Roof Ventilation

Roof ventilation moves outside air through the space between the insulation and the roof covering. In a vented attic or cold roof, its main jobs are to carry away moisture that leaks or diffuses up from the house, reducing condensation and decay, and, in cold climates, to keep the roof deck cold enough to limit snow melt and ice dams. It also lowers attic temperatures in hot weather, although insulation and air sealing usually have a bigger effect on cooling loads.

Intake and exhaust

Most systems work on natural stack and wind effects: cooler air enters low, at the eaves, and warmer air leaves high, near the ridge.

• Intake vents: perforated or slotted soffits, continuous strip vents, over-fascia vents (common in the UK) and drip-edge or roof-edge intake vents.
• Exhaust vents: continuous ridge vents, individual static roof vents (box or louvre vents), gable-end louvres, wind-driven turbine vents, and powered attic ventilators.
• Baffles and eaves trays keep insulation from blocking the air path at the eaves.

Balanced ventilation and code ratios (US)

International Residential Code section R806.2 sets a minimum net free ventilating area of 1/150 of the vented space. This can be reduced to 1/300 where, among other options, 40% to 50% of the required area is provided by upper vents located no more than 3 ft (914 mm) below the ridge, with the rest at the eaves. At eave vents, a clear space of at least 1 in (25 mm) must be kept between insulation and sheathing. Vent products are rated by net free vent area so their capacity can be compared.

Mixing exhaust types on one roof (for example, a ridge vent alongside gable louvres or a powered fan) is widely discouraged because one outlet can draw air through another instead of through the eaves, short-circuiting the flow. Powered attic fans can also depressurise the attic and pull conditioned air from the house through ceiling leaks.

UK and European practice

In the UK, BS 5250 governs moisture control and ventilation. Cold roofs with high-resistance underlays need loft ventilation; with low-resistance (vapour-permeable) underlays and air-tight coverings, the batten space itself may need ventilating, with guidance citing openings equivalent to a 25 mm slot at the eaves and 5 mm at the ridge.

Unvented alternatives

Warm roofs and unvented (conditioned) attics place insulation at the roof plane and do not rely on ventilation of the roof space; these are allowed under specific conditions.