What is the difference between insulating from inside vs outside walls in NB? | Insulation IQ?

When a New Brunswick homeowner decides to improve wall insulation, they face a fundamental choice that shapes the entire project: add insulation from the interior or from the exterior. Both approaches are legitimate and widely used across NB, but they have profoundly different costs, disruption levels, impacts on moisture management, and effectiveness at addressing thermal bridging. Understanding the trade-offs helps you choose the right strategy for your home and budget.

Interior wall insulation involves working from inside the house. The classic approach is to strip drywall from exterior walls, add insulation between or in front of the studs, update the vapour barrier, and re-drywall. A less invasive interior option for existing walls with some insulation already in place is dense-pack cellulose or fibreglass blown through small holes drilled through the interior drywall — or from outside through the cladding — without full teardown. Interior approaches give direct access to the stud cavities and allow easy inspection and remediation of any rot or wiring issues found during the work.

The biggest limitation of interior insulation is thermal bridging. A standard 2×6 stud wall filled with R-22 spray foam or R-20 fibreglass still loses roughly 20–30% of its theoretical assembly R-value because heat moves readily through the wooden studs themselves, which conduct far better than the insulation in between. In New Brunswick's Climate Zone 6 environment, where the NB Building Code targets an effective wall assembly R-value of approximately R-24 effective (not just nominal cavity R-value) for new construction, relying on cavity fill alone often falls short of optimal performance without adding a continuous insulating layer inside or out.

To address bridging from the interior, contractors sometimes add a continuous layer of rigid foam — typically 1.5 to 2 inches of polyisocyanurate or EPS — to the interior face of the studs before drywalling. This adds R-10 or more continuously across the entire wall plane, bridging the studs thermally. The downside: this process furs the wall inward by 1.5–2 inches, reducing room dimensions, requiring all electrical boxes to be extended, and complicating window and door trim depths. In a kitchen in Fredericton or a tight bedroom in a Moncton bungalow, losing even an inch of depth on every exterior wall is a real consideration.

Exterior wall insulation — often called an exterior continuous insulation (ci) retrofit — flips the approach. Rigid foam board, mineral wool board, or structural insulated sheathing is installed over the existing wall exterior before new cladding goes on. Popular products include EPS, XPS, and mineral wool (stone wool) board in thicknesses from 1 to 4 inches. A 3-inch layer of EPS board adds roughly R-12 continuously across the whole wall, eliminating thermal bridging entirely at the cladding plane.

Exterior insulation does not disrupt the interior living space at all — no furniture is moved, no rooms are taken out of service, and the work proceeds entirely outside. It also allows contractors to upgrade the weather-resistive barrier and window flashing details simultaneously, which is a significant opportunity in older NB homes where housewrap has failed or was never installed. The trade-off is cost and exterior appearance changes: adding 2–3 inches of exterior foam means all window and door trim must be extended (through the additional thickness), soffit and fascia details need revision, and the overall thickness of the wall assembly changes the home's visual profile. These details add labour cost and require skilled execution.

For a typical 1,500-square-foot Cape Cod or split-level in Saint John, interior wall re-insulation with dense-pack and vapour barrier replacement typically runs $8,000–$18,000 depending on scope. An exterior continuous insulation retrofit of similar scope — new foam, new housewrap, new cladding — typically costs $18,000–$35,000 or more, but often bundles cladding replacement that the homeowner would have been spending on anyway.

From a moisture management standpoint, exterior insulation has a significant advantage in NB's climate: it keeps the sheathing and framing warmer, pushing the dew point well into the foam layer rather than at the sheathing face. This dramatically reduces the risk of condensation-driven rot and mould in the wall assembly over time — a real concern in NB where older homes were built without the vapour barrier placement strategies understood today.

For guidance on which approach best suits your specific home and budget, consult with an experienced insulation contractor listed through New Brunswick Insulation or the New Brunswick Construction Network.

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

• Gionetterenovations
• Thirty Four Renovations
• Brunswick insulation & roofing
• Arctic Fox Construction Inc.
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