Built Right Detail 08: Where the Window Sits in the Wall

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

In a wall with exterior insulation, the window can sit at the sheathing, in the insulation or at the outer face. This Detail follows on from Detail 07 and shows how each position changes heat loss at the frame edge, how the window is supported, and how the opening is flashed.

Thick walls give you a choice

Once a wall has exterior insulation, the window no longer has one obvious place to go. It can sit at the sheathing, out in the insulation layer, or at the outer face of the insulation. The choice affects heat loss around the frame, condensation risk, how the window is supported, how it is flashed, how easy it is to replace, and how the opening looks inside and out. Make the decision at design stage, before framing and before the windows are ordered, because it can change the window type, the rough opening and the trim.

The three positions

A buck is a plywood or lumber box, or a manufactured frame, that extends the rough opening out to where the window sits.

Heat loss at the edge: the installation psi-value

A window’s real heat loss in the wall is more than its label suggests. Energy modellers combine four parts: the glass, the frame, the glass edge (spacer) and the installation joint. The installation psi-value (psi-install) is the extra heat that flows through the joint between window and wall, per metre of window perimeter, in W/(m·K). It is calculated with two-dimensional heat-flow software for the actual position and insulation around the frame.

• A frame that sits outside the insulation lets heat flow around its edge, so psi-install is high.
• A frame lined up with the insulation layer, with insulation overlapping the outer frame, loses much less. The insulation should not cover the sash or the glass.
• In a split-insulation wall the sheathing sits between the cavity insulation and the exterior insulation, so an innie can perform well if the exterior insulation returns tight to the frame.

A Passive House consultant writing in the Journal of Light Construction found that, for the windows on his project, setting them in the middle of the wall’s insulation gave the best result, with outside placement next and inside placement worst. The best spot depends on the frame, so ask your energy modeller or window maker for the psi-value at the position you plan to use.

Carry the weight

Middle and outie windows sit out in the insulation, which cannot carry load. Fix plywood box bucks, lumber bucks or engineered brackets back to the framing, and set the window on them. Triple-glazed units are heavy, so for large or heavy windows follow the window maker’s support details or have an engineer design the support. Insulate around the buck and seal it into the air barrier so it does not become a cold, leaky frame of its own.

Flashing at each position

The rough-opening sequence from Detail 07 still applies at every position: sill pan first, shingle laps, sill flange left free to drain and an air seal all the way round. What changes is where the water barrier runs and how far the flashings must reach.

• Innie: the window is sheltered in a deep recess. Line the reveal with membrane that laps down to the sill. The head flashing must span the insulation and the rainscreen cavity and reach past the cladding with end dams and a drip. A sloped sill extension carries water out over the insulation with a drip.
• Middle: the most three-dimensional flashing. Wrap the buck in membrane that laps onto the water-resistive barrier (WRB) so the sill pan drains out over it.
• Outie: the flashing is simpler, but the head is more exposed to weather and must lap correctly into the WRB.

Builders’ guides on thick walls agree that the WRB location largely decides which position works best. Under Part 9 of the BC Building Code, flashings over openings must extend at least 50 mm up behind the sheathing membrane, slope to the outside, end in end dams and finish with a drip. A 2025 BC Building Code Appeal Board decision upheld the 50 mm lap behind the membrane over a taped-on alternative, so talk to your building official early if your detail differs.

Comparing the positions

Confirm your chosen detail with the window maker, your designer and your envelope consultant.

Site checklist

• Window position shown on the drawings before framing
• Window maker’s installation details for that position on site
• Psi-install value from the energy modeller for the chosen position
• Bucks or brackets fixed back to the structure; nothing hung on foam
• Support for heavy units confirmed by the maker or an engineer
• Insulation tight to and overlapping the frame, clear of sash and glass
• Head flashing at least 50 mm up behind the WRB, with end dams and drip
• Buck or reveal wrapped in membrane lapping onto the WRB
• First opening mocked up, reviewed and hose-tested

Test yourself

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

Next: Detail 09, Doors, Sliders & Thresholds. See also our Windows, Doors & Water Management Field Guide.