Exploded wall diagram shows plasterboard, service void, purple VCL, insulated timber frame, sheathing, breather membrane, cavity and brickwork.

Where does the vapour control layer go in the wall build-up? Layer by layer (UK)

If you already know the vapour control layer belongs on the warm side, the next practical question is: where exactly does it sit relative to every other layer? This post is a quick layer-by-layer reference for the most common UK build-ups — each listed from the inside (warm) out to the outside (cold), with the VCL's position in bold.

For the reasoning behind why the VCL goes where it does, see our companion guide: which side of the insulation does a vapour control layer go? This post focuses on the running order.

The rule of thumb: plasterboard → service void → VCL → insulation → breather membrane → outer finish. The VCL sits behind the service void and in front of the insulation.

Timber frame wall (warm side insulation)

From inside to outside:

  1. Internal finish / plasterboard
  2. Service void (battens, 25–50mm) — for cables and sockets
  3. Vapour control layer (sheet VCL or liquid airtight vapour control paint)
  4. Structural timber frame + insulation between studs
  5. Sheathing board
  6. Breather membrane
  7. Ventilated cavity / battens
  8. External cladding or brick

Keeping the VCL behind a service void means screws and cables don't puncture it — follow the two-thirds / one-third rule (no more than a third of the thermal resistance on the warm side of the VCL).

Insulation diagram places the VCL between no more than one-third of thermal resistance on the warm side and at least two-thirds on the cold side.
A rule of thumb based on thermal resistance, not thickness: at least two-thirds outside the VCL and no more than one-third inside. Confirm the proposed assembly with a condensation-risk assessment.

Internal wall insulation (IWI) on existing masonry

From inside to outside:

  1. Internal finish / plasterboard
  2. Service void (optional but recommended)
  3. Vapour control layer
  4. Insulation (e.g. insulated plasterboard or boarded insulation)
  5. Adhesive / parge coat
  6. Existing masonry wall
  7. External render or brick face

IWI deliberately makes the original masonry colder, so a continuous warm-side VCL is critical — any gap lets indoor moisture reach the now-cold wall. This is a build-up where a seamless liquid VCL earns its keep.

Warm pitched roof (insulation at rafter level)

From inside (room) to outside:

  1. Plasterboard ceiling / internal finish
  2. Service void
  3. Vapour control layer
  4. Insulation between and/or over the rafters
  5. Breather membrane (or counter-battens + membrane)
  6. Battens
  7. Tiles or slates

Watch the eaves, ridge and rooflight junctions — these are where roof VCLs most often leak.

Cold roof (insulation at ceiling level)

From inside (room) up:

  1. Plasterboard ceiling
  2. Vapour control layer (at ceiling level, above the plasterboard)
  3. Insulation between/over the ceiling joists
  4. Ventilated cold loft space
  5. Breather membrane / felt
  6. Battens and tiles

In a cold roof the loft void above the insulation must be ventilated so moisture can escape — the VCL at ceiling level limits how much gets up there in the first place.

Warm and cold flat-roof sections show charcoal-grey waterproof coverings and purple vapour control layers, with a clear ventilation void in the cold roof.
In a warm flat roof, insulation sits above the deck. In a cold flat roof, insulation sits below the deck with a designed ventilation space above it. Both require correctly detailed vapour control.

Ground floor

Floors work a little differently, because the main moisture threat is from the ground below rather than warm air above. A typical insulated floor, top to bottom:

  1. Floor finish
  2. Screed or floor deck
  3. Vapour control layer / airtight layer, lapped and sealed to the wall VCL at the perimeter
  4. Insulation
  5. Damp-proof membrane (DPM)
  6. Concrete slab
  7. Hardcore / ground

The crucial detail is continuity at the floor-to-wall junction: the floor's airtight/vapour line must tie into the wall VCL so there's no gap at the perimeter.

Floor-to-wall section shows a blue floor DPM and wall DPC connected at the junction, with the purple wall VCL sealed into the overlap.
Connect the floor DPM and wall DPC continuously, then seal the wall air and vapour layer into the specified junction. These layers have related but distinct jobs.

The one thing every build-up has in common

Whatever the construction, the VCL only works if it's continuous — sealed at every lap, corner, junction and service penetration. A perfectly positioned VCL with gaps is a leaky VCL. This is why so many UK builders use a liquid-applied airtight vapour control paint: it forms one seamless warm-side layer that self-details around every awkward junction, with nothing to come unstuck over time.

Related reading

Frequently asked questions

Does the VCL go in front of or behind the insulation?

In front — on the warm, room side of the insulation. Working inside to out: finish, service void, VCL, then insulation.

Should the VCL be behind the plasterboard or in front of it?

Behind it — the plasterboard (and ideally a service void) sits on the warm side of the VCL so fixings and cables don't puncture the layer.

Is the VCL position the same in a floor?

The principle (a sealed, continuous airtight/vapour line) is the same, but a floor also has a DPM below the insulation to manage ground moisture. The key is tying the floor layer continuously into the wall VCL.

Want a second pair of eyes on your build-up order? Send it to our technical team and we'll check the layers and VCL position for you.

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