The importance for Roofs to Breathe
Now it's usually accepted, perhaps within conservation circles, it is important never to restrict the ability of traditional building materials and structures to ‘breathe’. However, attention has tended to focus on the damage caused by the use of impervious modern paint systems and cement-rich mortars and renders, and the one portion of a classic building where the evaluation of efficiency and a focus to detail is usually neglected often is the roof. Though this is one of the main areas where air flow could readily take place, especially in traditionally detailed tiled or slated roofs.
These days, the performance of so many historic roofs continues to be impaired of the come out of roofing felts and insulation. Although some roofing felts are now marketed being vapour permeable, until recently almost all felts were impervious. Roofing felt was created mainly to act as a secondary barrier from wind-driven snow and rain, however its use also causes a reduction in air movement within the roof space, specially if the roofing felt is impervious, this kind of effect is usually compounded by the introduction of insulation. Fibreglass quilt or resin fibre material, for instance, are often laid over eaves and put on to the bottom of the roof, in contact with the roofing felt. check 
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Common types of a physical secondary barrier against wind-driven snow and rain contain reeds laid between tiles and battens and a coating of mortar popular as ‘torching’ to the underside of the tiles or slates. Torching is most commonly encountered to the underside of old stone slate roofs. Both processes give roofs to breathe.
Roofing felt was initially introduced in the 1930s, when it mostly made up of thin building paper. After the Second World War high quality bitumen and plastic felts were commonly used, with ever more impervious supplies more and more common as time continued, until relatively recently when more vapour permeable felts started being introduced.
Insulation created being presented on a massive scale because to the need to make buildings, in selected domestic dwellings, more energy efficient. Insulating our properties is also seen as one of the most effective ideas of reducing the interest for fossil fuels, cutting pollution and climatic change caused by carbon dioxide.
The lessons that are being learnt from experience of the problems made by earlier progress to the energy output of older buildings now are being heeded, especially in view of the proposed revision of Part L of the Building Regulations which will increase insulation needed yet again.
It is recommended that a holistic method is adopted for the efficiency of a building to be understood, as the roof and walls shouldn't be taken in isolation: they are an necessary part of the building and have an active and continuing connection for the rest of the building, its environment and its occupants.