The envelope — envelope.gap

rule	level
envelope.gap	caution

Between two spaces that touch, a wall is derived whether or not you write one. Forget it and the wall still stands. But a boundary to the outside is never derived — no default boundary is drawn against a space with no region, because naming the other side is itself information.

The consequence of that asymmetry is that a forgotten boundary to the outside becomes a silently missing wall. You only notice by looking at the drawing. This rule puts it into words.

envelope.gap — part of the outline faces nothing

caution

koyu 1.0
grid X 0 4000 8000
grid Y 0 5000
level L1 0 h:2700 slab:150
space /out exterior
space /L1/a room X1..X2 Y1..Y2
space /L1/b room X2..X3 Y1..Y2
boundary /L1/a /out edge:W t:200
  door w:900
boundary /L1/a /L1/b
  door w:900
boundary /L1/b /out t:150

⚠ [envelope.gap] main.muro:line 6: Perimeter not faced by any envelope: /L1/a — S 4000mm / N 4000mm (8000mm over 2 run(s)). Write a boundary to the exterior
Validation — 0 violations / 1 caution

/L1/a has four sides. The east side meets /L1/b, so it has a counterpart. One boundary was written on the west. The north and the south face neither another space nor a declared boundary. No wall stands there.

The message names the sides. A total length alone would not tell you which edge to fix in a drawing that writes its edges separately. Runs covered by a boundary that does not name an edge come out grouped as N/S or E/W.

What is checked

A stretch of the outline is a hole unless it is one of these:

1. facing another space on the same level, or
2. covered by a boundary declared for that space — type:open and air:1 both cover it, and all of them differ from writing nothing.

Boundaries that join levels (type:stair, type:shaft, type:void) do not cover a horizontal outline, so they do not count as cover.

What is checked is "finish what you started", not completeness

A level with no boundary to the outside at all says nothing. That level has simply not modelled its envelope yet, and nagging a two-room example would be pointless.

The test for "has started its envelope" is concrete: somewhere on that level, a boundary is declared against a space with no region — the kind of exterior written as space /out exterior, with no extent. Which means that a model whose outside is written with a region, tiling the site as space /out/n exterior X1..X3 Y2..Y3, has boundaries between two spaces that both have regions, and its levels are never taken to have started an envelope.

Some spaces are never reported:

• spaces with no region (the outside itself)
• type:exterior spaces
• semi-outdoor spaces (derived from meeting an exterior across type:open or air:1)
• the site tiles under a zone carrying site:1

Not being enclosed is normal for all of them.

Fix

Write boundaries for the remaining edges. Pick them with edge:N/E/S/W, or catch the whole remainder with one boundary that names no edge.

koyu 1.0
grid X 0 4000 8000
grid Y 0 5000
level L1 0 h:2700 slab:150
space /out exterior
space /L1/a room X1..X2 Y1..Y2
space /L1/b room X2..X3 Y1..Y2
boundary /L1/a /out edge:W t:200
  door w:900
boundary /L1/a /out edge:N t:200
boundary /L1/a /out edge:S t:200
boundary /L1/a /L1/b
  door w:900
boundary /L1/b /out t:150

✔ Nothing caught by validation (this is a judgement, not a guarantee about the composition)

If an edge is genuinely open, write type:open; if it is a railing, write air:1. The hole closes not because you wrote a wall, but because you named the other side.

See also

• Reachability — close the envelope and the next problem is getting out. Doors live there
• The validation ledger — all fifteen rules, and why Finding is not Diagnostic