How the two differ in kind
Both are spun mineral fibre, both come sized for standard bays, and both are counted the same way: area divided by the coverage printed on the package, plus waste. What separates them is density. Stone wool is spun from melted rock and packed several times heavier, so the batt behaves like a stiff slab you can stand on its edge. Glass wool is spun from melted glass, packed light and squeezed hard into the bag, so the batt behaves like a blanket. Most of the differences below are downstream of that one — and the exception is the other one: rock and glass soften at temperatures a long way apart, whatever density either is packed to.
Density buys two things, and they are worth naming narrowly because the marketing widens them. It buys stiffness: a batt cut slightly oversize wedges itself into the bay and is still there, at full thickness, long after a tucked blanket has crept down a tall stud space. And it buys resistance to airflow, which is what a cavity absorber actually does for sound. The third advantage on the mineral wool side is not density's at all but the rock's: stone wool holds together at flame temperatures that have already softened glass, and packing glass wool denser does not move that number — which is why firestopping and head-of-wall listings name the material rather than a density. What none of it buys is a transformation in thermal performance: at the same cavity depth standard stone wool usually edges standard glass wool, and high-density glass wool closes most of that gap.
So the honest form of this decision is not which batt is better but what this particular cavity is being asked to do. Most bays in a house are asked for one thing — R-value, in a dry, ordinary, vertical space where either product delivers its label if it is fitted properly and the wall is air-sealed. The premium is charged against area, so it scales with the size of the job: spent across a whole house it buys, over most of that area, nothing you could measure. Spent on the garage separation, the party wall, the plant room and the bays near the flue, it is small in absolute terms and buys precisely what it is for. The deciding question is which of your cavities has a second job.
The factors that actually differ
| Mineral wool batts | Fiberglass batts | |
|---|---|---|
| Stiffness in the bay | Semi-rigid — cut slightly oversize it wedges in and holds full thickness on its own. | Limp: held by stapled flanges or a good tuck, and tall bays creep and sag over the years. |
| Cutting and fitting | Serrated blade, one clean square pass; a split made for a cable stays exactly where you cut it. | Compressed under a straightedge and sliced, or torn by hand around a wire — forgiving both ways. |
| R at the same depth | Standard product usually edges standard glass wool in an identical cavity depth. | Standard sits below it; high-density product closes most of the gap. Read the label, not the material. |
| Facings | Unfaced on the residential shelf, so any vapour retarder the wall needs is a separate material and a separate pass. | Kraft-faced product insulates and retards vapour in one operation, with flanges to staple. |
| Packages per area | Dense and close to finished thickness in the bag: less area a package, more pallet space and more trips. | Compressed hard and lofts on opening — much more area a package, but it must recover thickness to count. |
| As the temperature climbs | Holds together at flame temperatures; firestop, safing and head-of-wall listings name it. | Unfaced glass is no fuel either, but it softens and slumps far sooner — and kraft facing burns. |
| After a wetting | Hydrophobic: water drains through it and the batt recovers its loft once the cavity dries. | Mats down when soaked and often never regains full thickness — and thickness is the R-value. |
| Sound in the cavity | The denser mat resists airflow better and is the measurably better absorber of the two. | An adequate absorber, and the cheap way to fill a cavity whose real acoustics are mass and decoupling. |
| Stock and formats | Unfaced batts in the common depths; odd widths and thicknesses are an order, not a shelf. | Every merchant, every width, faced or unfaced, and long rolls for uninterrupted attic runs. |
Which one, and when
Choose mineral wool batts when…
- The cavity has a second job: a garage separation, a party or tenancy wall, a plant room, bays near a flue.
- The assembly you must build is a tested one that names mineral wool — glass is not a substitute in a listing.
- Sound is a stated requirement rather than a hope, and the mass and decoupling are already dealt with.
- Positions a limp batt loses: tall bays, sloped ceilings, overhead work, steel studs with nothing to staple to.
- Somewhere a single wetting is plausible and you would rather the insulation dried out than matted down.
Choose fiberglass batts when…
- Ordinary dry stud bays whose only job is R-value, which is most of the house.
- Large areas, where the premium is charged against every square metre and buys nothing measurable in most of them.
- The kraft facing is doing work you would otherwise buy and install as a separate layer.
- Attic floors and long uninterrupted runs, where rolls beat batts and density buys nothing.
- The budget is finite and the air-sealing is not done — that spends better than the material swap does.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Is mineral wool worth the extra?
- Per cavity, not per house. This site publishes no invented figures, but the shape of the cost is clear enough to decide on: mineral wool is the material-heavy side, the difference is charged against area, and the labour per bay is broadly similar — sometimes lower, because a stiff batt cuts clean and stays put, sometimes higher, because it is unfaced and a vapour retarder becomes its own operation. Since the premium scales with the size of the job, buy it by the wall rather than by the house: the walls that need it are usually a small fraction of the total area, and the absolute difference on those is minor. Run both calculators on the areas you actually intend to treat with each and price them against local quotes.
- Is mineral wool actually warmer?
- Modestly, and less than the aisle suggests. In the same cavity depth, standard stone wool typically reaches a slightly higher R than standard glass wool, but high-density glass wool closes most of that gap, so the label on the pack settles it rather than the material class. In practice the fit and the air-sealing dominate anything either fibre offers: a batt compressed behind a wire, a bay left short at the top plate, or an unsealed top plate above it all cost more performance than the gap between these two products. Choose on the second job the cavity has, then buy whichever product hits the R-value your assembly needs at the depth you have.
- Which is better for soundproofing a wall?
- Mineral wool, genuinely — and by less than the money implies. Its higher density gives more resistance to airflow through the cavity, which is what a cavity absorber does, and lab comparisons of the same partition show it ahead. But the gain is a couple of points, sometimes barely that, while decoupling the two faces, adding a layer of board, or sealing the perimeter and the electrical boxes each move the number far more. If the wall matters acoustically, fix the structure first and treat the batt choice as the last few points, not the first ones.
- Is fiberglass a fire risk?
- Unfaced glass wool is not a fuel — the difference is what each does as the temperature climbs, not whether it burns. Stone wool holds its form at flame temperatures that have already softened glass fibre, which is why firestop systems, safing and head-of-wall gap listings specify mineral wool by name and why substituting glass into one voids it. Two separate cautions on the fiberglass side: a kraft facing is combustible and must not be left exposed, and non-combustible is not the same as fire-rated. The rating of a wall comes from the tested assembly — the board layers, the fasteners, the joint treatment — never from the insulation on its own.
- Can I use both in the same house?
- Yes, and for this reader it is usually the right answer: mineral wool in the cavities with a second job, fiberglass everywhere else. Two things to keep straight. Do not improvise inside a tested fire or acoustic assembly — the listing governs what goes in it, including the insulation. And keep the vapour-retarder strategy consistent across a single plane of the building: mixing faced fiberglass with unfaced mineral wool in one wall means deciding, deliberately, what the retarder is over the wool rather than discovering later that the wall has one on part of its area and nothing on the rest.
- Is one of them nicer to handle?
- Different unpleasantness, not less of it. Both want long sleeves, gloves, eye protection and a respirator — the dust is respirable in each case and neither is the safe one. Mineral wool sheds coarse, gritty dust and makes visible debris when it is cut, but many installers find it less itchy on the skin; glass wool sheds finer fibres and is the classic itch, though modern products use fibres designed to clear from the lungs. The practical difference on site is weight and cleanup: mineral wool is heavier to carry up a ladder and leaves crumbs to sweep, while glass wool is light to lift and gets everywhere.