How does blown-in rockwool compare to blown-in fibreglass for fire resistance in New Brunswick attics?

Blown-in rockwool (mineral wool) offers significantly superior fire resistance compared to blown-in fibreglass, and this difference is meaningful in NB attic applications — though both materials are acceptable under the NB Building Code for residential attic insulation.

Rockwool (sold under brand names like Rockwool/ROXUL) is made from basalt rock and recycled slag, which gives it an extraordinary melting point of approximately 1,000°C (1,800°F). In a fire, blown-in mineral wool holds its position in the attic assembly, maintains its structure, and does not contribute fuel to the fire. Blown-in fibreglass, while non-combustible itself, has a much lower melting point — fibreglass fibres begin to soften and lose structural integrity around 500-600°C — and the binders used in some fibreglass products can off-gas during a fire event. Neither material is a fire hazard under normal conditions, but the performance gap in an actual fire is real.

For most NB homeowners insulating an attic to R-50 or R-60, this fire resistance difference is a secondary consideration. The more common reason homeowners choose blown-in mineral wool over fibreglass is its dimensional stability and moisture resistance. Mineral wool is hydrophobic — it repels water rather than absorbing it. In NB attics where ice dam leaks, condensation events, or roof penetration failures occasionally introduce moisture, mineral wool will dry out and retain its R-value. Wet fibreglass batts or blown fibreglass can mat down, lose R-value, and take a long time to dry. Blown-in cellulose, for comparison, absorbs moisture readily but has good drying potential if the moisture source is corrected.

The Practical Trade-Off in NB Attics

The honest trade-off is cost and availability. Blown-in mineral wool costs noticeably more than blown-in fibreglass in the NB market — expect to pay roughly $2.00-$3.50 per square foot for blown mineral wool versus $1.25-$2.75 per square foot for blown fibreglass at comparable R-values. Blown-in cellulose typically falls in the middle at $1.50-$3.00 per square foot and is the most commonly specified product for NB attic top-ups because of its recycled content, good fire resistance (borate treatment gives it a Class 1 fire rating), and excellent coverage characteristics.

Blown-in fibreglass is lighter than both cellulose and mineral wool, which matters in older NB homes where ceiling joists may be undersized or already carrying significant load. If your attic has 2x4 or small 2x6 ceiling joists and you're adding significant depth of insulation, your contractor should assess the load capacity before specifying a heavier product.

What Actually Matters Most for NB Attic Performance

Fire resistance is worth considering, but the factors that will have the greatest impact on your attic's real-world performance are air sealing and proper ventilation baffles — regardless of which blown-in product you choose. An attic with R-60 of any blown-in product but unsealed pot light housings, an uninsulated hatch, and buried soffit vents will underperform an R-50 attic that has been properly air-sealed and baffled. In NB's climate with 4,800-5,200 heating degree days per year, those air leakage points drive ice dams, moisture problems, and heating bills far more than the choice between fibreglass and mineral wool.

If fire resistance is a genuine priority — for example, if you're insulating an attic above a garage or a space with specific fire separation requirements — blown-in mineral wool is the clear choice. For a standard NB residential attic top-up where you're chasing R-50 to R-60 performance and the best value, blown-in cellulose is the most popular and cost-effective option, with blown-in fibreglass a close second. Mineral wool earns its premium in wall assemblies, rim joists, and fire-separation applications more than in open attic floors.

If you'd like help finding a local insulation contractor to assess your attic and quote a blown-in upgrade, New Brunswick Insulation can match you with professionals in your area through the New Brunswick Construction Network — the service is completely free.