The Short Answer
Fiberglass handles the majority of residential and commercial insulation needs. It's cost-effective, widely available, and meets energy code requirements in all climate zones.
Mineral wool is required when an assembly must meet fire-resistance ratings, high STC performance, or non-combustibility under ASTM E136. It's also the better choice for below-grade and high-humidity applications.
Side-by-Side Comparison
| Property | Fiberglass | Mineral Wool |
|---|---|---|
| Thermal performance | R-3.0–3.8 per inch | R-3.0–4.2 per inch |
| Fire resistance | Melts at high temps; does not meet ASTM E136 | Non-combustible; meets ASTM E136 |
| Sound (STC) | STC improvement 35–45 depending on assembly | STC improvement 45–65 depending on assembly |
| Moisture resistance | Can absorb moisture; loses R-value when wet | Hydrophobic; drains moisture, retains R-value |
| IBC fire-rated assemblies | Not suitable | Required |
| Typical applications | Exterior walls, attics, floors, interior partitions | Party walls, corridors, hotel rooms, high-humidity areas |
When You Must Use Mineral Wool
IBC Fire-Rated Assemblies
The International Building Code requires non-combustible insulation in 1-hour and 2-hour fire-rated wall and floor/ceiling assemblies. Mineral wool (AFB/SAFB) meets ASTM E136 non-combustibility. Fiberglass does not.
Applications that typically require mineral wool:
- Apartment and condo party walls (unit-to-unit separations)
- Hotel corridor walls
- Commercial tenant separation walls
- Mixed-use building floor/ceiling assemblies
- Any assembly with a published UL fire-resistance rating specifying mineral wool
High-STC Requirements
Building codes for multifamily construction often require STC 50+ for separation walls between units. Mineral wool SAFB (Sound Attenuation Fire Batts) is the standard product used to achieve high-STC assemblies. A double-stud wall with SAFB can reach STC 65+.
Below-Grade and High-Humidity Applications
Mineral wool is hydrophobic — it repels water and retains its thermal performance even when exposed to moisture. For below-grade installations, crawl spaces with moisture issues, or any assembly subject to condensation, mineral wool outperforms fiberglass significantly.
When Fiberglass Is the Right Choice
- Exterior wall cavities in residential construction (zones 1–5)
- Attic floor insulation (all zones)
- Floor assemblies over unconditioned crawl spaces
- Interior partition walls (no fire or STC spec)
- Any project where energy code compliance is the primary driver
Can I Mix Both on the Same Project?
Yes — and most multifamily and commercial projects do. Common split: fiberglass on exterior walls (thermal performance), mineral wool on party walls (fire + sound). We can quote both products in a single order with separate line items for each assembly type.
Products We Stock
- Fiberglass: R-11, R-13, R-15 HD, R-19, R-21 HD, R-30, R-38, R-49 — kraft, unfaced, FSK
- Mineral Wool AFB: R-15, 15" and 23" widths — for fire-rated and sound-critical assemblies
- Mineral Wool SAFB: R-15 to R-23 — highest-STC mineral wool batt
- Mineral Wool R-30: For cathedral ceilings and attic applications
Call (929) 466-1426 to confirm current stock and get a same-day quote for your project.