A pliable building membrane may be required for different reasons such as weatherproofing purposes, energy efficiency (i.e. part of the Total R-Value of the envelope) or managing condensation. Typically, it is your first or second line of defence against external moisture, represented by your water control layer.
Falling into two key groups as per AS4200.1 (2017) - Pliable building membranes and underlays Part 1: Materials, a pliable building membrane is either open to vapour diffusion and termed vapour permeable or closed and a vapour barrier.
Reflective Vapour Barriers
Vapour barriers are specially designed to stop water vapour passing through the membrane. As such, they are used when the dominant water vapour transfer is from the exterior of the building envelope, not the interior. In cold and temperate climates, vapour barriers should not be used as they increase the risk of condensation and mould.
Single-sided vapour barriers are usually a woven polymer mesh material with a reflective aluminium foil adhered to one side. The emissivity of the reflective aluminium foil face is typically 0.04, and the dull side 0.9. As a result of the low emissivity, single-sided vapour barriers can be referred to as radiant barrier insulation, reflective insulation or reflective foil laminates. Under ideal conditions, where no dust or dirt occurs, the effectiveness of the reflectivity is directly related to the angle and orientation in which the product is installed. Due to the unavoidable presence of dust and dirt, stated performance is often a best-case scenario, with actual performance much lower.
Double-sided antiglare vapour barriers have reflective foil on each side of an inner paper-based lining, one face being coated with an antiglare ink to reduce glare for installers. The emissivity of the bright foil face is typically 0.04, and the antiglare side 0.08.
In roof systems, where heat is reflected out, they can provide an effective means of keeping a roof cooler in summer. For wall systems, their effectiveness is highly variable. When the reflective surface faces externally, they are only useful when exposed to direct solar radiation. When the reflective surface faces internally, they can successfully reflect heat from the interior but introduce a condensation and mould risk as they place a cold surface directly in front of the thermal control layer.
Vapour Permeable Membranes
Vapour permeable membranes are specially designed to allow water vapour to pass through the membrane and are ideal for vapour open building envelop systems.
A vapour and air control layer is typically bonded between protective inner and outer layers to form a vapour-permeable membrane which also maintains barrier properties for liquid water and air. When used correctly, this can allow for drying of construction systems. These membranes do not typically contain aluminium foil layers or low emissivity surfaces.
Double-sided reflective foils with an inner core material thickness. One side coated with antiglare to reduce glare. Centre core material, typically mm thick enclosing a single layer of individual air bubbles or closed-cell foam. The emissivity of the bright foil face is assumed to be 0.06, and the antiglare side 0.1.