Is mineral wool batt insulation better than fibreglass for New Brunswick homes? | Insulation IQ?

Mineral wool batt insulation — sold under brand names like Rockwool (formerly Roxul) and Thermafiber — offers several meaningful performance advantages over standard fibreglass batts, and those advantages are particularly relevant to New Brunswick's cold, damp climate. That said, fibreglass still has a strong case in the right applications. The honest answer is that neither is universally better — the right choice depends on where you're insulating and what your priorities are.

Where Mineral Wool Has a Clear Edge

Higher R-value per inch. Mineral wool batts deliver approximately R-4.2 per inch, compared to R-3.7 to R-3.8 per inch for standard fibreglass. In a 2x6 stud cavity (5.5 inches), a mineral wool Comfortbatt achieves a genuine R-23, while a standard fibreglass batt tops out at R-20 (with high-density fibreglass reaching R-21 or R-22). Over the full wall assembly, that difference in cavity insulation narrows the performance gap — but in thick assemblies and when combined with continuous exterior insulation, it adds up.

Water resistance. Mineral wool fibres are hydrophobic — they repel water rather than absorbing it. In New Brunswick's climate, where basements in Fredericton or Saint John are susceptible to seasonal moisture, this is a real advantage. Fibreglass batts that get wet lose R-value, can harbour mould on the facing or surrounding framing, and may not fully recover their performance after drying. Mineral wool will drain and dry without permanent performance loss.

Dimensional stability. Mineral wool batts are denser and stiffer than fibreglass. They hold their shape between studs without sagging and stay in place without stapling. This matters most in wall cavities where fibreglass can settle slightly over decades, leaving cold spots near the bottom of the cavity. In an attic floor, this difference is less relevant.

Fire resistance. Mineral wool is made from slag or basalt — genuinely non-combustible materials. Standard mineral wool products are rated to withstand temperatures exceeding 1,000°C without melting. Fibreglass melts at high temperatures, though it doesn't fuel flame readily. In party walls, mechanical rooms, or anywhere fire resistance is a code or insurance consideration, mineral wool has a substantial edge. For most residential exterior wall applications in Moncton or Fredericton, this is a secondary concern — but for basement mechanical rooms or attached garages, it carries more weight.

Sound attenuation. Mineral wool's density gives it significantly better sound transmission class (STC) performance than fibreglass at equivalent thicknesses. If you're insulating interior walls between bedrooms, bathrooms, or a home office in a Saint John home, mineral wool batts will reduce noise transmission noticeably more than fibreglass.

Where Fibreglass Still Makes Sense

Cost is the primary argument for fibreglass. Standard R-20 fibreglass batts for 2x6 walls run approximately $0.30 to $0.55 per square foot for materials, while comparable mineral wool batts cost $0.60 to $1.00 per square foot — roughly double. In a large attic floor top-up project where moisture risk is low and the application is horizontal (reducing dimensional stability concerns), the premium for mineral wool may not be justified. Fibreglass also has a longer track record in Canadian residential construction and is more widely available at local suppliers in Moncton, Fredericton, and Saint John.

For attic floors in particular, many NB insulation professionals recommend blown-in cellulose or blown-in fibreglass over batts entirely — it fills around obstructions without gaps, requires less labour, and reaches the NB code target of R-50 (RSI 8.8) more easily. In that context, the mineral wool vs. fibreglass batt debate is moot.

The NB Bottom Line

For exterior walls in new construction or gut renovations in Climate Zone 6, mineral wool batts are a premium upgrade that pays off through slightly better whole-wall R-values, moisture resilience, and long-term stability. For attic top-up, blown-in products are often more practical and cost-effective. For basement walls near concrete with potential moisture exposure, mineral wool's hydrophobic properties make it the safer choice.

Both the Canada Greener Homes Grant and NB Power's Home Energy Efficiency Program apply to qualifying insulation upgrades regardless of whether you choose fibreglass or mineral wool — the key is meeting the performance threshold, not the product type.

For a specific recommendation on your home in Fredericton, Moncton, Bathurst, or elsewhere in New Brunswick, the insulation professionals listed through New Brunswick Insulation on the New Brunswick Construction Network can assess your assembly and recommend the best product for your budget and performance goals.

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Looking for experienced contractors? The New Brunswick Construction Network connects homeowners with qualified professionals:

• 3Tone Construction Ltd
• Arctic Fox Construction Inc.
• Gionetterenovations
• Thirty Four Renovations
• Brunswick insulation & roofing