Several factors must be taken into account when selecting an air barrier membrane for a wall design. The main purpose of an air barrier is to prevent air leakage through the building envelope. Uncontrolled air circulation can cause heat loss and high resultant energy costs, not to mention increase the risks of condensation and mould growth in the assembly.
VARIOUS TYPES OF AIR BARRIERS FOR VARIOUS NEEDS
Irrespective of their air leakage rate, permeable air barrier membranes are categorized according to their water-vapour permeance. The degree to which water vapour passes through a wall is specific to each type of air barrier. The diagrams below show the effects of water-vapour diffusion, per type of air barrier, for hot summer weather and cold autumn temperatures. Note that the more permeable an air barrier, the more it allows water-vapour diffusion in the assembly.
DRYING TIME OF A WALL
Water-vapour permeance of materials used in an exterior wall assembly has an impact on the drying time of the assembly. Based on hygrothermal simulations, this graph demonstrates the influence of air barrier permeance on the drying time of a wall.
Increasing membrane permeance above 10 US perm only provides marginal benefits to drying. Simulations illustrated* above demonstrate that the drying time of a wall is only 12 days faster when permeance is increased from 10 US perm to 50 US perm. In other words, a membrane that is five times more permeable will not allow the wall to dry five times faster. However, a 50 US perm membrane will allow faster moisture accumulation inside the assembly through inward vapour drive, thereby increasing risk of condensation.
In addition, the drying dynamics of any wall assembly will be dictated by the material that has the lowest water vapour permeance. Consequently, the potential benefits of a highly permeable air barrier will often be mitigated by another material that determines the assembly’s drying.
GOOD ADHESION IS CRITICAL
Among the permeable air barrier lines available on the market, prefabricated self-adhesive membranes have varying adhesive properties. An effective permeable air barrier must readily adhere to most building surfaces. The SOPRASEAL STICK VP permeable air barrier membrane meets and exceeds this basic specification. In fact, it stands out from the competition thanks to its unmatched adhesive qualities. Moreover, no primer is required for its installation.
IN CONCLUSION, HOW TO CHOOSE AN AIR BARRIER MEMBRANE ?
Several criteria must be considered when selecting an air barrier. In addition to the above mentioned criteria, other factors are also taken into account, including:
- UV resistance,
- Insulation present in the cavity,
- Insulation present outside of the envelope,
- Water storage capacity of the sheating.
All of these factors complicate the drying of a damp wall. Consequently, it is essential to choose a membrane by considering the wall type in which the membrane will be installed, and not only by membrane permeance.