Mineral Wool vs Fiberglass vs Spray Foam: Insulation Trade-offs
Mineral wool, fiberglass, and closed-cell spray foam compared — R per inch, fire and water behavior, air sealing, sound, and DIY practicality.
Updated August 20, 2026
Fiberglass, mineral wool, and closed-cell spray foam all insulate by trapping air (or a blowing agent) in small pockets — but they differ so much in density, chemistry, and installed form that they behave like three different building products that happen to share an R-value label. Per inch, the two batts overlap almost completely, while closed-cell foam packs roughly half again more resistance into the same depth.
The rows that actually separate them are the ones a bare R-value hides. Stone wool is spun from molten rock: it is non-combustible, sheds bulk water, and is dense enough to deaden sound and stand friction-fit without sagging. Fiberglass is the price floor and the ubiquitous default — lighter, softer, and easier to compress badly. Closed-cell foam is the only one of the three that is simultaneously insulation, an air barrier, and (at sufficient depth) a vapor retarder, which is why it owns the geometrically awkward, leakage-dominated corners of a house.
The R-per-inch cells below are mapped directly from this site’s insulation R-value reference chart — the same DOE- and manufacturer-adjudicated ranges, not retyped — so this page and that chart can never quietly disagree.
Side by Side
| Spec | Mineral wool batt | Fiberglass batt | Closed-cell spray foam |
|---|---|---|---|
| R-value per inch (published range)Ranges from the insulation R-value reference chart (DOE/PNNL and manufacturer data); the batts overlap almost entirely — density, not material, sets where a given product lands. | 3.1–4.3 | 3.1–4.3 | 5.5–7.0 |
| Fire behavior | Non-combustible; stone-wool fiber melts around 2150 °F (ROCKWOOL published data) — usable as fire-stopping in many assemblies | Non-combustible glass fiber, but kraft facing is flammable and must be covered; fibers soften at far lower temperatures than stone wool | Combustible foam plastic — code requires a thermal barrier (typically ½-in gypsum) or a listed ignition barrier over it (IRC R316) |
| Water & moisture behavior | Hydrophobic — sheds liquid water and keeps most of its R when damp; dries in place | Loses R while wet as water displaces trapped air; can dry, but sagging and settling after soaking are common | Closed cells reject bulk water; at typical installed depth it also functions as a Class II vapor retarder — placement must respect the wall’s drying direction |
| Air-sealing capability | None — fibrous batts filter air rather than stop it; pair with a separate air barrier | None — same as any fibrous batt, and gaps from sloppy fitting cost disproportionate R | The defining strength: expands into cracks and cures into a rigid air barrier in one operation |
| Sound attenuation | Best of the three — the density advantage is audible in party walls and ceilings | Good and cheap — the standard acoustic-batt filler | Poor for airborne sound — rigid foam couples panels; do not buy it for quiet |
| Installed form & DIY-ability | Friction-fit batts; cuts crisply with a bread knife and stands in place — very DIY-friendly, though heavier and dustier than glass | Batts and rolls everywhere, lightest to handle — the classic DIY material; itch and compression errors are the price | Two-component professional spray rig with cure chemistry, off-gassing window, and PPE; small DIY kits exist but per-board-foot cost is high and rework is brutal |
| Cost drivers | Typically ~1.5–2× fiberglass material cost for the same bay; labor similar to fiberglass | Cheapest R on the shelf; the budget baseline everything else is measured against | By far the highest installed cost per R — justified where air sealing, moisture control, or thin assemblies do the paying |
Which One for the Job
Sound-critical interior partition
Mineral wool batt
Density is what absorbs airborne sound, and stone wool carries the most of it per bay while friction-fitting tightly enough to avoid the voids that leak noise. It buys audibly more attenuation than fiberglass for a modest premium, and foam is the one material that can make acoustics worse.
Rim joist and crawlspace air-sealing
Closed-cell spray foam
The rim is dozens of small, irregular, leaky bays where the enemy is air and moisture-laden air at that — exactly the geometry batts fit badly and exactly the job foam does in one pass: air seal, vapor control, and R in a couple of inches. Fibrous batts against a cold rim can let condensation form behind them.
Standard attic top-up on a budget
Fiberglass batt
An open, accessible attic floor with an existing air-sealed ceiling plane is the least demanding insulation job in the house — maximum area, no moisture drama, R purchased by the square foot. Cheap fiberglass (batts, or blown loose-fill from the same family) delivers the code R for the least money; premium materials repay nothing up there.
Bays around a wood stove alcove or where fire performance drives the spec
Mineral wool batt
A non-combustible batt that holds together at temperatures where glass fiber slumps is the conservative fill anywhere fire separation matters, and building codes lean on stone wool for fire-stopping details for that reason. Foam is disqualified here without its thermal barrier, and often even with it.
Frequently Asked Questions
Is mineral wool worth the extra money over fiberglass?
Only when one of its distinctive properties is on the job description: sound deadening, fire performance, water shedding, or a stiff batt that friction-fits without sagging. On pure R per dollar in a dry, quiet, protected wall, fiberglass wins — the two overlap almost completely in R per inch, so paying the stone-wool premium purely for warmth buys little.
Does closed-cell spray foam need to be covered inside a house?
In living space, yes — foam plastics must be separated from the interior by a thermal barrier, typically half-inch gypsum board, under IRC R316. Attics and crawlspaces entered only for service may instead use a listed ignition barrier or specifically tested coating. Leaving cured foam exposed in occupied space is a code violation, not a style choice.
Can batt insulation air-seal a wall if it is fitted really well?
No — fibrous insulation slows air but cannot stop it, no matter how carefully it is cut. A perfectly fitted batt in a leaky bay performs far below its label because moving air carries heat straight through the fiber. The sequence that works is caulk, gasket, or foam the leaks first, then fill the bay with the batt; the two jobs are different materials for a reason.
Why would spray foam ever be limited to certain walls if it performs so well?
Because at typical depths closed-cell foam is also a vapor retarder, and a wall needs a coherent drying strategy. Foam on the wrong side of a cold-climate assembly, or foam trapping a wall that also has an exterior vapor barrier, can lock moisture in. It also bonds permanently to sheathing and wiring, making future alterations destructive — power in exchange for reversibility.
Try the Calculators
Sources
- Insulation R-value reference chart on this site (DOE/GO-10095-060, PNNL Building America, DOE Energy Saver archive, and manufacturer datasheets) — the mapped R-per-inch ranges
- ROCKWOOL published fire data — non-combustible classification and ≈1177 °C / 2150 °F stone-wool melting point
- IRC R316 (Foam Plastic) — thermal barrier and ignition barrier requirements over spray foam
- DOE Energy Saver insulation pages — material types, moisture behavior, and air-sealing guidance