Continuous Exterior Insulation Added During Re-Siding

The Problem It Solves

An insulated wall is only as good as its weakest path, and in a wood-framed house that path is the framing. Batts fill the stud cavities, but every stud, plate, header and corner post runs straight from the drywall to the sheathing with nothing but wood in the way, and wood resists heat flow far less than the insulation beside it. Thicker batts leave those lines untouched, because the studs are still there. Short of gutting the house from the inside, the fix is a continuous layer across the outside of the framing.

The outside of the framing is only reachable when the siding is off. That makes a re-siding job, whether it follows hail, failing vinyl or tired stucco, the one practical moment to add continuous insulation. Once new fibre cement or vinyl goes on, the chance is gone for as long as that cladding lasts. Owners who miss it usually do so because nobody raised it before the tear-off, or because trim had already been ordered to suit the old wall depth.

Done carelessly, exterior insulation can make a wall worse. Thicker foam boards behave as vapour retarders, so moisture moving out of a heated house can stall at the back of the foam. If that foam is too thin for the insulation inside the wall, the sheathing stays cold enough for the moisture to collect. The building code’s own worked example shows a foam-sheathed wall over a 140 mm batt-filled cavity reaching a ratio of only 0.28, which would not pass in Calgary. A thin layer of foam over a 2×6 wall can be worse than none.

How It Works

Continuous insulation moves part of the wall’s insulating value outside the framing, so the studs, plates and headers sit on the warm side instead of forming a bridge to the cold. A warmer sheathing is also a drier one. We size the layer with the code’s outboard-to-inboard ratio: in Calgary, Airdrie and Cochrane the exterior layer needs at least 30% of the thermal resistance of what’s inboard of it, and in Okotoks at least 20%. A shallow 89 mm cavity is easier to satisfy than a 140 mm one, so the existing wall decides how much board is needed.

Material choice changes the arithmetic. EPS, GPS and XPS are efficient per inch but close up to vapour as they thicken, so the ratio governs them. ROCKWOOL Comfortboard 80 is stone wool that tests far above the code’s permeance limit, is non-combustible to CAN/ULC-S114, and lets the wall dry outward. The code exempts an insulation with a permeance of at least 30 ng/(Pa·s·m²) and at least RSI 0.7 from the ratio in places under 6000 degree-days, which covers Calgary, Airdrie, Cochrane and Okotoks. We confirm the actual assembly against that article rather than assuming it applies.

What Decides the Specification

Board type is driven by four things: insulating value per inch, vapour permeance, fire performance and how the wall must dry. Where trim depth limits thickness, GPS or XPS packs more resistance into fewer millimetres. Where a wall must dry outward, or non-combustible materials are wanted near a property line, Comfortboard 80 is the stronger candidate. XPS in Canada is rated on its long-term thermal resistance, so compare published LTTR values, not first-day figures from older product sheets.

Thickness is driven first by the ratio, then by the house. A 2×6 wall with full batts needs noticeably more exterior board than a 2×4 wall to keep the sheathing warm, and Calgary’s 0.30 ratio asks for more than Okotoks’ 0.20. The code’s ratios also assume mechanical ventilation and indoor humidity of about 35% in winter; a home kept more humid than that for long periods may need more margin or a design professional. Window depth, soffit overhang and the added screw length all follow from the thickness chosen.

Around Calgary

The houses that gain most are the ones built before anyone thought about thermal bridging. In southwest Killarney/Glengarry, where many homes are post-war bungalows from around the 1950s, and in northwest Varsity, much of it built through the 1960s and 1970s, original cladding may be due for replacement, and where walls are 2×4 framed the ratio is easier to meet. In Okotoks the lower degree-day count puts the town in climate zone 6 with a 0.20 ratio, so a wall there can meet the rule with less exterior insulation than the same wall in Calgary.