Where an Internal Wall Meets an External One

An air barrier is not a product but a line: the connected set of surfaces nominated to resist bulk air movement, together with everything joining those surfaces. On a section it should be possible to trace it around the enclosure without lifting the pen, and the same should hold on plan.

An internal wall arriving at that line is exactly the kind of interruption the line is vulnerable to. Where the detail does not say whether the layer runs past or is joined to the partition, the line simply stops there, and the enclosure behaves as though the barrier ended even though both planes are individually complete.

A framed partition's cavity is continuous along and over the wall unless something deliberately interrupts it. An external wall's cavity is a designed space, drained at its base and trayed at interruptions. Connecting the two creates a path that neither drawing shows.

In masonry, the equivalent is a partition bonded into the inner leaf, which makes the interruption of the external wall's layers permanent. Whether the partition is bonded, tied or stopped is settled by the envelope designer rather than at the abutment.

The same continuity that makes a cavity useful for drainage makes it a path through a building, which is why barriers subdivide concealed spaces at defined positions. A barrier only interrupts what it is continuous and tight against on both sides, and one with a gap around it has interrupted nothing.

Barriers are installed as the cavity is built and are concealed immediately, so whether they are continuous is very hard to verify later. What barriers were installed, and where, is worth recording because the answer is not observable afterwards.

Where a lining or a retrofit insulation layer is applied internally, an intersecting partition built into the external wall carries the outside condition inward past the insulation. The room-side surface at that junction stays close to the temperature of an untreated wall.

Carrying insulation along those returns lengthens the path but never removes it, and each return consumes reveal depth or room dimension that may not be available. Those junctions are commonly the coldest interior surfaces after the work and often where condensation first appears.

Where the external wall has already been lined, a partition meeting it lands on the lining rather than on the structure, unless the lining is stopped and the partition taken through to the wall behind. Which was intended affects the fixing, the junction, and whether the lining's void runs on past the partition.

A lining void open at floor and ceiling connects into the floor and ceiling zones, so the question is not only what happens in plan at the abutment but what the void does above and below it.

The practical move is to put the abutment on one drawing and ask each party the same question: does your layer run past this partition, stop at it, or join it. The answers rarely conflict once they are written on the same sheet, and they very often conflict when they are not.

Where the partition also runs into a separating construction, the decision about whether it is stopped, broken or taken through belongs with the separating construction's designer rather than with the partition's.