How much blown-in insulation do I need for a 1500 sq ft attic in Rothesay? | Insulation IQ?

Rothesay sits in the Saint John River valley, where winters are reliably cold and heating seasons are long — a typical climate zone 6 location where attic insulation depth has a direct and measurable impact on energy bills. Calculating how much blown-in insulation you need for a 1,500 sq ft attic involves understanding your current depth, your target R-value, and the settled depth per bag for the product you are using.

Target R-value for Rothesay attics

New Brunswick follows the National Building Code (NBC 2015 as adopted provincially), which sets a minimum attic insulation requirement of RSI 8.67 — equivalent to R-50 — for new residential construction. That said, many Rothesay homes built in the 1970s through early 2000s have R-20 to R-28 of existing insulation. Energy advisors and NB Power typically recommend upgrading to R-50 minimum, with R-60 providing incremental additional savings in colder stretches of the valley near Sussex or Norton.

Calculating the volume needed

The math starts with the difference between where you are and where you need to be. If your attic currently has R-20 of old fibreglass batts and your target is R-50, you need to add approximately R-30 of blown-in over the existing material.

For blown-in cellulose: one bag of cellulose (typically 25 lb) covers approximately 40 to 45 sq ft at a settled depth providing R-3.5 per inch. To achieve R-30 of added cellulose you need roughly 8.5 inches of depth. For 1,500 sq ft, that works out to approximately 65 to 75 bags of cellulose.

For blown-in fibreglass: fibreglass is lighter and less dense. One bag (typically 19 to 22 lb) covers approximately 35 to 40 sq ft at R-2.5 per inch settled. Achieving R-30 of added fibreglass requires about 12 inches of depth. For 1,500 sq ft, expect approximately 75 to 90 bags.

If starting from scratch in a completely empty attic and targeting R-50, the totals scale up accordingly: cellulose to R-50 requires roughly 14 inches of depth, meaning 110 to 130 bags for 1,500 sq ft. Fibreglass to R-50 requires approximately 20 inches, meaning 140 to 160 bags.

Practical cost estimates for Rothesay

For a professional blown-in installation in Rothesay, including pre-installation air sealing of penetrations (the most critical step before any blown-in work), labour, rental equipment, and materials, typical costs run:

• Topping up from R-20 to R-50 (1,500 sq ft attic): approximately $1,800 to $2,800 for cellulose; $2,200 to $3,200 for fibreglass
• Full installation from bare attic to R-50: approximately $2,800 to $4,200 depending on attic accessibility, number of penetrations, and existing obstructions

Attics with complex geometry — multiple dormers, low slopes, obstructions from HVAC equipment or ductwork — add to labour time and cost.

Air sealing: non-negotiable before blowing

In Rothesay, as throughout the Saint John area, many homes have older construction with unsealed electrical boxes, plumbing chases, and attic hatches. Before any blown-in material goes in, these penetrations must be sealed. Air sealing a typical 1,500 sq ft attic adds $300 to $700 to the project but delivers as much energy benefit as the insulation itself in older homes. Do not allow a contractor to skip this step.

Rebate programs that apply in Rothesay

NB Power's Home Energy Savings Program covers attic insulation upgrades and pays based on the RSI improvement per unit area. For a 1,500 sq ft attic upgraded from R-20 to R-50, rebates in the $400 to $750 range are typical (exact amounts depend on current program parameters — check nbpower.com for the latest). The Canada Greener Homes Grant previously offered up to $600 for insulation improvements when paired with a pre- and post-retrofit EnerGuide home assessment.

Settling allowance

Both cellulose and fibreglass blown-in products settle after installation. Cellulose typically settles 15 to 20 percent within the first year. Contractors should install to a marked depth that accounts for this settling — do not accept a job installed to the settled R-50 depth without the settling allowance baked in.

For a precise quote and product recommendation tailored to your specific attic in Rothesay, reach out to a local insulation professional through New Brunswick Insulation or browse contractors on the New Brunswick Construction Network.

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