Exterior Insulation

Most of the insulation talk homeowners hear is about attics, because attics are easy. Walls are where the harder gains live. The wood-frame walls that make up the vast majority of Lower Mainland housing leak heat in a way that no amount of cavity insulation can fully solve, because the framing itself is the leak. Exterior insulation, sometimes called continuous insulation, attacks that problem from the outside by wrapping the whole wall in an unbroken thermal layer. And there is exactly one moment in a house's life when adding it is straightforward and economical: when the cladding is already coming off for a siding replacement.

This page walks through the building science, the material choices, the moisture questions specific to coastal BC, and the detailing that separates a wall that performs from a wall that merely looks upgraded.

Thermal Bridging: The Problem Inside Framed Walls

A typical 2x6 wall is insulated between the studs, and between the studs the insulation does its job. But the studs themselves conduct heat many times faster than the batts beside them. Add up the studs, the doubled studs at openings, the headers, the plates at floor lines, and the corners, and roughly a fifth to a quarter of a typical framed wall is solid wood running straight from your warm drywall to your cold sheathing. Each of those members is a thermal bridge, a highway for heat to bypass the insulation entirely.

The result is that a wall's real-world performance is well below the rated value of the batts inside it. You can sometimes see thermal bridging with your own eyes: ghosting stripes on interior walls where dust has deposited on the cooler drywall over each stud, or frost melt patterns on sheathing during a cold snap. Thermal cameras make it obvious, showing the framing glowing through the wall like a skeleton.

Continuous insulation fixes this by putting an insulating blanket outboard of everything, framing included. Even a modest thickness of exterior insulation raises the whole assembly's effective performance disproportionately, because it covers the bridges, not just the bays. It also warms the sheathing, which turns out to matter enormously for moisture, as we will get to shortly.

Rigid Foam vs Mineral Wool

Two families of board insulation dominate exterior wall work, and they behave differently enough that the choice deserves real thought.

Rigid Foam Boards

Foam boards come in three common types. Expanded polystyrene is the economical option, moderately vapour open in thinner layers, and stable over time. Extruded polystyrene carries a higher rating per inch and sheds water well, but it is quite vapour closed, which constrains how the rest of the wall must be built. Polyisocyanurate offers a strong rating per inch of thickness and usually comes foil faced, which makes it effectively a vapour barrier on the outside of your wall, a significant design decision in our climate.

Foam's advantages are cost per unit of thermal value, light weight, and easy cutting. Its limitations are combustibility, which building codes address through assembly rules, and the vapour question: put enough foam of the wrong type outside a wall that also has polyethylene sheeting inside, and you have built a moisture trap with no drying direction.

Mineral Wool Boards

Semi-rigid mineral wool boards, made from spun stone fibre, take a different approach. They are highly vapour open, so the wall keeps its ability to dry outward. They are hydrophobic, shedding liquid water while letting vapour pass. They are non-combustible, which simplifies code compliance and adds genuine fire resilience. And they are dimensionally stable across temperature swings.

The trade-offs are higher material cost, more weight, and a compressible surface that demands more care when furring is fastened over it. For coastal BC wall assemblies that already contain an interior vapour retarder, mineral wool's ability to let the wall breathe outward is a strong argument, and it is the option we find ourselves recommending most often on renovation work where the interior construction is an unknown.

Vapour and Drying Dynamics in Coastal BC Walls

The Lower Mainland sits in a marine climate where walls get wet and must be able to dry. Rain exposure is long and frequent, outdoor humidity is high through the cool months, and our winters are mild rather than deeply cold. This shapes the physics.

In winter, indoor moisture pushes outward through the wall. Where it reaches a surface below the dew point, typically the back of the sheathing, it can condense. A cold sheathing surface in a poorly performing wall spends much of the winter close to that threshold. Here is the elegant part of exterior insulation: by placing insulation outboard of the sheathing, you move the sheathing to the warm side of the thermal gradient. Warm sheathing rarely reaches dew point, so the condensation plane that threatens conventional walls effectively disappears when the exterior layer is thick enough relative to the cavity fill.

The ratio matters. A thin exterior layer over a deep, heavily insulated cavity can leave the sheathing only slightly warmed and still vulnerable, which is why the exterior thickness should be selected in proportion to the cavity insulation, not picked off a shelf. It is also why vapour permeability of the chosen board matters: a vapour open exterior layer forgives imperfection, letting incidental moisture escape outward, while a vapour closed layer demands that everything else in the assembly be right.

Every assembly we propose is designed around what is actually in your wall, which we can verify during cladding removal rather than guess at from inside.

Why a Re-Side Is the Right Time

Adding exterior insulation to a finished wall means removing and reinstalling the cladding, which is most of the cost of a siding project on its own. But when the siding is being replaced anyway, the wall is already stripped to sheathing, and the incremental work is the insulation boards, longer fasteners, furring, and extended trim details. The economics change completely. You are buying decades of improved wall performance for the marginal cost of materials and detailing, on scaffolding that is already up, with a crew that is already there.

This is also the moment when the water resistive barrier gets renewed, sheathing damage gets found and repaired, and the rainscreen gets built to current practice. Bundling insulation into that sequence produces a wall assembly that is better than the house has ever had, in one mobilization.

Furring and Fastener Detailing

Cladding cannot hang directly on soft insulation board, so the load path runs through vertical furring strips fastened with long screws through the insulation into the framing. This is where field competence shows, because the details are unforgiving.

Screws must land in studs, not just sheathing, which means the stud layout is mapped before boards cover it. Fastener length is calculated from the stack of cladding, furring, insulation, and sheathing with proper embedment beyond. On compressible mineral wool, screws are driven to snug alignment rather than crushing the board, and the furring is shimmed straight because the finished wall will telegraph every hump. The screw pattern itself is engineering, not habit: cantilever loads on the fasteners depend on insulation thickness and cladding weight, and heavier claddings on thicker insulation require tighter spacing.

The furring cavity doubles as the rainscreen gap, which coastal BC building practice requires behind cladding. Air enters at the base, exits at the wall head, and any water that gets past the cladding drains down the face of the water resistive barrier. Insect screen closes the openings. The system is the same principle as any rainscreen installation, extended outward by the insulation thickness.

Penetrations get planned rather than improvised: hose bibs, vents, light blocks, and electrical fixtures all need mounting that reaches back to structure and flashing that respects the drainage plane.

Window Extension Jambs and Openings

Thickening a wall from the outside moves the cladding plane outward, and windows and doors are where that becomes visible. There are two honest approaches. Windows can remain where they are, in which case the jambs are extended outward with insulated boxed returns, new sills, and head flashings adapted to the deeper reveal. Done well, this creates attractive deep-set openings; done poorly, it creates leak paths, which is why we treat each opening as its own small flashing project with sloped sill pans, lapped membranes, and back dams.

Alternatively, if your windows are near the end of their life, replacing them during the project lets the new units be positioned in the plane that suits the new wall thickness, simplifying the detailing and pairing the thermal upgrade of the wall with the thermal upgrade of the glass. We handle that coordination through our windows and doors service, and it is worth an honest conversation before scope is finalized, because window placement decisions are far cheaper on paper than on site.

Comfort and Energy Outcomes

The measurable outcome is lower heat loss, and heating energy is where the savings accrue. But homeowners consistently report that the change they notice is comfort. Interior wall surfaces run warmer in winter, which eliminates the radiant chill of sitting near an exterior wall. Cold-corner condensation and the mould spotting that follows it fade because interior surfaces stay above dew point. Rooms hold temperature more evenly, and the house rides through cold snaps and heat waves with less swing. The added layers and the decoupling effect of the furred cladding also damp outside noise noticeably, something people living near busy roads mention unprompted.

For those considering a heat pump, a better envelope shrinks the equipment size the home needs and improves its cold weather performance, so wall insulation and mechanical upgrades reinforce each other.

Rebates and Incentive Programs

British Columbia has, for several years, run provincial and utility programs that offer incentives for home envelope improvements, and exterior wall insulation has historically been among the eligible measures, alongside federal programs that come and go. Program rules, eligibility, required documentation, and incentive levels change frequently, and most programs require specific pre and post upgrade steps, often including an energy evaluation before work begins. We deliberately avoid quoting amounts here because they shift. What we can tell you is what documentation from the contractor side the programs typically want, and our free written quotes are line-itemed in a way that separates insulation from cladding, which is the format rebate paperwork generally asks for. Check the current CleanBC and utility program details before you schedule work, and sequence the energy evaluation first if a program requires it.

Talk Through Your Wall Before You Re-Side

If a re-side is anywhere on your horizon, the insulation conversation should happen now, not after the new cladding is on. Berkana Construction Management has been building high performance wall assemblies across the Lower Mainland since 2019, licensed, insured, and WorkSafeBC compliant. Call (672) 771-6868 for a free written, line-itemed quote, and we will lay out exactly what your wall could become.