Self-Build & Passivhaus — Airtightness Made Achievable
Why Airtightness Matters for Your Self-Build
Airtightness is one of the most impactful things you can do for your self-build home. A well-sealed building uses significantly less energy to heat, stays warmer without draughts, is less prone to condensation and mould, and is far more comfortable to live in year-round.
The good news: with Passive Purple, achieving excellent airtightness is within reach of any self-builder. The brush-applied version requires no specialist equipment — just a brush, a bucket, and a bit of patience at the critical junctions. For larger builds, the spray-applied version can be hired out or applied by one of our approved installers.
🏆 What is Passivhaus?
Passivhaus is a voluntary building standard originating in Germany that delivers homes with exceptionally low energy bills, excellent indoor air quality and outstanding thermal comfort. The five principles are: airtightness, continuous insulation, thermal-bridge-free construction, high-performance windows, and mechanical ventilation with heat recovery (MVHR).
To achieve Passivhaus certification, a building must reach ≤0.6 air changes per hour (ACH) at 50 Pascals. Passive Purple is the world's first Passivhaus-certified liquid-applied membrane and has helped dozens of UK self-build projects achieve certification.
Products for Self-Builders
Passive Purple Brush (5kg)
The self-builder's best friend. Brush-applied, fibre-reinforced, and suitable for all substrates. Use it at junctions, around windows, and on any area that's difficult to spray. No primer, no specialist equipment needed.
Passive Purple Spray/Roller (11.4kg)
For those with an airless spray machine or willing to roller — covers large surfaces quickly. One person can do 300m²/day by spray. The Handy Pack is ideal for smaller self-builds.
IM Passivhaus Sealant
Flexible, Passivhaus-certified sealant for sealing around window frames, service entries, and movement joints. Works alongside Passive Purple for a complete system.
IM Fleece Tape
Airtight tape for membrane-to-frame transitions and areas where tape is more practical than liquid. Use in combination with Passive Purple for a belt-and-braces approach at critical junctions.
Wöhler Smoke Stick
Find air leaks before your blower door test with smoke testing. Produce a pressure differential (or use a fan) and run the smoke stick along junctions to identify leakage paths for remediation.
Product Bundles
Our self-build bundles combine Passive Purple Brush and Spray with sealant and tape — everything you need for a complete airtight system at a bundle price.
How to Achieve Excellent Airtightness — Step by Step
- Design the airtight layer into your build-upDecide where your airtight layer will be (warm side of insulation, internally) and plan for continuity around windows, doors, service entries and floor–wall–ceiling junctions.
- Seal all penetrations first with Passive Purple BrushBefore applying the main coat, use Passive Purple Brush around all cables, pipes, MVHR ducts, and windows. Let it cure before the spray coat.
- Apply Passive Purple Brush at all junctionsWall–floor, wall–ceiling, external corners, around door and window reveals — anywhere two surfaces meet is a potential leakage point. Brush at least 100mm each side of the junction.
- Apply the full-surface coatSpray, roller or brush apply Passive Purple over the full wall, floor and ceiling surface to the specified dry film thickness (see coverage rates).
- Do a pre-test smoke checkBefore booking your official blower door test, create a pressure differential with a fan or window fan and run a Wöhler smoke stick along all junctions to find any remaining leakage paths. Remediate before the official test.
- Book your blower door testUse a UKAS-accredited air tightness tester. If you're going for Passivhaus, your certifier will advise on the test procedure. Results of ≤0.6 ACH are readily achievable with a careful Passive Purple application.
Airtightness Targets for Self-Builds
| Standard | Target | Achievable with Passive Purple? |
|---|---|---|
| UK Building Regulations Part L 2021 | ≤5 m³/h·m² @ 50 Pa (new dwellings) | ✅ Yes — easily |
| SAP Design Target (typical new build) | ≤3 m³/h·m² @ 50 Pa | ✅ Yes |
| Passivhaus Classic | ≤0.6 ACH @ 50 Pa | ✅ Yes — multiple certified projects |
| EnerPHit (deep retrofit) | ≤1.0 ACH @ 50 Pa | ✅ Yes — with Passive Purple Retrofit |
Free Technical Support for Self-Builders
Our team is happy to advise self-builders on product selection, application technique, junction detailing, and what to expect from the blower door test. Contact us — there's no obligation and no minimum order.
Ready to Start?
Order direct from our online store, use the heat loss calculator to estimate your savings, or call us to talk through your self-build.
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