Built Right Detail 10: Exterior Insulation & Cladding Attachment

A builder’s net zero envelope course by Xerox Builders · October 2026

Insulation outside the sheathing cuts thermal bridging and keeps the sheathing and air barrier warm and protected. This Detail shows how to hang cladding over it with wood strapping and long screws, how to choose the board, and how to keep the layer continuous.

Why insulate outside the frame

A continuous layer of insulation outside the sheathing covers studs, plates and headers, so far less heat bypasses the cavity insulation. It keeps the sheathing warmer and drier, and it lets the air barrier sit on the sheathing, behind the insulation, where trades can’t damage it. The cost is that the cladding now has to hang through the insulation, and every opening and penetration gets deeper. (For why warm sheathing matters, see the Building Science Deep Dive.)

The wall, from inside out

1. Drywall (and interior vapour control, as designed)
2. Studs with cavity insulation
3. Sheathing with the air and water barrier membrane on its face
4. Exterior insulation boards
5. Vertical strapping, screwed through the insulation into the studs
6. Cladding

The spaces between the straps form the drained and vented rainscreen cavity that Part 9 requires behind cladding in wet coastal climates like ours (confirm for your location). Where furring is applied over sheathing, Part 9 (Article 9.27.5.3) asks for at least 19 × 38 mm lumber, securely fastened to the framing and spaced no more than 600 mm apart (confirm the current edition). Over thick exterior insulation, take the strapping size, screw size and spacing from the engineer or the insulation or cladding maker; cladding makers often want closer spacing.

Choosing the board

Vapour-open boards let the wall dry outward. A low-permeance board outside the sheathing must be thick enough to keep the sheathing warm, or moisture can collect behind it. Have your designer or envelope consultant confirm the ratio of outside to inside insulation for your wall.

How long screws carry the cladding

Strapping screwed through the insulation into the studs works like a small truss: the screw holds in tension and the insulation board resists in compression. In RDH’s 2017 laboratory tests, with high-density (128 kg/m³) mineral wool from 76 mm to 305 mm thick (and 152 mm of XPS) and 8 mm screws fully embedded in framing, the strapping moved less than 1 mm (about 0.35 to 0.66 mm) at a load of about 9 kg (25 lb) per screw, which is small next to normal wood shrinkage. Fixing into 19 mm plywood sheathing behaved much like fixing into framing; 13 mm plywood did not. Heavy claddings such as stone veneer and stucco need an engineered attachment design.

Attachment choice and R-value

Every metal path through the insulation leaks heat. An RDH comparison of attachment systems (mostly for commercial walls) gave these approximate ranges for how much of the exterior insulation’s value is kept:

For wood-frame houses with light to medium cladding, wood strapping and long screws is the usual and best-performing choice.

Fastening on site

• Snap stud lines on the membrane before the insulation goes on.
• Choose screws long enough to pass through strapping and insulation with a solid bite into the stud.
• Take spacing from the engineer or the insulation or cladding maker.
• Use at least two screws per strap; re-drive any that miss framing.
• Drive snug, not crushing, so the cladding plane stays flat.

Keeping it continuous

Use two layers with staggered joints where you can, butt boards tight, fill any gaps, and lap the layers at outside corners. Carry the insulation past the floor lines and rim joists. Screen the cavity at the bottom and top against insects while keeping it open to drain and vent.

What usually goes wrong

• Screws that miss the studs. No stud lines were marked, so the crew guessed. Each miss leaves the strap relying on its neighbours.
• Crushed boards. Over-driven screws dish the strapping into soft insulation and the cladding goes wavy.
• Gaps at corners and floor lines. Short boards and unfilled joints leave cold bands that show up on a thermal camera.
• Blocked cavities. Insulation offcuts, mortar or horizontal blocking dam the cavity so water can’t drain.
• Late fire questions. Foam is ordered before anyone checks the limiting distance to the property line.

Site checklist

• Air barrier inspected or tested before insulation covers it
• Stud lines marked on the membrane
• Boards tight, joints staggered, gaps filled
• Insulation carried past floor lines and rim joists
• Strapping straight, plumb and on the studs
• Screw length and spacing as specified; misses re-driven
• Cavity clear, with insect screens at top and bottom
• Openings bucked and flashed out to the cladding face
• Fire limits near property lines confirmed
• Every elevation photographed before cladding

Test yourself

Try to answer each question, then click it to check.

Next: Detail 11, Double-Stud & Deep Walls. See also our Building Envelope & Rainscreen Systems Field Guide.