Mineral Wool vs Fiberglass Insulation — Ontario Guide

R-value per inch is close on modern products. The real differences are density, fire behaviour, and how each handles water and sound.

Mineral wool is the right pick when:

1. party walls between dwelling units in a multiplex, townhouse, or basement apartment — the density gives 5-8 STC points over fiberglass, which is the difference between hearing your neighbour's TV and not
2. mechanical rooms, boiler closets, and any cavity within 6 inches of a chimney, vent, or recessed light fixture — non-combustible products are required by OBC in many fire-separation assemblies
3. basement perimeter walls where bulk water is a real risk — the hydrophobic fibres drain and dry without losing R-value or growing mould
4. commercial or institutional projects that require ULC fire ratings on assemblies
5. home theatres, music rooms, and any sound-sensitive partition
6. bathrooms and kitchens with vapour exposure.

For any wall where fire, water, or sound matter more than budget, mineral wool wins.

Fiberglass is the right pick when:

1. standard above-grade walls in a single-family home — modern high-density fiberglass batts hit R-22 in a 2x6 cavity for about half the cost of mineral wool
2. attic flat ceilings using blown loose-fill — blown fiberglass is the cheapest path to R-60 and installs in hours
3. renovations on a tight budget where the cavity isn't sound-sensitive or fire-critical
4. DIY work — fiberglass is more forgiving for first-time installers
5. rim joists and floor cavities away from below-grade exposure.

For 60-70% of typical GTA residential cavity insulation, fiberglass at modern R-22 spec is adequate and meaningfully cheaper.

Material cost is roughly 2x for mineral wool. Total installed cost is closer to 1.6-1.8x because the friction fit and faster trim of mineral wool slightly reduces install time.

Both products meet OBC SB-12 prescriptive R-values when sized correctly. Climate Zone 6 (most of southern Ontario) requires R-22 for above-grade walls and R-60 for attics, and both products achieve those numbers in standard cavity depths.

For fire separations between dwelling units (a 1-hour ULC rating is the OBC minimum for party walls in multi-unit residential), the assembly drawings often specify mineral wool by name. Substituting fiberglass without a re-engineered assembly will fail inspection. If you're framing a multiplex, garden suite, or basement apartment, confirm the assembly spec before ordering.

For sound between dwelling units, OBC 9.11 requires STC 50 minimum. A standard 2x4 wall with single-layer 5/8" Type X drywall and fiberglass batts measures around STC 39-42, below code. The same assembly with mineral wool batt and resilient channel typically measures STC 50-54. Mineral wool is the standard solution for hitting OBC's sound separation requirement without resorting to staggered-stud or double-stud construction.