Airtightness Membranes and Vapour Barriers

What This Requirement Covers

Airtightness membranes and vapour control layers (VCLs) are essential components of energy-efficient construction. They serve two distinct but related functions: preventing uncontrolled air leakage (airtightness) and preventing moisture vapour from penetrating the building fabric (vapour control).

Key Requirements

Airtightness Membranes

1. An airtight layer prevents uncontrolled air movement through the building envelope
2. In timber frame construction, a dedicated airtightness membrane is applied to the warm side of the insulation
3. In masonry construction, the plaster or pargeting serves as the primary airtight layer
4. The airtightness membrane must be continuous across all walls, floors, and ceilings, with all joints taped and sealed
5. Penetrations for services (pipes, cables, ducts) must be sealed with appropriate grommets or sealant

Vapour Control Layers (VCLs)

1. A VCL restricts the passage of water vapour from the warm interior into the cold part of the construction, preventing interstitial condensation
2. VCLs are placed on the warm side of the insulation (the same position as the airtightness membrane)
3. In many constructions, a single membrane serves both functions (airtightness and vapour control)
4. VCL resistance is measured in MNs/g; a typical VCL has a resistance of 100-250 MNs/g

Breather Membranes

1. A breather membrane is placed on the cold side of the insulation, protecting the construction from wind and rain while allowing moisture vapour to pass outwards
2. Breather membranes have low vapour resistance (high breathability), allowing any moisture that enters the construction to evaporate to the outside
3. The combination of a VCL on the warm side and a breather membrane on the cold side creates a construction that is airtight, weatherproof, and free from condensation risk

Intelligent Membranes

1. Variable-permeability (intelligent) membranes adjust their vapour resistance depending on humidity conditions
2. In winter (high humidity inside, low outside), they act as a VCL, restricting vapour movement
3. In summer (lower humidity inside, higher outside), they become more permeable, allowing the construction to dry out
4. Suitable for retrofit insulation of existing buildings where the moisture dynamics are more complex

Practical Compliance Tips

1. Identify the airtightness line on the drawings and ensure it is continuous; every gap undermines the whole system
2. Tape all joints in the airtightness membrane with compatible tapes (not generic duct tape)
3. Seal all service penetrations through the membrane; use proprietary grommets or airtightness sealant
4. Do not puncture the membrane after it is installed; coordinate with electricians and plumbers to plan penetrations before the membrane is completed
5. Carry out a pre-plasterboard airtightness test (smoke pencil or preliminary pressure test) to identify leaks before finishes conceal them
6. In timber frame construction, the airtightness membrane is critical; a single missing tape joint can significantly increase the air leakage rate
7. For retrofit projects on existing buildings, consider intelligent membranes to manage moisture risk