What is the R-value per inch for blown-in fibreglass vs cellulose in NB? | Insulation IQ?

Understanding R-value per inch is essential when comparing blown-in insulation options for a New Brunswick home, because the two most common loose-fill products — blown-in fibreglass and blown-in cellulose — perform very differently in terms of thermal resistance per unit of depth, and that difference directly affects how much material you need to buy and how much attic space the installation will consume.

Blown-in cellulose delivers approximately R-3.5 to R-3.8 per inch at typical installed densities for open attic applications. The slight variation in that range depends on the specific product brand, moisture content at the time of installation, and installed density (measured in kg/m³). At the standard reference temperature (10°C per ASTM C518 testing protocols), most Canadian cellulose products sold through retailers in Fredericton, Moncton, and Saint John are tested and labelled around R-3.6 to R-3.7 per inch. To reach the R-50 minimum required for Climate Zone 6 attics under the New Brunswick Building Code, you'd need roughly 14 to 15 inches of settled cellulose.

Blown-in fibreglass performs at approximately R-2.2 to R-2.5 per inch at typical open attic densities. Fibreglass is a lower-density material and its loose-fill form has more air space between fibres, which reduces its per-inch thermal resistance compared to cellulose. To reach R-50 with blown fibreglass, you'd need approximately 20 to 23 inches of depth — significantly more than cellulose. That depth difference is meaningful in attics with low truss heels, vents near the eaves, or limited clearance.

There's an important nuance here that affects real-world performance in New Brunswick winters: cold temperature performance. Fibreglass can lose some of its rated R-value at very low temperatures because cold air can move more freely through the low-density fibre structure — a phenomenon sometimes called thermal drift or wind-washing. In extremely cold attics (common across New Brunswick from December through February), blown fibreglass may underperform its rated value unless a wind baffle or air-impermeable layer is present. Cellulose, being a denser material, is somewhat less susceptible to wind-washing, though proper attic ventilation baffles near the eaves should be used with either product.

From a cost standpoint, cellulose is generally less expensive per R-value achieved. In New Brunswick, installed costs for blown-in cellulose in an open attic typically run in the range of $1,500 to $3,500 depending on attic area, access difficulty, and any air sealing work included. Blown-in fibreglass may cost similarly per bag but requires more bags to reach the same R-value, so the total installed cost is often higher for equivalent performance. That said, pricing varies by contractor and market conditions, so getting quotes from multiple insulation companies in your area is always the right approach.

One area where blown-in fibreglass holds an advantage is in situations with moisture risk. Fibreglass is inorganic and does not absorb moisture or support mould growth, whereas cellulose — though treated with borate fire retardants that also resist mould — can absorb moisture and mat if exposed to liquid water or persistent high humidity. In attics with any history of condensation issues, a roof leak, or inadequate vapour barrier, some contractors prefer fibreglass for that reason.

Both products are eligible for NB Power Home Energy Savings Program rebates and the Canada Greener Homes Grant as long as the upgrade meets the minimum R-value improvement thresholds and is documented with an EnerGuide audit. Neither product is inherently preferred by rebate programs — what matters is achieving the target R-value and having it verified by a certified energy advisor.

Dense-pack cellulose — used for blowing into enclosed wall cavities rather than open attics — operates at higher density (around 55 kg/m³) and a correspondingly lower R-value per inch of about R-3.4 to R-3.5, but it provides excellent air resistance in that application.

For most New Brunswick homeowners doing an open attic upgrade, cellulose offers the better combination of depth efficiency, cost, and air resistance. The right choice for your specific home depends on attic geometry, existing conditions, and your contractor's recommendation.

The New Brunswick Construction Network lists qualified insulation professionals across the province who can evaluate your attic and recommend the best product for your situation.

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

• Thirty Four Renovations
• moose luxury painting
• Brunswick insulation & roofing
• 3Tone Construction Ltd
• Gionetterenovations