Should I add house wrap or insulation board under new siding in New Brunswick?

Yes — absolutely. In New Brunswick's climate, proper housewrap and adding rigid foam insulation board under new siding is one of the highest-return upgrades you can make to an older home, and skipping either one is a decision you will almost certainly regret within a few years. The short version: housewrap is mandatory, and insulation board is strongly worth the additional investment.

Housewrap — products like Tyvek HomWrap, Typar, or their equivalents — serves as the primary weather-resistive barrier (WRB) between your siding and your sheathing. It blocks liquid water that penetrates behind your siding while still allowing water vapour to escape outward. Without it, water infiltrating around fasteners, trim joints, window edges, and any siding seam finds its way to your OSB or plywood sheathing, where it sits, saturates the wood, and eventually causes rot, mould, and structural damage. In NB's wet Maritime climate with persistent wind-driven rain off the Bay of Fundy and along the Northumberland Strait coast, this isn't a theoretical risk — it's a near-certainty over 15 to 20 years without a proper WRB. Many NB homes built in the 1970s and 1980s have no housewrap at all, and siding replacement projects routinely uncover significant sheathing decay underneath.

When installing new siding, always use a quality housewrap applied to clean, smooth sheathing with lapped seams and properly taped joints around all penetrations. The wrap goes over the sheathing, and all window and door openings need carefully integrated flashing — either peel-and-stick membrane or proper sill pan flashing — before the windows are reinstalled or new casings applied. This is where most water intrusion problems originate on NB homes: improperly flashed window rough openings, not failures of the siding material itself.

The Case for Rigid Foam Insulation Board

Adding a layer of rigid foam insulation board — typically 1-inch to 2-inch extruded polystyrene (XPS) or polyisocyanurate — under new siding accomplishes two things that are difficult to achieve any other way. First, it adds meaningful R-value to your wall assembly without any interior disruption. A 1-inch XPS board adds R-5; 2-inch adds R-10. For NB homes with 2x4 walls stuffed with original fibreglass batts (often settled and degraded), this exterior continuous insulation layer dramatically reduces thermal bridging through your studs and meaningfully improves your wall's overall performance. Second, it creates a slight drainage plane that helps any water behind the siding drain downward rather than pooling against the sheathing.

The added cost of 1-inch rigid foam runs approximately $1.50 to $2.50 per square foot for materials, plus some additional labour for fastener length adjustments and window extension jambs (which need to be extended to accommodate the added wall thickness). On a typical NB home, that might add $2,500 to $5,000 to a full siding project. Given that you're already paying $15,000 to $30,000 for siding replacement, this incremental investment can meaningfully improve your home's thermal performance for decades at a fraction of what interior wall insulation upgrades would cost.

If your siding project is planned for exterior vinyl over existing vinyl or any scenario where you're not removing the old siding entirely, the options narrow — but even a drainable housewrap with a built-in drainage mat is far better than bare siding against sheathing. For NB homes near the coast or in the Saint John River valley where wind-driven rain loads are significant, this is not optional if you want your exterior envelope to perform well.

Confirm with your contractor that the housewrap installation includes proper window and door flashing integration — this is the single most important detail in any NB siding project, and it separates a proper installation from one that will leak within a few years. Ask to see the flashing details before the new siding goes on. Once the siding is installed, you cannot fix what's hidden behind it without starting over.

---

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

• Thirty Four Renovations
• Gionetterenovations