Exterior Wall Protection and Insulation in Ontario: House Wrap, Rainscreens, Flashing, Water Behind Siding, Exterior Insulation at Re-Siding, and ICF vs Wood Frame

Exterior FinishesWall Protection

The Wall Under the Siding: The Layer That Actually Keeps the House Dry, the Flashing That Feeds It, and the Insulation You Can Only Add When the Skin Is Off

The Ontario Building Code calls it the second plane of protection. Every siding quote should call it something, and most do not.

Water gets behind every cladding there is. Wind-driven rain goes through the laps of vinyl, the joints of fibre cement, the hairline cracks in stucco and the mortar of brick, and the wall is dry anyway because of what is behind the cladding: a membrane that sheds it, flashing that directs it, and a path that drains it. When that layer is missing or the flashing is wrong, the siding looks perfect and the sheathing rots behind it for fifteen years. These pages are about that layer: what it is, what the code wants, how to tell when it has failed, what a real diagnosis costs, and the insulation you can add while the wall is open.

Price the wall and the skin Water behind siding Exterior insulation

The numbers

$0.85house wrap, per sq ft, as a line
$2.50rainscreen strapping, per sq ft
$4.501.5 in. foam at re-siding, per sq ft
$6.50rotten sheathing replaced, per sq ft

The guides

Exterior insulation cost

Adding continuous insulation at re-siding: foam vs mineral wool, the jambs and soffits that change, and the payback.

Rainscreen siding guide

What a rainscreen is, when the code and the product want one, the strapping, and what it costs.

House wrap, air barrier, vapour barrier

The three jobs, which layer does which, and where the poly goes in an Ontario wall.

Siding flashing details

Head flashing, sill pans, Z-flashing, kick-outs and transitions, and what to photograph before the wall closes.

Water behind siding

Stains inside and nothing visible outside: where it gets in, the $90,000 scare at the door, and what a real diagnosis costs.

Cladding on wood frame vs ICF

Fastening to webs, strapping, stucco on foam, brick ledges: what changes when the wall is concrete and foam.

Exterior wall moisture inspection

What a qualified inspection does, meters and probes, what it costs, and when it is worth paying for.

Four rules for the wall under the skin

  1. Two planes, always. The cladding sheds most of the water; the membrane behind it sheds the rest. One without the other is a wet wall on a schedule. The layers.
  2. Flash every opening, every transition, every roof-to-wall. Sill pan, head flashing, Z-flashing over trim, kick-out at the eave. Photograph them before the siding covers them. Flashing.
  3. Caulking is not flashing. It is what gets applied after the flashing was skipped, and it lasts three winters. Where water gets in.
  4. If the siding is coming off anyway, insulate. It is the only cheap time to do it, and the jambs and soffits have to be planned for the thicker wall. Exterior insulation.

Have quotes or plans in hand? Let a builder read them first

For $99, I put your siding, stucco, masonry or eavestrough quotes on one grid, check the wall area against your house, name the product each one actually promised, price the tearoff, wrap, flashing, strapping and rot repair that was left as “extra”, and tell you which one to sign. For a new build or an addition, $249 turns your elevations into a wall-by-wall takeoff of cladding, stone, stucco details, eaves and flashing, with a budget by grade, that three contractors can price the same way.

Exterior quote review, $99 Exterior finish takeoff, $249 Check the quotes yourself first