Vapour Barrier or VCL? Vapour Control Layers Explained

A vapour control layer (VCL) is a layer on the warm side of the insulation that limits how much water vapour passes from inside a building into the structure. It is what used to be called a vapour barrier. The name changed because the thinking changed: the aim is to control vapour, not to stop it dead.

This guide covers what the initials mean, where the layer goes, how a VCL differs from an air barrier, and what goes wrong when a build-up gets it wrong.

What does VCL stand for?

VCL stands for vapour control layer. You will also see AVCL — air and vapour control layer — used where a single product is doing both jobs at once.

Vapour barrier or vapour control layer — what is the difference?

On site the two terms get swapped freely and usually mean the same layer. The distinction underneath is real, though, and it decides what you should actually specify.

• A vapour barrier aims to stop vapour: very high resistance, effectively closed.
• A vapour control layer aims to regulate it: enough resistance to keep moisture out of the cold part of the construction, while still letting the build-up dry.

Why that matters: a wall that cannot dry in either direction is a wall that traps whatever moisture does get in — and some always does, through a service penetration, a missed junction, or the fabric itself. Modern practice leans towards control rather than a sealed barrier, particularly in retrofit, where the existing structure needs to keep drying inwards.

Vapour resistance is measured as an sd value in metres — the thickness of still air that would offer the same resistance. A higher sd means a more closed layer. Some membranes are intelligent, or humidity-variable: more closed in winter when the risk is vapour driving into the structure, more open in summer so the build-up can dry back. The Passive Purple family is tested at an sd of roughly 0.8–40 m to EN ISO 12572 depending on conditions, which is what “intelligent” looks like in test data.

Why a vapour control layer matters

Warm indoor air holds moisture. If that air reaches the colder parts of a construction, the moisture condenses inside the build-up where nobody can see it. That is interstitial condensation, and over time it produces mould growth, insulation that no longer performs as specified, timber decay, and corrosion of fixings and metalwork.

A VCL reduces the amount of moisture that gets in there in the first place.

How moisture actually gets into a structure

There are two routes, and they are not equal.

Air movement — by far the bigger one

Air leaking through gaps, cracks and unsealed junctions carries a great deal of moisture with it. This is the primary cause of condensation problems in practice, and it is why airtightness and vapour control are so often discussed together.

Vapour diffusion — the slower one

Water vapour also passes through materials themselves, driven by the difference in vapour pressure between inside and out. It is a slower process, and it is the one the sd value describes.

The practical consequence: a perfectly specified VCL with a hole in it around a soil pipe is not doing its job, because the air route dwarfs the diffusion route.

VCL or air barrier — are they the same thing?

No, though one product can be both.

• An air barrier stops air moving through the envelope. It is tested by pressurising the building — the ACH or air permeability figure.
• A vapour control layer limits vapour diffusing through materials. It is described by its sd value.

A liquid-applied membrane such as Passive Purple performs both roles in one coat — it is Passivhaus certified class A with an air permeability of 0.02 (±0.002) m³/(hm²) to EN 12114, and it carries the intelligent sd range above. But the two properties are still tested separately, and a product that is good at one is not automatically good at the other. Check both.

Where does a vapour control layer go?

The general rule is on the warm side of the insulation — the inside face in a UK heated building — within walls, roofs and floors, and continuous across the whole envelope.

Two qualifications matter more than the rule:

1. The build-up decides, not the rule. Warm roofs, cold roofs, internal wall insulation and external wall insulation all put the layer in different places. On a retrofit especially, the position should come out of the moisture design for that wall, not a default.
2. Continuity is the whole game. A VCL that stops at the skirting, at the joist zone, or at the edge of a service penetration is not a layer. This is where sheet membranes tend to lose: every lap, staple and pipe is a detail somebody has to get right by hand.

What a liquid-applied VCL changes

A liquid membrane is sprayed or rolled onto the substrate — blockwork, concrete, OSB, plasterboard, timber — and cures into a continuous film. There are no laps to tape, no staples to seal, and complex detailing is covered as the coating passes over it. Reveals, joist ends, wall-to-ceiling junctions and penetrations are detailed first in a brush grade, then the field is coated, and the two cure as one material.

You can see what a finished layer looks like across a whole room in our 360° site tours.

Common mistakes

• Relying on the plasterboard lining. Board on dabs sits clear of the wall, and that void runs behind sockets, past skirtings and into the ceiling. It looks sealed and behaves like a plenum — see why blockwork is not an air barrier.
• Sealing a penetration before the service is installed. The following trade cuts it open. Detail it so the cable or pipe comes through the seal — the key-shaped liquid grommet is one way.
• Specifying a fully closed barrier on a retrofit that still needs to dry inwards.
• Treating the VCL as the last job. It has to be in place, and inspected, before anything covers it.

Frequently asked questions

Is a vapour barrier the same as a vapour control layer?

In everyday use, yes — most people mean the same layer. Strictly, a vapour barrier aims to stop vapour and a vapour control layer aims to regulate it while allowing the build-up to dry. Current practice favours control.

Which side of the insulation does a VCL go?

The warm side — the internal face in a UK heated building. The exact position depends on the wall or roof build-up and should follow the moisture design for that construction.

Do I still need a VCL if the building is airtight?

Airtightness deals with air-carried moisture, which is the larger route, but not with diffusion through materials. Most build-ups want both, which is why combined air and vapour control layers are common.

Can a vapour control layer be painted on?

Yes. A liquid-applied membrane is sprayed or rolled on and cures to a continuous film that covers junctions and penetrations without laps or tapes.

What is an intelligent VCL?

One whose vapour resistance changes with humidity — more closed when vapour is driving into the structure, more open when the build-up needs to dry back.

Getting the junctions right

A vapour control layer is only as good as its connections. At frames, board joints and changes of material, the layer has to connect to whatever comes next. Our airtightness tape guide covers the selection checks and the window and door tape guide covers preparation and inspection. IM Fleece Tape suits those junctions where the surfaces and the moisture design allow.

Ask our technical team to review your wall or roof build-up and the proposed junctions.