Building Science Attic Venting

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.

Building science attic venting.

It s hard to screw this up. 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. Entire attic or if you re venting only the roof deck.

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 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. This is a vented attic and it s probably one of the most unappreciated building assemblies we have in the history of building science.

Attic ventilation works on the principle that heated air naturally rises primarily utilizing two types of vents. 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. A well ventilated attic has almost no impact on radiant transfer of heat between the underside of the roof deck and the top of the insulation. Building science fundamentals see all events a building science consulting and full service architecture firm specializing in building technology for all types of buildings including commercial institutional and residential.

The applicable physics impacts the design of attic or roof systems as does the applicable building code but neither limit the choice. Intake vents located at the lowest part of the roof under the eaves allow cool. 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. The model building codes allow both vented and unvented roof assemblies.