Which side of the insulation does a vapour control layer go? (UK guide)
One of the most common questions we get from builders, self-builders and renovators is a deceptively simple one: which side of the insulation does the vapour control layer go on? Get it right and your wall, roof or floor stays warm, dry and healthy for decades. Get it wrong and you can trap moisture inside the structure, causing condensation, rot, mould and a steady loss of thermal performance.
This guide gives you the short answer first, then explains the "why" so you can apply the rule confidently to any build-up — masonry or timber frame, wall, roof or floor.
The quick answer: In the UK, a vapour control layer (VCL) goes on the warm side of the insulation — that is, the inside of a heated building, directly behind the plasterboard or internal finish. Warm, moist indoor air should meet the VCL before it reaches the cold insulation and structure.
Why the warm side? Dew point and interstitial condensation
Warm indoor air holds a lot of moisture — from cooking, showering, drying clothes and simply breathing. That water vapour naturally wants to move from the warm, humid inside towards the cold, drier outside. As it travels through a wall or roof, it passes through progressively colder material. At some point it reaches the dew point: the temperature at which the air can no longer hold its moisture and it condenses into liquid water.
If that dew point sits inside your insulation or against the cold structure, you get interstitial condensation — hidden water forming within the build-up, where you can't see it and it can't easily dry out. Over time this soaks insulation (destroying its performance), corrodes fixings and rots timber.
A vapour control layer on the warm side dramatically slows how much moist air can get into the structure in the first place. Less vapour getting in means far less risk of it condensing somewhere cold. This is why the VCL always belongs on the warm, internal side — never buried in the middle or placed on the cold outer face.
VCL vs breather membrane: don't confuse the two
A wall or roof build-up often has two membranes, and they do opposite jobs:
- Vapour control layer (warm/inside): highly resistant to vapour. Its job is to keep moisture out of the structure.
- Breather membrane (cold/outside): resists liquid water but lets vapour escape. Its job is to let any moisture that does get in dry outwards.
The golden rule is "vapour-tight inside, vapour-open outside." The build-up should get progressively more breathable as you move from warm to cold. Putting a vapour-tight layer on the cold side as well traps moisture between the two — a classic and damaging mistake.
The service void: how to fit the VCL and still run cables
A real-world problem: if the VCL sits directly behind the plasterboard, every screw, socket and cable run punctures it. Each hole is a leak in your airtight, vapour-tight line.
The standard solution is a service void — a battened cavity (typically 25–50mm) on the warm side of the VCL, in front of it, where electrics and pipework can run without penetrating the layer. This follows the widely used two-thirds / one-third rule: keep at least two-thirds of the insulation's thermal resistance on the cold side of the VCL and no more than one-third on the warm side (the service void counts as warm-side space). Staying within that ratio keeps the dew point safely on the cold side of your control layer.
Walls, roofs and floors: applying the rule
Warm walls (timber frame and internal wall insulation)
The VCL goes immediately on the internal (room) face of the insulation, behind the service void and plasterboard. On internal wall insulation (IWI) retrofits this is especially critical, because you're deliberately making the existing masonry colder — a continuous warm-side VCL is what stops that masonry getting wet.
Warm roofs and room-in-roof
For a warm pitched roof or a loft conversion, the VCL sits on the warm side of the rafter insulation, under the plasterboard. Continuity at the eaves, ridge and around rooflights is where most roofs fail — these junctions must be sealed, not just butted.
Ground floors
Here the "warm side" logic still applies, but the detail differs: a damp-proof membrane (DPM) under or over the slab manages ground moisture, and the airtight/vapour line is tied into the wall VCL at the floor perimeter. The key is continuity — the control layer must be unbroken where floor meets wall.
The hardest part isn't the side — it's continuity
Knowing the VCL goes on the warm side is the easy bit. The failures we're called out to almost always come down to gaps: unsealed laps in sheet membranes, tears behind dot-and-dab, unsealed service penetrations, and the awkward junctions where wall meets roof, floor or window reveal. A VCL is only as good as its weakest, leakiest point.
This is exactly why many UK builders now use a liquid-applied vapour control layer instead of (or alongside) sheet membranes. A fluid-applied coating like Passive Purple Internal Airtight & Vapour Control Paint is rolled or sprayed straight onto the warm-side substrate, forming one continuous, seamless layer that self-details around every penetration, corner and junction — with no laps or tapes to fail. For projects where a white finish is preferred, our White Airtight Vapour Control Paint does the same job.
Key takeaways
- The VCL goes on the warm (inside) side of the insulation.
- Vapour-tight inside, vapour-open outside — never trap insulation between two tight layers.
- Use a service void and the two-thirds / one-third rule to run services without puncturing the layer.
- Continuity matters more than anything — seal every lap, junction and penetration.
Related reading
- Do I need a vapour barrier? A UK guide
- What is a vapour control layer (VCL)? Explained
- Builder forgot the vapour control layer? How to fix it
Frequently asked questions
Is a vapour control layer the same as a vapour barrier?
In everyday UK usage the terms are used interchangeably. Strictly, a "barrier" implies near-total vapour resistance while a "control layer" slows vapour to a safe level; both go on the warm side.
Can I ever put a VCL on the cold side?
No — in a UK heating-dominated climate the VCL belongs on the warm side. A vapour-resistant layer on the cold outer face traps moisture and is a leading cause of interstitial condensation.
What if my build-up already has a breather membrane outside?
That's correct and expected. The breather membrane on the cold side lets the wall dry outwards; your VCL on the warm side limits how much moisture gets in. The two work together.
Do I still need a VCL if I use airtight paint?
An airtight vapour control paint such as Passive Purple is your VCL — it provides both the airtight and vapour-control functions in one continuous, warm-side layer, which is why it's a popular replacement for sheet membranes on Passivhaus and high-performance retrofits.
Not sure which product suits your build-up? Get in touch with our technical team and we'll help you specify the right warm-side vapour control solution.