Fiberglass vs Cellulose Insulation — Ontario Guide

Cellulose is recycled paper treated with borate fire retardants; fiberglass is melted glass spun into fibres. Both are blown into attics with similar equipment, but they perform differently over time.

Fiberglass is the better choice when:

1. install needs to be DIY-friendly, fiberglass blowers are widely available at rental yards, and the material is forgiving of inconsistent depth
2. settling is a concern, fiberglass holds its installed depth for decades, so the R-value at year 1 is the same as year 30
3. attic ventilation is marginal, fiberglass tolerates damp summer attics better than cellulose because it doesn't absorb water
4. cost is the priority and you only need R-60, fiberglass at $1.20–$2.20/sq ft installed is the cheapest path to OBC minimum.

For a typical homeowner doing a one-time attic top-up, fiberglass is usually the right call.

Cellulose wins when:

1. air leakage is a known issue, cellulose's higher density (3.5 lb/ft³ vs fiberglass's 0.6 lb/ft³) packs tighter around penetrations and reduces air convection inside the insulation, often saving 15-25% on heating bills versus fiberglass at the same nominal R-value
2. you're insulating walls (dense-pack), cellulose is the standard wall blow-in product, fiberglass loose-fill doesn't dense-pack well
3. moisture buffering matters, cellulose absorbs and releases water seasonally without losing performance, useful in older homes with imperfect vapour control
4. fire performance is a priority, borate-treated cellulose meets Class A flame spread and is often used to upgrade older homes with knob-and-tube wiring concerns
5. you care about embodied carbon, cellulose has 75-85% recycled content and far lower embodied energy than glass production.

Cellulose's settling is real but accounted for by professional installers, who blow extra depth at install to compensate.

Material cost differences are small; the bigger gap is in install labour and how aggressively a contractor compensates for cellulose settling.

Both products qualify for the Canada Greener Homes Grant ($1,800 max attic rebate) when installed to OBC SB-12 R-60 attic minimum. Both meet OBC fire spread and smoke developed indices when correctly installed. Most GTA insulation contractors carry both products and switch based on project specifics, if you're getting quotes, ask each contractor which they prefer for your specific attic and why.

Mixing the two in a single attic isn't recommended unless one is being used as a topup over an existing layer of the other, settling rates differ, which can cause depth inconsistencies over time.