The vapour barrier is an important part of the roof structure as it controls the condensation in the roof. It's actually responsible for controlling the amount of diffusion of moisture from inside the building into the roof and to the outside.
Because it is installed on the warm side of the insulation, the temperature in the roof will not fall below the dew point so condensation will not form in the roof.
There are a host of different types of vapour barrier to choose from, most of which are good enough to also stop driving rain and strong winds from getting through your surface structure.
In the past, most vapour barriers weren't breathable. But this further exacerbated the problem with condensation and only caused further damage to the roof. Now, with the introduction of modern vapour barriers that don't let rain in and still stop condensation forming, it's become much easier to protect your roof.
There are also modern hybrid products like the Actis Boost'R Hybrid which combine an insulation layer with a breathable membrane to give you an easy to install product that does both jobs at the same time. This hybrid product is sufficient for most applications and can even be upgraded with the addition of the Actis sister product, the Hybris reflective insulation to improve the heat retention of your roof still further.
While vapour barriers can allow a large quantity of moisture to pass through from the loft space, they can be helped by the addition of tile vents which allow greater airflow into the spaces under the tiles, reducing the moisture still further and lowering the risk of moisture damage to your rafters, batten and interior walls.
Every roof, whether new or old, should have some sort of vapour barrier in place just underneath the tiles so that the roof's lifespan will be lengthened and maintenance time on the roof will be reduced.
