What is the best type of underlay to use?

There are several forms of underlay commonly used in roof construction, the most popular being traditional bituminous felts, vapour permeable underlays and impermeable plastic products. 

Vapour permeable underlays, more commonly referred to as ‘breathable’, give many advantages over the more traditional bituminous based underlays and are rapidly becoming the first choice of designers, builders and roofers. Breathable underlays let the building breathe by allowing the passage of water vapour and so cut down, or in some cases eliminate altogether, the need for roof space ventilation. This in turns makes the insulation more efficient because there is less cold air circulating around the roof space. 

Impermeable plastic underlays can be used with adequate roof space ventilation and are particularly good in situations where the roof tiles are being laid on a roof which is below the recommended minimum roof pitch by creating a waterproof sub roof below the tiles.