ONE SIZE DOES NOT FIT ALL
For the vast majority of roof systems, attic ventilation is required by the four model building codes (BOCA National Building Code, Uniform Building Code, International Building Code, and Standard Building Code). Despite these code standards, attic ventilation is not a "one size ﬁts all" application. For example, in hot and humid climates, air intake can actually increase moisture problems in attics, or if you plan to build a very complex roof design, it many not be possible to ventilate every 'nook and cranny' of the attic. In fact, all of the model building codes allow for the design of vented and unvented roof systems (http://www.buildingscience.com/documents/digests/bsd-102-understanding-attic-ventilation/). Because of the complexities with this issue, Raynproof Rooﬁng recommends you consult with a local building expert or contact your local building department before modifying your ventilation design.
The National Housing Agency ﬁrst recognized the importance of attic ventilation in 1942. Since then, the amount of attic ventilation required by building codes has steadily increased. During summer months, attic ventilation can reduce excessive heat and humidity build-up, while in the winter, proper ventilation can reduce problems associated with condensation and ice dams. Proper attic ventilation also promotes energy savings by reducing air conditioning costs during the summer and maintaining insulation performance during the winter.
At the current time, the ratio of 1 square foot of net free ventilation for every 300 square feet of ﬂat attic area is the typical standard, with the stipulations that ventilation must be split evenly between intake and exhaust, and that the ceiling must have a properly installed vapor barrier that separates any 'conditioned' air space in the house from the unconditioned air space of the attic. For steeper roofs, or for roofs with more complex designs, sometimes the ratio of 1/150 is required. Read More...