My Builder Forgot the Vapour Control Layer? Don’t Panic - Here’s How to Fix It

Few things make a builder’s heart sink like realising the vapour control layer has been missed. Hand on head, kettle on, but don’t panic.

A forgotten vapour control layer (VCL) is one of the most common things we get calls about. Because our membranes are liquid-applied, it’s almost always fixable without ripping anything out. The right solution depends on how far the build has gone — and on getting the prep right first.

First, why it matters

A vapour control layer sits on the warm side of the insulation and limits how much moisture moves into the structure. Skip it and you risk interstitial condensation, mould and reduced insulation performance — and it’s a detail building control will look for. So it’s well worth putting right.

Will building control accept a liquid-applied fix?

Yes — and this is where third-party certification earns its keep. Both Passive Purple and Airtight White are BBA approved (and Passivhaus certified), giving building control documented, independent assurance that the retrofit vapour control layer performs as claimed. That paperwork is often the difference between a smooth sign-off and an awkward conversation on site — so keep the BBA certificate to hand.

Prime the substrate first for a lasting bond

Before any liquid membrane goes on, the surface has to be sound. On bare, dusty, porous or lime-based substrates — concrete block, calcium silicate, old render or lime plaster — prime first with Lime Prime. It consolidates the surface, regulates absorption and dramatically improves adhesion, so the membrane keys properly instead of pinholing or lifting. Skipping prep on a porous wall is the single most common reason a retrofit coating disappoints.

One quick distinction: Lime Prime preps the substrate before the membrane; Supergrip preps the cured membrane before plaster (see Option 2). Two different primers, two different jobs.

Option 1: The walls are finished and painted

If you’ve completely finished - plastered and painted - the answer is Airtight White.

It’s a VOC-free, water-based airtight vapour control membrane that dries white, so it blends into the finish. Simply roll or spray it over the existing painted surface, let it dry, and paint straight over the top in your chosen colour. Airtightness restored, no demolition.

Option 2: It’s plasterboarded but not yet plastered

If the boards are up but the skim coat hasn’t gone on, reach for Passive Purple®.

Spray or roll Passive Purple across the boards and junctions to form a continuous airtight layer. Then apply Supergrip - a primer that gives plaster a key to grip to — and you can plaster straight over the top as normal.

Either way, you’re covered

Both routes are retrofit-friendly, seal awkward junctions and penetrations, and — with BBA-approved membranes and the right primer underneath — get your airtightness back on track without tearing the room apart.

Frequently asked questions

Do I always need to prime? On a sound, sealed, painted surface, no. On bare, dusty, porous or lime substrates, yes — use Lime Prime first so the membrane bonds.

Will it satisfy building control? The membranes are BBA approved and Passivhaus certified — share the certificates with your building control officer for sign-off.

Lime Prime or Supergrip? Lime Prime goes on the substrate before the membrane; Supergrip goes on the cured membrane before plaster.

Not sure which option fits your stage of build? Contact us today and we’ll point you to the right product.