Vapour barrier is a component that retards
vapour movement but does not totally prevent its transmission. Vapour barriers are
commonly of plastic film or aluminum. The barrier needs to be installed as close to the
surface exposed to the higher water vapour pressure or warm air as possible, typically
just behind interior surfaces as shown in Figure 4. Table 7 shows the performance in
vapour resistance of some common vapour barriers.
To select vapour barriers, the following properties are
(a) vapour permeance,
(b) mechanical strength in tension, shear, impact and
(e) thermal stability,
(f) fire and flammability resistance,
(g) inertness to other deteriorating elements, and
(h) ease of fabrication, application and joint sealing.