Is blown-in fibreglass better than cellulose for moisture resistance in Shediac NB? | Insulation IQ?

Shediac sits at the intersection of two moisture challenges: the high relative humidity of the Northumberland Strait coastline and the deep cold of a climate zone 6 winter. Choosing between blown-in fibreglass and cellulose for an attic or wall upgrade there is a genuinely nuanced decision, not a simple one-wins-all comparison.

What moisture resistance actually means for loose-fill insulation

When contractors or product salespeople say fibreglass is "more moisture resistant" than cellulose, they are usually referring to liquid water absorption. Blown-in fibreglass does not wick or hold water in its fibres. If a small roof leak reaches the insulation, the water drains through rather than being retained. Fibreglass fibres themselves do not rot, mould, or decompose. This is a real advantage in a scenario involving liquid water intrusion.

Cellulose, however, is treated with borate compounds during manufacturing, which gives it strong mould and pest resistance. Despite being paper-based, cellulose in a properly detailed attic with intact vapour control and no liquid intrusion does not typically develop mould. What it does do is absorb and buffer atmospheric moisture — it can take in a modest amount of humidity from air and release it again as conditions change. This buffering actually helps moderate moisture fluctuations within the assembly. The concern arises when that buffering capacity is exceeded — sustained wetness will cause cellulose to mat, compact, and lose both depth and R-value permanently.

The Shediac-specific context

In Shediac, the primary moisture threat in a properly built attic is not liquid water from leaks but vapour-driven condensation on the cold sheathing during winter. Warm, humid indoor air rises into the attic, passes through inadequate air barriers, and hits the cold roof deck. Both fibreglass and cellulose are vulnerable to this mechanism if vapour control on the warm side is inadequate. The NB Building Code requires a continuous 6 mil polyethylene vapour barrier on the warm side of insulation in climate zone 6. Without it, neither product performs well in Shediac's humid winters.

Where fibreglass has a practical edge in Shediac is settling and air permeability. Blown-in fibreglass settles more than cellulose over time, which reduces the effective depth. However, its low density means it is slightly more permeable to air movement within the insulation layer. Dense-pack cellulose — installed at higher density into wall cavities or enclosed spaces — can dramatically reduce air movement through the insulated assembly, which translates to measurably better thermal performance in windy coastal conditions.

Cost comparison in Shediac

For a typical 1,200 sq ft attic in Shediac being topped up to R-50 (RSI 8.67, the NB Building Code minimum for new construction), material and labour costs generally run:

• Blown-in cellulose: approximately $1,600 to $2,600


• Blown-in fibreglass: approximately $1,800 to $3,000

Fibreglass is typically 10 to 20 percent more expensive per installed R-value than cellulose. The premium is sometimes justified by the liquid-water-tolerance advantage, but for a well-maintained roof with intact vapour control, the practical performance difference in Shediac is marginal.

What matters most in the real world

Installation quality eclipses product choice in almost every real-world moisture outcome. An attic with cellulose blown to proper depth, with all penetrations air-sealed first and a continuous vapour barrier below, will outperform a fibreglass job where pot lights were left unsealed or the barrier has gaps at the perimeter. In coastal Shediac, pre-installation air sealing — foam around plumbing penetrations, acoustical sealant at junction boxes, rigid insulation over the attic hatch — is the single most impactful step for long-term moisture performance.

NB Power rebates apply to both products equally under the Home Energy Savings Program. Both cellulose and fibreglass blown-in are eligible insulation types for the Canada Greener Homes Grant when paired with an EnerGuide assessment.

For most Shediac homeowners with a sound roof and intact vapour control, dense-pack cellulose offers excellent value and slightly superior air-sealing characteristics. For homeowners with older roofs or a history of minor leaks, the liquid-water tolerance of fibreglass may tip the balance. A licensed insulation contractor doing a pre-installation attic inspection can assess your specific situation. Explore qualified local insulation professionals through New Brunswick Insulation or the New Brunswick Construction Network.

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• Arctic Fox Construction Inc.
• Brunswick insulation & roofing
• 3Tone Construction Ltd
• Gionetterenovations
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