Both mineral wool and fiberglass are sold as cavity insulation batts. They look similar and fit the same stud bays — but they perform very differently in fire, sound, and moisture. Here's when to use each.
Use fiberglass for most exterior wall, floor, and attic applications where the assembly doesn't require fire ratings or exceptional sound attenuation. Use mineral wool where code requires non-combustibility (IBC party walls, fire-rated corridors), where STC performance matters (apartments, hotels), or where moisture resistance is important (below-grade or high-humidity applications). Mineral wool costs more per bag.
| Factor | Fiberglass | Mineral Wool |
|---|---|---|
| Thermal performance | Excellent — most common choice | Excellent — comparable R-values |
| Fire resistance | Melts at ~1100°F, not non-combustible | Non-combustible (ASTM E136) to 2000°F+ |
| Sound attenuation | Good — STC improvement ~3–5 pts vs empty | Better — STC improvement ~6–10 pts vs empty |
| Moisture resistance | Can absorb moisture; loses R-value when wet | Hydrophobic — repels water, maintains R-value |
| IBC fire-rated assemblies | Not suitable for most IBC assemblies | Required — UL-listed, non-combustible |
| Material cost | Lower | Higher (typically 20–40% more per sq ft) |
| Best for | Exterior walls, attics, floors, interior partitions | Party walls, fire-rated corridors, sound assemblies |
There are assembly types where mineral wool isn't just better — it's what the code or the UL assembly requires. IBC fire-rated party walls between dwelling units, corridor walls in hotels and multifamily buildings, and floor/ceiling assemblies with specific STC requirements all commonly require mineral wool (SAFB or AFB). Fire-rated insulation →
Exterior walls, attic floors, crawlspace floors, and interior partitions that don't need fire ratings or enhanced STC are all good fiberglass applications. Fiberglass is available in every R-value from R-11 to R-49, covers a wider range of applications, and costs less. For most residential new construction, fiberglass handles the bulk of the job — mineral wool handles the specific assemblies where it's needed. All fiberglass products →
Yes — and this is the most cost-effective approach for multifamily and commercial projects. Spec fiberglass for exterior walls and attics, mineral wool for party walls and corridors. We supply both and can stage delivery to match your framing sequence. Apartment insulation guide →
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