Airtightness and Vapour Control for Light Gauge Steel Construction

Airtightness & Vapour Control for Bedford Academy

Section: Airtightness / Vapour Control Membranes

System: Intelligent Membranes Passive Purple Airtightness System

Products: Passive Purple Internal, Passive Purple Brush, IMAF Tape (Intelligent Membranes Airtight Fleece), Waterproof Blue Liquid DPM, Airtight Grommets

Project Overview

Bedford Academy’s two-storey extension, delivered by Ashe Construction for Bedford Borough Council and designed by GSS Architecture, required Passive House-grade airtightness to meet Net Zero Carbon in Operation standards. The scheme included 16 classrooms, changing rooms, dining facilities, a community café, and flexible multi-use spaces for community use.

Project Goals

• Achieve airtightness well beyond standard Building Regulations
• Ensure continuity of the vapour control layer across multiple substrates (masonry, steel, OSB, plasterboard)
• Integrate airtightness measures with high-performance insulation, MVHR, ASHP, and solar PV systems
• Deliver long-term durability for a high-traffic educational environment

System Components

1. IMAF Tape (Intelligent Membranes Airtight Fleece) – Used to bridge large gaps and seal challenging junctions between different materials, including wall-to-roof and window reveals. Applied in multiple widths (75mm–200mm).
2. Passive Purple Brush – Fibre-reinforced liquid membrane for detailed, high-risk areas (pipe penetrations, beam ends, floor-to-wall, wall-to-steel, OSB joints, fixings).
3. Passive Purple Internal – Spray- or roller-applied airtight and vapour control membrane to the warm side of the external walls, creating a seamless airtight layer.
4. Waterproof Blue Liquid DPM – Applied in moisture-prone areas and under door linings for additional vapour and damp control.
5. Airtight Grommets – Used for cables and pipe penetrations, ensuring repeatable and verifiable airtight seals.

Performance Achieved

• Airtightness: 0.43 ACH@50Pa (Passive House levels)
• Vapour Control: Continuous and integrated with airtightness layer
• Durability: Long-term resistance to environmental factors, maintaining performance for the lifespan of the building
• Energy Efficiency: Significant reduction in heating/cooling loads, supporting Net Zero operation

Installation Methodology

1. Surface preparation – Substrates cleaned and primed where necessary.
2. Moisture protection – Waterproof Blue Liquid DPM applied in damp-prone zones.
3. Junction sealing – IMAF Tape installed at all critical interfaces.
4. Detailing – Passive Purple Brush applied to penetrations and irregular surfaces.
5. Full coverage – Passive Purple Internal sprayed or rolled to create a seamless airtight membrane.
6. Penetration sealing – Airtight Grommets fitted to cables and pipes.