Where a Window Sits in the Wall Depth

Anchorage is the first. A frame set further out is further from the structure it is fixed back to, so the fixings lengthen and the load sits further from its support. The record asks whether the anchorage of the units has been assessed for the loads they will carry in that location, which is a structural question rather than a fitting one.

The other three follow from the same move. The external reveal deepens or shallows, changing how far water runs along a surface before it reaches the frame. The thermal line either can or cannot be carried onto the frame, which the bridging record describes as producing a different junction from the same window in the same opening. And the perimeter gap, which is where tolerance, seal movement and insulation all live, changes shape.

Here the opening cuts the void on every side at once, and the frame is normally fixed to the inner leaf. The cavity wall record describes the head as needing interception with outlets, the jambs as needing a closer that carries no water across, and the sill as having to throw water clear of the leaf below. The seal between frame and closer is also where the wall's air line passes.

The position of the unit therefore decides how much of the reveal is closer and how much is masonry, and what the air seal has to bond to. The record also warns that the closer sits where stopping air, shortening the thermal path and not reconnecting the leaves all meet, which is why reveals are among the first places internal staining appears.

In a framed wall the opening is trimmed rather than spanned, the sheathing membrane is the drainage plane, and the perimeter is formed before any cladding arrives. The record is explicit that the order in which membrane, pan and frame are installed is fixed by the build-up itself rather than chosen on the day.

That makes the unit's position partly a sequencing decision. The wall's drainage plane has to be lapped around the reveal and terminated at the frame in the correct order, upper over lower, and the record names this as the sequence most often reversed on site because the frame arrives after the plane it has to lap onto. Where the unit sits decides how much membrane has to be dressed into the reveal and returned.

The steel record identifies a condition peculiar to a conducting frame. Openings are trimmed with additional sections that concentrate metal around the perimeter, which is the coldest part of the frame, and the window is fixed into that zone. Frame anchorage, the thermal detail and the seal to the air and vapour control layer therefore become one problem rather than three.

The bridging record asks the complementary question for any wall: where does each frame sit in the depth, and can the insulation be carried onto the frame there? Where a concentration of metal already runs around the opening, the answer to that question carries more weight than it does elsewhere in the same wall.

Here the decision is made earliest of all and is hardest to revisit. The record states that openings arrive already cut, so the reveal is a timber face and the barrier's returns at head, jamb and sill are tape lines on that face. Where the frame sits relative to the insulation decides whether the reveal is inside or outside the thermal line, and that position is fixed long before the window arrives.

The insulation on this wall sits entirely outboard of the panel, so a frame set inboard leaves reveal depth outside it and a frame set outboard has to be carried on something crossing the thermal layer. Either way the choice is made at the point the panels are drawn rather than when the units are ordered.

On an existing wall wrapped from outside, the wall gets thicker and the unit usually does not move. The render record describes the consequence: the frame ends up recessed and the reveal has to be insulated, reinforced and finished around it, and where the render meets the frame there is a joint between a rigid unit and a coat applied over a compressible board that has to be able to move.

The record also names reveals as the hardest place to maintain continuity in that build-up, and notes that beads, stop profiles and the render itself then sit between any future inspection and the seal. This is the one case in the family where deciding not to move the unit is itself the decision, and where its consequences fall on the reveal rather than on the frame.

Whichever position is chosen, the record insists on one thing being uniform. The perimeter is either designed to be closed at the outer face or drained back to the outside, and the two strategies cannot be mixed by accident. Sealing the outside continuously while leaving gaps inboard turns the perimeter into a vessel, because anything that does get past has no route out and no way of drying.

The three edges are also one water path rather than three details. What the head diverts runs down the jambs, and what the jambs collect arrives at the sill, so if the jamb membranes stop short of the pan instead of lapping into it, the water the jambs gathered is delivered into the wall at precisely the place the pan was formed to protect. The pan then looks blameless because it is dry.

Earlier than most people expect, and earlier in some walls than in others. In a mass timber wall the reveal exists before the window is ordered. In a masonry wall the closer and the interception above the head are built into the masonry as it rises. In an externally insulated facade the reveal detail governs how much insulation can be returned, which is a survey-stage question about how much depth the existing reveal offers.

The published questions for professionals put it as two: where does the unit sit in the wall depth, and what drove that decision on this project? The second is the more useful of the two, because it reveals whether the position was chosen for the elevation, for the thermal line, for the anchorage or by default.