What is the fire rating difference between mineral wool batts and standard fibreglass batts for interior walls in New Brunswick?

Mineral wool batts are significantly more fire resistant than fibreglass batts — mineral wool remains intact and continues to insulate up to 2,150°F (1,177°C), while fibreglass batts begin to melt and lose structural integrity at temperatures as low as 1,100°F (593°C).

This difference matters in real NB construction scenarios, particularly in fire separation assemblies between attached garages and living spaces, between dwelling units in duplexes, and in mechanical rooms — all of which are common in NB's housing stock.

How the Materials Behave in Fire

Mineral wool (sold under brands like Rockwool/Roxul) is made from basalt rock and recycled slag spun into fibres. Because it is literally made from stone, it does not combust, does not melt at typical building fire temperatures, and does not contribute to flame spread. In a fire, mineral wool batts hold their position in the wall cavity, continuing to act as a thermal and smoke barrier even as surrounding materials burn. This is why mineral wool is used in fire-rated wall assemblies and is often specified by NB building inspectors for garage-to-house separations.

Fibreglass batts, by contrast, are made from spun glass fibres bonded with a resin binder. The glass itself is non-combustible, but the resin binder burns off at relatively low temperatures, and the fibres begin to soften and lose their structure well below the temperatures reached in a typical building fire. Standard kraft-faced fibreglass batts add another concern — the kraft paper facing is combustible and must always be covered with drywall or another thermal barrier. Unfaced fibreglass is better in this regard, but neither product matches mineral wool's fire performance.

What the NB Building Code Actually Requires

Under the NBC (as adopted in New Brunswick), fire separation assemblies between an attached garage and a living space require a minimum 30-minute fire resistance rating. This is typically achieved with 5/8-inch Type X drywall on the garage side, but the insulation within that wall cavity contributes to the overall assembly performance. Mineral wool batts are frequently specified in these assemblies because they maintain cavity integrity longer than fibreglass, giving occupants more time to evacuate and slowing fire spread.

For party walls between dwelling units (duplexes, semi-detached homes — very common in Moncton, Saint John, and Fredericton), the code requires a 1-hour fire separation. Mineral wool is often the insulation of choice here, combined with double-layer drywall, because it adds meaningful fire resistance while also providing excellent sound attenuation — a dual benefit in shared-wall construction.

Flame Spread and Smoke Development Ratings

Both materials carry a Flame Spread Index (FSI) of 25 or less and a Smoke Developed Index (SDI) of 50 or less when unfaced, which meets the NBC's interior finish requirements. However, mineral wool's FSI is typically rated at 0-5, while fibreglass unfaced batts typically rate at 10-25 depending on the product. The lower the number, the better — and in a fire separation assembly, that gap in performance is meaningful.

Practical Guidance for NB Homeowners

For standard interior partition walls with no fire separation requirement, fibreglass batts are perfectly acceptable and the cost difference (roughly $0.50-$0.75 per square foot more for mineral wool) may not be justified. But for any of these situations, mineral wool is worth the premium:

Garage-to-house walls and ceilings — code-required fire separation, mineral wool adds meaningful protection beyond the minimum drywall requirement. Party walls in duplexes or semi-detached homes — fire and sound performance both improve substantially. Mechanical rooms containing furnaces, hot water heaters, or electrical panels. Any wall where soundproofing is also a priority — mineral wool's density (typically 1.7-2.5 lbs/cubic foot versus 0.5-1.0 for fibreglass) makes it dramatically better at blocking airborne sound transmission.

Mineral wool is also dimensionally stable — it does not sag or shift in the cavity over time the way fibreglass can — and it is hydrophobic, meaning it repels water rather than absorbing it. In NB's humid Maritime climate, that moisture resistance is a genuine long-term advantage in exterior walls and basement applications.

If you're planning a renovation involving fire separations or party walls, it's worth having a conversation with your contractor about specifying mineral wool. Find insulation contractors familiar with fire-rated assemblies through the New Brunswick Construction Network directory.