Design Window Perimeter

A window perimeter is not three details. It is one sequence, built from the bottom up by trades arriving in an order, and every lap either sheds water or catches it depending on which one went in first. Drawn as three separate sections, it is usually three details that individually work and together do not.

Workflow

Step 1. Take the wall, the window, and its position in the depth

Ask, in one message:

1. The wall build-up, inside to outside, with the four control layers located.
2. The window type and frame, and how it is anchored to what.
3. Where in the wall depth does the window sit: aligned with the structure, with the insulation, or with the cladding face? This decides most of the rest.
4. Is the opening punched, part of a ribbon, or a unitised system? A punched opening in a drained wall and a unitised curtain wall are different problems and only the first is what this skill is shaped for.
5. Is there an exterior sill, a projecting head, a recessed reveal, a shading element?
6. What is the sequence of trades: does the window arrive before or after the weather barrier is on and the cladding is up?
7. What is already fixed: a window schedule, a facade module, a head height, a reveal depth already drawn?

Question 6 is not a construction question. If the window arrives after the weather barrier, the laps run one way; if before, they run the other. That is a design decision and it belongs on the drawing.

Step 2. Locate each control layer's transfer at the opening

One row per control layer, all three sides at once, because the answer has to be consistent around the opening:

Control layer	Plane in the wall	Where it meets the window	Head	Jamb	Sill

• Water. The wall's drainage plane has to be brought to the window and terminated on it, not near it. Name the physical component at each of the three sides.
• Air. The air seal from the wall's air barrier to the window frame, continuous around all four sides including under the sill. Name the material and say whether it is accessible after the window is in.
• Vapor. Whether the perimeter seal is vapor-tight, vapor-open, or different on the inside and the outside, and whether that matches the wall's drying strategy.
• Thermal. Where insulation meets the frame, and whether the frame's thermal break lines up with the wall's insulation plane. If they do not line up, say plainly that there is a thermal bridge around the full perimeter of every opening and that it has not been quantified.

Alignment between the frame's thermal break and the wall's insulation plane is the single decision that most often gets made by the window schedule rather than by anyone looking at a section.

Step 3. Build the sequence, sill first, and read the gaps

Not three details. One numbered sequence of who does what, in order:

#	Trade / component	What it does	Laps over	Value needed from you

Start at the sill, because everything above laps onto it.

• Sill. The pan and what closes its ends. Which way it drains and where it discharges to. Whether the pan is continuous or made in pieces, and what happens at the joints if it is.
• Jambs. How the jamb flashing laps into the sill pan, and how the wall's drainage plane laps over the jamb flashing.
• Head. How the head flashing laps over the window and under the wall's drainage plane, and where it drains out at each end.

Then read between the rows:

• Does every lap run so that water on the outside of one layer stays on the outside of the next one down?
• Is any lap dependent on a sealant bead rather than on geometry?
• Does the sequence require a trade to come back after another has finished, and is that realistic?
• Is the air seal reachable at the moment the sequence asks for it?

Step 4. Work the corners, ends, and what interrupts the opening

• The four corners of the opening, where two flashings meet and neither is continuous.
• The ends of the sill pan and the ends of the head flashing, and where each discharges.
• Any mullion, transom, or coupled unit inside the opening.
• Where the opening meets something else: a spandrel below, another opening beside it, a corner of the building, a floor line.
• Any projecting sill, canopy, or shading element that penetrates a control layer.
• Interior finishes returning into the reveal, and whether they conceal the air seal.

Step 5. Name where water that gets past the window goes

Some water always gets past. State plainly:

• The path water takes from the window frame's own drainage to the outside.
• Whether that path passes through the wall's cavity, and if so what it lands on on the way.
• Whether the sill pan discharges to the outside face or into the cavity, and what receives it there.
• What happens if the discharge point blocks: where the water then goes and whether anyone would see it.

An opening whose drainage terminates inside a cavity with no named receiver is not detailed, however many sections have been drawn.

Step 6. Report, and hand off

The control layer table, the sequence table with its between-row notes, the corners and interruptions, then the drainage path. Close with the hand-offs, every time:

• The perimeter thermal bridge has not been quantified, and it runs around every opening in the building.
• Anchor and structural design is open.
• Air barrier continuity around the building belongs to air-barrier-trace, and openings are the junction it fails at most.
• The window's own tested performance and the manufacturer's required installation are open and have not been read.

Count the open values on the face of the output. Then ask whether to write the detail to a file and where, and write it directly, tables included.

Rules

• Never supply a number. Not a shim space, not a pan slope, not an end dam height, not a joint width, not as a placeholder.
• Never recall a code or a manufacturer's installation requirement. Both come from documents you quote.
• Refuse to proceed without the wall assembly. The perimeter is a junction.
• Work the sill first, always. Sequence is the design.
• State the trade sequence explicitly and put it on the drawing. It is not the contractor's business alone.
• Name the alignment between the frame's thermal break and the wall's insulation plane, every run.
• Name where water past the window goes, every run.

Anti-patterns

• Drawing head, jamb and sill as three independent sections. They are one sequence and the laps only work in one order.
• Detailing the sill without a pan, or with a pan whose ends are open.
• Relying on a sealant bead where geometry should be shedding the water.
• Assuming the window frame is watertight and detailing no secondary path.
• Recalling a required pan slope or end dam height because the figure feels standard.
• Quoting a manufacturer's installation instruction found by web search.
• Locating the window in the wall depth by elevation appearance alone, without saying what it does to the thermal break alignment.
• Calling the opening resolved with no named receiver for the sill pan discharge.
• Running air-barrier-trace from inside this skill. It is a hand-off.

Resources

None. This skill is one file. Output is written directly at the path you choose.