How to Terminate a Cavity Drain Membrane -- and Why Many Details Get It Wrong

Direct answer

A cavity drain membrane creates a controlled air gap between the external structure and the internal lining. If that air gap is left open — at the wall head, at window reveals, at service penetrations, or at the junction between the membrane and any above-membrane treatment — humid air from the wet cavity side can migrate into the internal space, contact a cooler surface, and deposit moisture as condensation. The result is damp staining or mould above the membrane line, with no obvious connection to the drainage system below. The termination must close the vapour pathway, not just the water pathway — and this is a design and specification requirement, not a site-level judgment.

Full explanation

What a cavity drain membrane system does

A cavity drain membrane (dimple membrane, studded drain mat) creates a drainage cavity between the external structure and the internal lining. Water that passes through the structure — whether through cracks, construction joints, or the pore network of the masonry or concrete — enters the cavity rather than penetrating the finishes. The cavity directs water down to a drainage channel and from there to a sump and pump.

This is an effective strategy for managing water in an existing basement where external waterproofing is impractical, or where the structure has been in service for many years and its integrity is uncertain. It is also used in new build where a Type C (drained protection) strategy is the design intent.

What the membrane does not do — and is not intended to do — is eliminate moisture from the cavity side. The wall or slab behind the membrane is wet. The air in the cavity is humid, particularly during and after periods of rainfall or groundwater rise.

The vapour pathway

This is where an unterminated or poorly terminated cavity creates a specific problem.

Humid air is not still. If the cavity air gap is open at the top — if the membrane terminates at the wall head without a sealed closure, or if there is a gap at a window reveal or service penetration — humid air from the cavity will migrate through that opening into the occupied space above. It will carry moisture vapour with it. When that moist air reaches a cooler surface — plasterboard, a ceiling soffit, a window reveal, a timber stud — it deposits moisture as condensation.

The symptom is damp staining or mould appearing at or above the level where the membrane terminates. The staining will not correlate with rainfall in the way that direct water ingress would, because the mechanism is vapour migration rather than liquid water. Moisture investigation will typically show elevated moisture readings in the plasterboard or finishes at and above the termination level, while the membrane and drainage system below appear to be functioning normally.

If this symptom is not recognised as a termination problem, the investigation may misattribute the damp to: a defect in the membrane itself (which is dry); a services leak (where none exists); or internal condensation unrelated to the waterproofing (which is incorrect in terms of origin, even if the mechanism is technically condensation).

This is not a theoretical failure mode. It occurs on installations where the cavity termination was not specified, not detailed, or not installed correctly — and it is among the more commonly misdiagnosed damp problems in refurbished basement and below-grade spaces.

What a correct termination requires

The cavity air gap must be closed at every discontinuity — every point where the membrane approaches a material transition, an opening, or a change in system:

At the wall head — where the membrane terminates at the floor soffit or at the ceiling line, the cavity must be closed with a solid closure strip, sealed fillet, or the membrane dressed tightly against the structure with a compatible sealant. Air movement from the cavity side to the internal space above the closure must be prevented.

At window and door reveals — where openings penetrate the wall, the membrane termination must return into the reveal and seal against the window or door frame. A membrane that simply terminates at the edge of the wall, leaving the cavity open into the reveal, creates a direct vapour pathway through the reveal cavity to the internal space.

At pipe and duct penetrations — services that pass through the wall within the membrane zone must have sealed collars that prevent air and vapour movement from the cavity into the internal space around the penetration. Penetrations are a common location for this failure because they are numerous, often installed by a different trade, and frequently omitted from the membrane specification.

At upstands and level changes — where the floor construction changes level, the membrane termination at the upstand must be sealed against the upstand face. The junction between the membrane and an upstand is a standard detail on any cavity drain specification and should be shown on the drawings, not left to site judgment.

At the junction with rising-damp treatment or above-membrane finishes — where the cavity drain terminates mid-height and a different treatment is applied above (rising damp treatment, tanking coat, plain plasterboard), the transition must seal the cavity against the back of the above-membrane material. A gap at this transition allows vapour from the cavity to travel up behind the material above and deposit as damp at a height well above where the membrane terminates.

Materials and specification

Termination materials must be:

• Compatible with the cavity drain membrane — typically the manufacturer’s own termination strip, batten, or closure profile.
• Specified by the manufacturer for the specific application (wall head, reveal, penetration).
• Applied to a prepared substrate — the substrate should be clean, sound, and primed in accordance with the manufacturer’s instructions before the termination material is applied.

These requirements should be in the specification. “Terminate membrane as required” is not a specification — it is an instruction to the installer to make a judgment that requires the designer’s input.

Why open terminations are common

The most common reason for inadequate cavity termination is that it is not shown on the design drawings and not described in the specification. Many cavity drain specifications concentrate on the membrane system, the drainage channel, the sump layout, and the pump — and describe the installation requirements for each. The termination at the wall head, at reveals, and at penetrations is assumed to be “standard practice” and is not specified.

This assumption is incorrect. Different installers have different practices, and in competitive tender conditions the lowest-cost installer is unlikely to include termination details that were not explicitly required. The result is a system installed correctly at its primary function (managing liquid water) and incorrectly or incompletely at its secondary function (preventing vapour migration above the installation).

The cavity termination should be specified explicitly, shown on design drawings at every relevant discontinuity, and included in the construction monitoring programme as a hold point before the internal lining is installed. Verifying that the termination has been correctly installed is straightforward if it is inspected before it is covered. It is much more difficult to establish after the lining is in place — and investigation to confirm the termination is the cause of observed damp requires removing finishes.