Exterior Insulation Cost in Ontario (2026): Adding Continuous Foam or Mineral Wool While the Siding Is Off, Jambs, Soffits, the Ratio Rule and the Honest Payback

The numbers

These are the lines the exterior cladding cost calculator adds under any siding when you tick the insulation box. The siding, the tearoff and the house wrap are priced on their own; the siding cost page has every cladding by grade.

What continuous insulation does that the batts cannot

A 2×6 wall with R20 batts is not an R20 wall. About a quarter of it is wood, at the studs, plates, headers and corners, and wood is about R1.2 an inch. Run the arithmetic the way the code does and the wall comes out at an effective R16 or so. The studs are where the drywall is cold in January, which is why the dust pattern on an old painted wall shows you exactly where the framing is.

A layer of insulation outside the sheathing covers the studs too. Add R6 continuous to that R16 wall and you get about R22, because nothing is bridging it. The second thing it does, and the one that matters more for a wall that will be closed up for forty years, is move the cold plane outward. The sheathing sits warm, inside the foam, so air that leaks into the wall in January finds no cold surface to condense on. Warm sheathing is dry sheathing. NRCan’s Keeping the Heat In has said this for thirty years; it is not a new idea, just an expensive one on any day but re-siding day.

The ratio rule, because foam on the outside can also rot a wall

Ontario wood-frame walls have the poly vapour barrier on the warm side, behind the drywall. Put a low-permeance foam on the cold side as well and you have a wall that can only dry in one direction, so the code limits how much of the insulation can be outboard of the sheathing unless the foam is thick enough to keep the sheathing above the dew point. OBC 9.25.5 does this as a ratio of outside R to inside R by degree-days. South of about Barrie, under 5,000 degree-days, the outboard share has to be at least 0.20 of the inboard; from 5,000 to 5,999, which takes in Muskoka and north, 0.30.

Translated: with R20 batts, 1.5 in. of XPS at R7.5 is a ratio of 0.375 and passes everywhere in Southern Ontario. 1 in. of XPS at R5 is 0.25 and passes south of Barrie but not north of it. 1.5 in. of EPS at about R6 is 0.30 and passes everywhere, just. Mineral wool is vapour-open, so the rule does not bite; the wall still dries outward through it, which is the main reason people pay $2.50 a foot more for it. If a quote offers 3/4 in. of foam as “exterior insulation”, it is a drainage board, not insulation.

A 2,400 sq ft wall worked through

Two-storey house, 2,400 sq ft of net wall, 22 windows and doors, 120 lf of shallow eave to build out, old vinyl off, mid-grade vinyl back on. Tearoff 2,400 at $2.00 is $4,800 and a bin is $550. House wrap over the sheathing, 2,400 at $0.85, is $2,040. Window and door flashing kits, 22 at $85, $1,870. That is the base re-side, $9,260, before the siding.

Now the insulation. 1.5 in. XPS, 2,400 at $4.50, $10,800. Extension jambs, 22 at $150, $3,300. Soffit rework, 120 lf at $18, $2,160. Insulation package $16,260. Vinyl on through the foam with 2.5 in. nails, 2,400 at $9.50, $22,800. Total $48,320 before HST, $54,602 with. Without the insulation the same job is $32,060 before HST, so the foam, the jambs and the eaves added $16,260, about half again. In 2 in. mineral wool the package is $16,800 plus $6,000 of strapping plus the same jambs and soffits, $28,260.

The honest payback

Walls are roughly a third of the heat loss of an older two-storey house; the attic, the windows, the basement and the air leaks share the rest. Taking the wall from R16 to R22 effective cuts the wall’s share by about a quarter, which is 8 to 10% of the heating bill. On a gas-heated house paying $1,800 a year that is $150 to $180 a year against $16,260. A hundred-year payback. On propane or electric baseboard, where the fuel costs two to three times as much, call it $400 a year and forty years. I am not going to dress that up; anyone selling exterior insulation on energy savings alone is selling you the brochure.

The reasons that do hold up: the siding is off and will not be off again for thirty years; the drywall at the studs stops being cold, which ends the mould line behind the dresser on the north wall; the sheathing stays dry; the house is quieter; and if you are changing the furnace to a heat pump in the next few years, a warmer wall is the difference between a heat pump that keeps up at minus 20 and one that does not. That last one changes the heat-loss design, and a stamped heat-loss calculation that reflects the new wall is what the HVAC contractor should size from, which is the job of OntarioHeatLoss.ca. Size the equipment to the old wall and you pay for the insulation twice.

Where it goes wrong

1. Nobody decides where the water plane is. The house wrap can go over the sheathing, under the foam, or over the foam with taped seams as the drainage plane. Either works. What does not work is the wrap on the sheathing and the window flanges on the foam, with a 1.5 in. gap between the two that every window drains into. Pick one plane, run every flashing to it, write it on the quote. What each layer does.
2. The windows stay where they were. With 1.5 in. of foam and 3/4 in. of strapping, the window is 2.25 in. back from the siding face and the sill is a trough. Extension jambs and a sloped sill extension at every opening, $150 each.
3. The eave is too shallow. An 8 in. overhang with 2.25 in. of new wall under it is a 5.75 in. overhang. The drip edge no longer clears the wall, the soffit vent is half covered, and the frieze lands on foam. That is the $18 a foot line.
4. Hose bibs, dryer vents, the gas meter, the deck ledger. Every one was set for the old wall thickness. Each is an hour, and the meter is a call to the utility.
5. Foam at grade. The siding still stays off the ground, 200 mm in the code and 6 in. in the fibre cement sheets, and the bottom edge of the foam needs a screened closure or the mice will insulate their own place with it.

If the house is ICF, none of this applies: the foam is already there, inside and out, and the cladding goes to the webs. What changes on an ICF wall.

Questions people ask

Sources and further reading

• Keeping the Heat In: Natural Resources Canada: walls, exterior insulation at re-siding, air and vapour barriers, and what changes at the windows and soffits when the wall gets thicker.
• Owens Corning Canada: rigid foam exterior insulation product data: thickness, R-value per inch, fastening through it.
• Rockwool: exterior mineral wool board (Comfortboard): R-value, vapour-open, how it is strapped and fastened.