Passive Purple — Liquid Airtight Membrane Paint
Passive Purple — Liquid Airtight Membrane Paint
The UK's only BBA and Passivhaus-certified liquid airtight membrane. Spray or roller-applied vapour control layer (VCL) paint that achieves 0.03 ACH — 20× tighter than the Passivhaus standard. Seamless, flexible, and certified for new builds and retrofits.
testing @ 50 Pa
Passivhaus standard
airless spray
& test documents
What Is Passive Purple?
Passive Purple is the world's first Passivhaus-certified liquid airtight membrane paint — a water-based, VOC-free coating that creates a seamless vapour control layer (VCL), airtight layer, and smoke-tight barrier in a single application. It replaces traditional polythene sheet membranes, foil VCLs, and airtight tapes with a flexible, spray or roller-applied coating that bonds permanently to the building fabric.
Applied to the warm side of insulation across walls, floors, and ceilings, Passive Purple fills micro-cracks, seals penetrations, and creates a continuous airtight building envelope. Unlike rigid sheet membranes, the liquid formulation adapts to irregular surfaces and building movement — delivering a certified performance result that sheet-based systems simply cannot match at junctions and transitions.
Backed by BBA Certificate 22/6002, Passivhaus Institut 2026 certification, ICC testing, M1 indoor air quality classification (the strictest in Europe), Fire Class C (two classes better than standard foil membranes), and an Environmental Product Declaration, Passive Purple is the most comprehensively certified airtight membrane paint available in the UK.
Why Liquid Airtight Paint?
The micro-crack problem that renders sheet membranes inadequate
The Physics of Building Movement
Every building contains multiple materials — brick, blockwork, timber, steel frame, concrete slab, plasterboard — each with a different thermal expansion coefficient. As the building heats and cools through daily and seasonal cycles, these materials expand and contract at different rates.
At the interfaces between materials — floor-to-wall junctions, wall-to-ceiling connections, around window frames and service penetrations — this differential movement creates micro-cracks in the plaster and substrate. These cracks are invisible to the eye but large enough to allow significant air infiltration.
Polythene sheet membranes and foil VCLs are rigid over joints and tear at fixings; the taped joints are the first point of failure. Airtight tape applied directly to substrates debonds as the substrate flexes. The result: every traditional airtightness system has inherent weak points at the exact locations where movement is greatest.
The Liquid Solution
A liquid airtight coating penetrates and seals the substrate surface before any plaster is applied, bonding at the molecular level across every material boundary. When cured, the flexible polymer film moves with the building rather than against it — cracks that would rupture a rigid membrane are accommodated by the elastic layer beneath.
The result is a seamless, continuous airtight building envelope with no overlaps, no taped joints, and no weak points — confirmed by independent air pressure testing to achieve just 0.03 ACH at 50 Pa.
Traditional sheet membrane failures
Six Reasons Specifiers Choose Passive Purple
Certifications & Third-Party Approvals
The most comprehensively certified airtight membrane paint available in the UK. Every claim is independently verified.
All certificates available to download in the Documents section below. BBA Certificate 22/6002 covers the Spray/Roller formulation; separate BBA cert covers the Brush formulation.
The Complete Passive Purple System
Two complementary products working together to create a fully sealed building envelope. As a guide: 1 tin of brush-applied is needed for every 5 tins of spray for a complete system.
Passive Purple Spray / Roller
11.4kg tin • £99.99 • Coverage: 1kg/m² (~11.4m²)
The primary liquid airtight membrane for all wall, ceiling, and floor large-area applications. Spray-applied with an airless rig at up to 300m²/day per installer, or applied with a short-pile roller for smaller areas. Creates the seamless VCL/airtight layer across all solid substrates.
- Spray or roller application — no specialist equipment required for roller
- 300m²/day with airless spray (one operator)
- Covers concrete, brick, blockwork, timber, OSB, steel
- BBA certified spray/roller formulation
- Apply to warm side of insulation before first coat of plaster
- Can be plastered, rendered, or painted over when cured
Passive Purple Brush (Fibre-Reinforced)
3.7kg or 6.9kg • from £39.99 • Fibre-reinforced formula
The fibre-reinforced brush formulation is used to seal junctions, penetrations, cracks, and transitions that the spray/roller cannot fully cover. Applied first to all detail areas before the spray coat, ensuring continuity of the airtight layer at every weak point in the building envelope.
- Seals cracks and voids 2–5mm wide
- Window and door frame perimeters
- Floor-to-wall junctions
- Wall-to-ceiling connections
- Service pipe and cable penetrations
- Separate BBA certification for brush variant
- Works with IM Fleece Tapes for additional reinforcement
Dot-and-Dab / Plasterboard: When plasterboard is to be applied over Passive Purple using a dot-and-dab technique, apply Supergrip primer over the cured Passive Purple layer before applying the adhesive dabs. This ensures adhesion bond strength is maintained through the airtight layer.
Substrate Compatibility
| Substrate | Spray/Roller | Brush Detail | Primer Required | Notes |
|---|---|---|---|---|
| Concrete (in-situ & precast) | ✓ Yes | ✓ Yes | No (clean substrate) | Ensure surface is free of laitance and loose material. Apply to sound, clean face. |
| Brick & Blockwork | ✓ Yes | ✓ Yes | No | Suitable for dense aggregate blocks, aerated (Aircrete) blocks, and clay brick. Fill major voids first with brush grade. |
| Timber & OSB Boards | ✓ Yes | ✓ Yes | No | Suitable for timber frame studs and OSB sheathing panels. Seal all board joints with brush grade first. |
| Steel Frame (LGS) | ✓ Yes | ✓ Yes | No | Light gauge steel frame system. Apply across cladding panels and at all frame junctions. Brush detail around every penetration. |
| Plasterboard (dot-and-dab) | ✓ Yes | ✓ Yes | Supergrip primer required | For dot-and-dab plasterboard: apply Supergrip over cured Passive Purple to maintain adhesion bond strength. Direct fix requires no primer. |
| Floor-to-Wall Junctions | Roller | ✓ Primary | No | Brush grade applied first to all junctions; spray/roller coat over. Critical airtightness weak point in all build types. |
| Wall-to-Ceiling Connections | Roller | ✓ Primary | No | Apply brush grade into the corner and up each face before overall spray coat. |
| Window & Door Frames | Not recommended | ✓ Primary | No | Use brush grade to seal frame-to-wall perimeters. Combined with Passivhaus Sealant for movement joints. |
| Service Penetrations | Not applicable | ✓ Primary | No | Brush grade applied around all penetrations to create a smoke-tight, airtight seal. Can be used with IM Fleece Tapes for reinforcement on larger penetrations. |
Technical Specification
| Property | Spray / Roller | Brush (Fibre-Reinforced) |
|---|---|---|
| Fire Classification | Class C (EN 13501-1:2018 cl.11) | Class C (EN 13501-1:2018 cl.11) |
| Airtightness Achieved | 0.03 ACH @ 50 Pa | As system (used with spray grade) |
| UK Building Regs Standard | ≤10 m³/h·m² @ 50 Pa (Part L) | |
| Passivhaus Standard | ≤0.6 ACH @ 50 Pa (PHI) | |
| Coverage Rate | 1 kg/m² (11.4kg = ~11.4m²) | Check TDS for application rate |
| Application Speed | 300m²/day (airless spray, 1 operator) | Detail/junction work rate |
| Base | Water-based polymer | |
| VOC Content | VOC-free | |
| Indoor Air Quality | M1 Class (strictest in Europe) | |
| Colour | Purple / White (Airtight White variant) | |
| Radon Barrier | ✓ Certified | ✓ Certified |
| Methane / Ground Gas Barrier | ✓ Certified | ✓ Certified |
| Smoke Tight | ✓ Yes | ✓ Yes |
| Vapour Control Layer (VCL) | ✓ Yes | ✓ Yes |
| WUFI Hygrothermal Data | Listed — EN 15026, ASHRAE 160, DIN 4108 | |
| BBA Certificate | 22/6002 (Spray/Roller) | Separate BBA cert (Brush) |
| Available Size | 11.4kg | 3.7kg / 6.9kg |
| Price | £99.99 per 11.4kg | From £39.99 |
Understanding Airtightness Standards
How UK Building Regulations and Passivhaus compare — and what Passive Purple achieves.
What this means for your project
Achieving a test result of 0.03 ACH with Passive Purple liquid VCL on a completed building means that even with construction tolerances, service penetrations installed after membrane application, and the inevitable minor imperfections of any building site, the final measured result is highly likely to satisfy both Passivhaus and Part L requirements comfortably.
For Passivhaus designers: WUFI Database listing means Passive Purple can be included directly in hygrothermal models run to EN 15026, ASHRAE 160, and DIN 4108 — simplifying the certification pathway.
Estimate the heat loss savings from improved airtightness for your project using our free heat savings calculator.
Passive Purple vs Traditional Airtightness Solutions
A like-for-like comparison of every major airtightness system available in the UK market.
| Attribute | Passive Purple (Liquid VCL) |
Polythene Sheet DPM |
Foil Membrane |
Self-Adhesive Membrane |
Airtight Tape Only |
|---|---|---|---|---|---|
| Fire Classification |
Class C EN 13501-1 |
Class E | Class E | Class E | Varies |
| BBA Certified | ✓ Yes (22/6002) | Some products | Some products | Some products | Some products |
| Passivhaus Certified | ✓ Yes (2026) | ✗ No | ✗ No | ✗ No | ✗ No |
| Seamless Coverage | ✓ Yes | ✗ No — overlaps & joints | ✗ No — overlaps & joints | Partial — joints remain | ✗ Linear only |
| Application Speed | 300m²/day | ~80–120m²/day | ~60–100m²/day | ~50–80m²/day | Linear metre only |
| Radon Barrier | ✓ Certified | Some grades | ✗ No | Some products | ✗ No |
| Methane Barrier | ✓ Certified | Some grades | ✗ No | ✗ No | ✗ No |
| All Substrates | ✓ Concrete, brick, timber, OSB, steel | ✗ Flat surfaces only | ✗ Flat surfaces only | Smooth surfaces | Linear junctions |
| Flexibility | ✓ Elastic polymer film | ✗ Rigid at junctions | ✗ Rigid, tears | Limited | Adhesion failure risk |
| VOC / Indoor Air | VOC-free • M1 Class | Low concern | Low concern | Adhesive VOCs possible | Adhesive VOCs possible |
| Smoke Tight | ✓ Yes | ✗ No | ✗ No | ✗ No | ✗ No |
| EPD Available | ✓ Yes | ✗ Rarely | ✗ Rarely | ✗ Rarely | ✗ Rarely |
Technical Documents & Certificates
All certificates, data sheets, and installation guides available to download free.
Frequently Asked Questions
Complete the System
Complementary products for a fully certified airtight building envelope.
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The UK's only BBA and Passivhaus-certified liquid airtight membrane paint. Delivered direct. Full technical support available.