Airtightness Explained

Airtightness Explained: A Complete Guide to Air Barriers, Junctions, Ventilation and Pressure Testing

Airtightness explained clearly for self-builders planning a continuous air barrier from the drawings through to the pressure test. This technical guide covers leakage paths, junctions, service penetrations, thermal bypass, ventilation coordination and the inspections that help prevent concealed defects.

Key Topics Covered

• Core Principles & Air-Barrier Continuity: Defines envelope airtightness as the resistance to uncontrolled air leakage (infiltration and exfiltration) through cracks, joints, and penetrations when intentional ventilators are closed. Establishes the “pen-on-paper” rule for drawing one continuous primary air barrier around the conditioned space.

• Air Leakage Mechanics & Thermal Bypass: Explores driving forces (wind pressure, stack effect, mechanical fan pressures) and complex leakage pathways behind dry-lining voids, joist zones, and connected cavities. Details thermal bypass risks where circulating air degrades nominal insulation performance.

• Material Strategies & Element Junctions: Compares primary barrier strategies, including continuous wet plaster/parge coats on internal masonry versus taped vapour-control membranes or sealed structural sheathing on timber framing. Outlines detailing for high-risk junctions at ground-floor slab edges, intermediate floor joists, eaves, party walls, and window/door reveals.

• Penetrations, Services & Testing Methodology: Evaluates group service routing, elastomeric grommets, pipe collars, and protected internal service voids. Details intermediate leakage detection (smoke testing, thermal imaging) prior to concealment and final compliance pressure testing (blower-door setup) in accordance with CIBSE TM23 standards.

• Compliance & Defect Prevention: Aligned with Building Regulations Approved Documents L (Energy Performance), F (Ventilation), C (Moisture Resistance), B (Fire Safety), J (Combustion Safety), Regulation 7, and CIBSE TM23 standards. Features an illustrated troubleshooting matrix for critical site defects (e.g., dot-and-dab drywall air paths, unsealed slab edges, unsupported membrane laps, relying solely on expanding window foam), a 5-stage inspection checklist with pre-concealment hold points, an airtightness glossary, and FAQs.

Airtightness and moisture control must be coordinated, so this guide works particularly well alongside Vapour Control Layers Explained. The official Approved Document L (2026) sets out the relevant energy-performance requirements for dwellings in England.