Built Right · Detail 07

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

Window openings are where many walls fail. This Detail walks through a flanged window in a wood-frame wall with a rainscreen, step by step: preparing the opening, the sill pan, setting the window, lapping the flashings and sealing the air gap. Every opening has two jobs: drain water out at the bottom and seal air all the way around.

The golden rule: shingle lapping

Every upper layer laps over the layer below it, like shingles on a roof, so water running down the wall always flows out and never behind. A reverse lap catches water and feeds it into the wall.

Step by step

  1. Check the opening. Size, square and level, with the shim gap the window maker asks for.
  2. Slope or dam the sill. A sloped sill or a back dam at the inside edge stops water reaching the interior. Cut the water-resistive barrier so the sides fold in, leaving a flap at the head.
  3. Build the sill pan. A flexible membrane across the sill, turned up the jambs about 150 mm (or as specified) and lapped out over the water-resistive barrier below. Seal the corners completely.
  4. Set the window. Shim under the jambs and mullions, level and plumb, and fasten as specified. Seal behind the head and jamb flanges, but leave the sill flange unsealed, or with drainage gaps, so water in the pan can drain out.
  5. Lap the flashings in order. Jamb tape over the side flanges, running up past the head; a sloped metal head flashing over the head flange, with a drip and end dams; then the water-resistive barrier flap folded down over the flashing, with the cuts taped.
  6. Seal the air gap. On the inside, join the window frame to the wall’s air barrier with backer rod and sealant, airtight tape or a low-expansion window foam, continuous on all four sides. On the outside, tape goes over the flanges.

With exterior insulation and a rainscreen

  • Plywood or engineered window bucks often carry the window out into the insulation layer.
  • The sill pan still laps onto the water-resistive barrier, so water drains into the rainscreen cavity.
  • The metal head flashing sits on the strapping, spans the cavity and projects past the cladding with a drip.
  • Window position also changes heat loss; that’s Detail 08.
Follow the instructions: the window maker’s installation guide and your envelope design come first, and can override general advice like this.

Site checklist

  • Rough opening size, square and level checked
  • Sloped sill or back dam in place
  • Sill pan up the jambs and lapped over the barrier below
  • Corners sealed, no pinholes or wrinkles
  • Head and jamb flanges sealed; sill flange left open to drain
  • Jamb tape, head flashing with drip and end dams, barrier flap lapped in order
  • Interior air seal complete on all four sides
  • Sample window hose-tested before cladding
  • Each opening photographed

Test yourself

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

Why is the window’s bottom flange left unsealed?

So any water that gets into the sill pan can drain back out instead of being trapped in the wall.

What is the shingle lapping rule?

Every upper layer laps over the layer below it, so water always runs out and never behind.

Where does the window air seal go?

Between the window frame and the wall’s air barrier, continuous on all four sides, usually on the inside.

Next: Detail 08, Where the Window Sits in the Wall. See also our Windows, Doors & Water Management Field Guide.

Please note: This page is general information, not legal, financial or professional advice. The facts, figures, bylaws, fees and code requirements described here were accurate to the best of our knowledge when it was published or last updated. These rules change often and differ from city to city, so some details may have changed since. Before you make decisions, please confirm the current requirements with your city’s planning and building department and with qualified professionals. We’re always happy to help you check what applies to your property.

Planning a high-performance home in Vancouver, Burnaby or Surrey? Talk to our team about getting the envelope right from day one.

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Sources: BC rainscreen and window installation guide (PDF); Illustrated Guide: Achieving Airtight Buildings; window maker installation guides.