Pliable Membranes

Pliable building membranes serve multiple purposes within the building envelope. They may be required for weatherproofing, to contribute to the Total R-Value for energy efficiency, or to assist with condensation management. Typically, they form part of the water control layer, acting as a first or second line of defence against external moisture.

According to AS 4200.1:2017, pliable building membranes fall into two primary categories:

1. Vapour Barriers – Closed to vapour diffusion

2. Vapour Permeable Membranes – Open to vapour diffusion

Reflective Vapour Barriers

Vapour barriers are designed to block water vapour movement through the membrane and are used when vapour transfer is predominantly from the outside in. They are not recommended in cold or temperate climates, where they can trap moisture and increase the risk of condensation and mould.

Single-Sided Vapour Barriers

These typically consist of a woven polymer mesh with a reflective aluminium foil bonded to one side:

• Reflective face emissivity: ~0.04

• Dull face emissivity: ~0.9

These barriers may be referred to as radiant barrier insulation, reflective insulation, or foil laminates. Their performance depends heavily on installation angle, orientation, and surface cleanliness. In real-world conditions, dust and dirt significantly reduce their reflectivity and effectiveness.

Double-Sided Anti-Glare Vapour Barriers

These feature reflective foil on both sides of a paper-based core, with one side coated in anti-glare ink for installer safety:

• Bright foil face emissivity: ~0.04

• Anti-glare face emissivity: ~0.08

In roof systems, these can reflect heat away and help keep buildings cooler in summer. In wall systems, effectiveness varies. Reflective surfaces facing outwards work only when exposed to direct sunlight, while inward-facing reflective barriers can reflect interior heat but introduce condensation risk, especially when placed in front of thermal control layers.

Vapour Permeable Membranes

These membranes are designed to allow moisture vapour to escape, making them suitable for vapour-open envelope systems. They typically consist of a vapour and air control layer sandwiched between protective outer layers, offering liquid water and air resistance while allowing drying of the structure.

Unlike vapour barriers, these membranes do not include aluminium foil or low-emissivity surfaces. When properly integrated, they help reduce moisture accumulation and improve long-term durability of the building envelope.

Bubble Foils and Foam Laminates

These systems combine double-sided reflective foil with an internal core of bubble wrap or closed-cell foam. One side features an anti-glare coating for safer installation.

• Bright foil emissivity: ~0.06

• Anti-glare foil emissivity: ~0.1

The core thickness varies but generally consists of a single layer of air pockets or foam. These products offer moderate insulation and radiant heat reflection, but like other reflective products, their real-world performance is impacted by dust, orientation, and airspace management.