Building Science Attic Venting

In mixed climates ventilation serves either role depending on the season.

Building science attic venting.

The amount of roof cavity ventilation is specified by numerous ratios of free vent area to insulated ceiling area ranging from 1 150 to 1 600 depending on which building code is consulted the 1 300 ratio being the most common. It s hard to screw this up. Attic ventilation works on the principle that heated air naturally rises primarily utilizing two types of vents. In a hot climate the primary purpose of ventilation is to expel solar heated hot air from the attic or roof to reduce the building s cooling load and to relieve the strain on air conditioning systems.

In hot climates the primary purpose of attic ventilation is to expel solar heated hot air from the attic to lessen the building cooling load. See energy smart details in fhb 218 for more on ice dams. In mixed climates ventilation serves either role depending on the season. In a cold climate the pri mary purpose of ventilation is to maintain a cold roof tempera ture to avoid ice dams created by melting snow and to vent any moisture that moves from the conditioned living space to the attic.

The applicable physics impacts the design of attic or roof systems as does the applicable building code but neither limit the choice. As such there is no energy benefit to vent an attic in a hot humid climate but there is alarge moisture penalty due to the moisture being brought into the attic by venting the attic. In a hot climate the primary purpose of ventilation is to expel solar heated hot air from the attic or roof to reduce the building s cooling load and to relieve the strain on air conditioning systems. Entire attic or if you re venting only the roof deck.

This is a vented attic and it s probably one of the most unappreciated building assemblies we have in the history of building science. For 20 of the effort it gets us to 80 of optimal performance and it works in hot climates in mixed climates the arctic the antarctic the amazonian rain. Intake vents located at the lowest part of the roof under the eaves allow cool. The choice of venting or not venting is a design and construction choice not a requirement determined by the physics or by the building code.