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 important:

(a) vapour permeance,

(b) mechanical strength in tension, shear, impact and flexure,

(c) adhesion,

(d) elasticity,

(e) thermal stability,

(f) fire and flammability resistance,

(g) inertness to other deteriorating elements, and

(h) ease of fabrication, application and joint sealing.