Is blown-in cellulose or fibreglass better for attics in New Brunswick? | Insulation IQ?

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New Brunswick Insulation · Accepted Answer

Both blown-in cellulose and blown-in fibreglass are legitimate choices for New Brunswick attics, but when you weigh their respective strengths against the province's Climate Zone 6 conditions, most building-science professionals give cellulose a modest edge for the majority of residential applications. That said, the right choice depends on your specific attic geometry, existing moisture history, and budget — and either material can perform excellently when properly installed.

Thermal performance per inch is the starting point for any comparison. Blown cellulose settles to an R-value of approximately R-3.5 per inch, while blown fibreglass lands at R-2.2–R-2.7 per inch depending on density and product. The NB Building Code requires R-50 minimum for attics in Climate Zone 6, meaning you need roughly 14–15 inches of settled cellulose or 18–22 inches of blown fibreglass to meet code. That difference in required depth has real implications in attics with shallow eave clearance — common in Fredericton-area bungalows and in many 1950s–1970s homes throughout the Saint John River Valley — where you simply may not have 20 inches of headroom available at the eave.

Air movement resistance is where cellulose tends to outperform fibreglass in NB's climate. Blown fibreglass is a relatively low-density material, and when wind enters an attic through soffit vents and flows along the attic floor, it can penetrate the fibreglass insulation layer and reduce its effective R-value — a phenomenon called wind-washing. Dense-pack cellulose, with its tighter fibre matrix and higher installed density, resists wind-washing more effectively. Given that New Brunswick regularly experiences Nor'easter winds driving cold air through attic ventilation systems, this is not a trivial concern, particularly for homes in exposed coastal communities near Saint John, Sackville, or along the Fundy shore.

Moisture behaviour is the most nuanced part of this comparison for NB specifically. Cellulose is hygroscopic — it absorbs and releases moisture — and treated cellulose can buffer moderate humidity swings without losing structural integrity, provided moisture levels stay below approximately 25–28% by weight. The borate treatment that fire-retards cellulose also inhibits mould. However, in attics with a history of ice-damming, bathroom exhaust fans venting into the space, or inadequate vapour control from below, repeated wetting and drying cycles will degrade cellulose over time. Fibreglass, being inorganic, does not absorb moisture in the same way and will not mould — but wet fibreglass loses essentially all its R-value temporarily and can compress permanently if saturated severely enough.

The practical implication for New Brunswick homeowners: if your attic has any history of moisture intrusion, identifying and correcting the source takes priority over insulation choice. Once the moisture issue is resolved, either material performs reliably. If there is ongoing condensation risk you haven't yet fully controlled, fibreglass may offer slightly more forgiving long-term behaviour.

Cost is a meaningful factor. In the Moncton, Fredericton, and Saint John markets, blown cellulose is typically $0.10–$0.25 per board foot cheaper than blown fibreglass, primarily because raw cellulose (recycled newsprint) costs less than glass fibre. For a 1,000 sq ft attic upgraded to R-50, that translates to a cellulose job costing roughly $1,500–$2,500 versus fibreglass at $1,800–$3,200 depending on the specific products and contractor overhead. The cost gap narrows for smaller jobs where setup time dominates.

Settling is a common concern raised about cellulose. Quality cellulose products today are pre-treated to limit settling, and a professional installer will blow to a depth that accounts for the expected 15–20% post-installation settlement, guaranteeing the contracted R-value at settled depth. Blown fibreglass also settles, though somewhat less. Either way, the performance guarantee in the contract — specifying R-value at settled depth — is what you should hold the contractor to, not the initial blown depth.

For rebate purposes, both materials qualify under the NB Power Home Energy Efficiency Upgrade programme and the Canada Greener Homes Grant (check current funding availability) when installed to the programme's minimum R-value requirements. There is no rebate preference between the two materials; the upgrade depth and resulting RSI improvement determine your rebate amount.

In summary, for most New Brunswick attics, blown cellulose offers a better value proposition — higher R-value per inch, better wind-washing resistance, and lower cost — while blown fibreglass may be preferable in attics with chronic moisture challenges or very limited depth at the eaves. Either way, thorough air sealing before the blow is more important than which material you choose.

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