Ventilated pitched roof

A pitched roof in which a continuous air layer, between the insulation and the tile covering, flows from eaves to ridge. This ventilated cavity sheds summer heat, expels winter vapour and keeps the build-up dry, extending the life of the roof and improving comfort in inhabited lofts.

A ventilated pitched roof is not an 'extra' roof, but a roof that manages air. Above the insulation, and below the tiles, runs a continuous air cavity open at the eaves and venting at the ridge: the convective motion that sets in - driven by the height difference and by solar radiation - turns the cavity into a thermo-hygrometric engine that works for free, all year round.

The air in the under-tile cavity, warmed by the sun beating on the covering, becomes lighter and rises towards the ridge, drawing fresh air from the eaves: the chimney effect. In summer this flow intercepts much of the solar energy before it reaches the insulation, cutting the inner surface temperature and the cooling demand. For it to work, the ventilation gap must be continuous and sized (typically a 4-8 cm layer, with free openings at eaves and ridge proportioned to the pitch): pinch points, nests or debris kill the draught.

The ventilated cavity is not waterproof: water tightness relies, beyond the discontinuous covering, on a breathable membrane laid over the insulation, which sheds infiltrating water and wind-driven snow while letting vapour out. Below the insulation, towards the inside, a vapour control layer governs how much moisture enters the build-up from the living space. The order of the layers follows Glaser's principle: vapour resistance decreasing towards the outside, so that condensation does not build up in the insulation.

Compared with an unventilated roof, micro-ventilation brings three measurable benefits. In summer it lowers the peak loft temperature and pushes the decrement delay forward, especially when paired with high-mass insulants such as wood fibre. In winter it keeps the insulation dry (wet insulation loses most of its insulating power) and prevents ice dams at the eaves caused by snow melting on the warm covering. All year round, finally, it draws moisture away from the timber structure, preventing rot and biological attack: dry timber is timber that lasts.