on a single product for the whole roof is selling a product rather than solving a problem.
Reading the performance claims
Mineral wool's thermal conductivity is a straightforward, well-standardised figure and the installed performance follows predictably from the depth. Multifoil is harder to compare, because its performance depends on the whole assembly — the foil, the two air gaps, and how well the laps are sealed — rather than on the product in isolation.
That is not a criticism of the product; it is a caution about the marketing. "Equivalent to 200mm of mineral wool" is a claim about a specific tested build-up under specific conditions, and it does not survive being fitted flat against a rafter with no air gap. Ask for the tested figure for the build-up being quoted. A good installer will have it. If the answer is a glossy leaflet and a confident tone, be careful.
The short version
Got depth? Use wool — it is cheaper and it is excellent. Got no depth? Use multifoil — it is the only thing that works. Got both conditions in one roof, which most London houses with converted lofts do? Use both, in the right places.
In the loft
The two materials in their proper places
Not competitors so much as tools for different jobs.
Questions & answers
Comparing insulation materials — common questions
Which is better, SuperQuilt or mineral wool?
Neither, in the abstract. They solve different problems. For a cold loft floor with unlimited depth available, mineral wool is better value by a wide margin. For a sloping ceiling where every millimetre comes out of your headroom, multifoil does something wool cannot. The question is not which is better but which situation you are in.
Can I use both?
Frequently, and in many homes it is the right answer: mineral wool across the cold loft floor, multifoil at rafter level over the part of the roof that sits above a room. They are not mutually exclusive and they are not competing for the same square metre.
Is multifoil's 'equivalent to' claim reliable?
Treat it carefully. Equivalence figures depend on the test method and the assumed air gaps, and independent testing has historically produced more conservative numbers than manufacturer literature. Ask for the tested performance of the complete build-up, including the gaps, rather than a headline comparison.
Does mineral wool irritate the skin?
It can, which is why installers wear gloves, long sleeves and eye protection. Once it is laid and the loft is closed up it is inert and causes no issue in the house below. Modern glass and rock wool are not classified as carcinogenic, unlike the asbestos-era products they are sometimes confused with.
Which is more environmentally friendly?
Both have manufacturing footprints and both repay them many times over through the heat they save. Mineral wool has high recycled content and is itself recyclable; multifoil is a composite and harder to recycle at end of life but uses far less material per square metre. If this is your deciding factor, the more useful question is which product will actually be installed correctly in your roof — a badly fitted layer of either saves nothing.
Which lasts longer?
Both should outlast the roof covering above them if kept dry. Mineral wool's failure mode is compression and moisture; multifoil's is torn or unsealed laps and lost air gaps. In both cases the enemy is installation quality and water, not age.
Written and fact-checked by the Loft Insulation London survey team, who assess loft spaces across the capital every working week. Last reviewed .
Not sure which your roof needs?
That is what the free survey is for. We will look, measure and tell you — including when the cheaper option is the right one.