20 May 2026 · Updated 10 August 2026 · Materials
Three names come up again and again when you're choosing insulation: mineral wool, Rockwool and PIR boards. They do the same basic job — slowing heat loss — but in quite different ways, at different prices and with different strengths. Here's how to tell them apart and pick the right one for your home.
Mineral wool (glass wool) is the soft, fluffy roll most people picture when they think of loft insulation. It's inexpensive, easy to fit between and over joists, and widely available, which makes it the default choice for an open loft floor. The trade-off is bulk: it needs a fair thickness to hit the recommended performance, so it's less suited to spaces where headroom is tight.
Rockwool is a type of mineral wool made from stone (often called stone wool or slab). It's denser and more rigid than soft glass-wool rolls, and it's particularly valued for its excellent fire resistance and sound-deadening. That makes it a strong choice where acoustic comfort or fire performance matters — for example between floors, in stud walls, or around a home, as well as in the loft.
PIR (polyisocyanurate) boards are rigid foam panels — the familiar foil-faced boards from brands like Celotex and Kingspan. Their big advantage is performance-per-millimetre: they achieve the same thermal result as mineral wool in a much thinner layer. That makes them ideal for loft conversions, sloped ceilings, boarded loft floors and anywhere space is at a premium.
You'll also come across slimline multifoil and SuperFOIL insulation, which reflects radiant heat and is often combined with the materials above where headroom is limited. It's worth considering alongside, rather than instead of, these three.
Here is how the three main insulation materials compare at a glance, plus multifoil for context. Use it as a quick guide, then let us match you with an installer who can advise on your exact loft.
| Material | Typical cost | Thermal performance per mm | Fire resistance | Soundproofing | Best for |
|---|---|---|---|---|---|
| Mineral wool (glass wool) | Low | Moderate, needs more thickness | Good | Moderate | Standard loft floors, budget upgrades |
| Rockwool (stone wool) | Medium | Moderate | Excellent | Excellent | Fire and sound, floors, stud walls |
| PIR boards | Higher | High, thin profile | Good | Moderate | Loft conversions, warm roofs, tight spaces |
| Multifoil / SuperFOIL | Medium | Reflective, use with others | Varies | Low | Rafters and tight spaces, in combination |
Cost and performance are general guidance only and vary by product, thickness and installation. Comparing quotes is the best way to see fair pricing for your home.
Not exactly. Rockwool is a brand and a type of mineral wool made from stone, often called stone wool, while the term mineral wool usually refers to softer glass wool rolls. Both are mineral wools, but Rockwool style stone wool is denser and gives better fire resistance and soundproofing.
There is no single winner. Mineral wool is the cheapest, Rockwool is best for fire and sound, and PIR boards give the most insulation for the least thickness. The right choice depends on your loft, your budget and the space available.
They suit different jobs. PIR insulates more per millimetre, so it wins where space is tight, such as loft conversions and sloped ceilings. Rockwool performs better on fire resistance and soundproofing. Many homes use a combination.
Usually yes. Rockwool style stone wool typically costs more than standard glass wool rolls because it is denser and more robust. PIR boards are generally the priciest of the three.
Stone wool is another name for Rockwool type mineral wool, made from volcanic rock rather than glass. It is valued for its fire resistance and sound deadening.
Yes. Installers often combine materials, for example PIR boards with slimline multifoil in a loft conversion, to get the best result where headroom is limited. Read our guide to multifoil insulation for more.
There's no single winner — the right material depends on your loft, your budget and whether the space is used for storage, a conversion or living. A good installer will often combine materials to get the best result. Tell us about your home and we'll advise on the right mix, then match you with vetted insulation installers so you can compare quotes. You may also find our guides on loft insulation depth and multifoil insulation helpful.
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