How much ventilation does an attic need?
The IRC requires 1 square foot of net free ventilation area to every 150 square foot of attic floor space (1/150), but does not specify the location of intake vents or exhaust vents, nor does it specify the ratio of intake to exhaust. (EXAMPLE: 2,200 square feet of attic space divided by 150 equals 14.67 square feet of required net free ventilation.) Net free ventilation is the amount of clear air space of a ventilation product after deductions have been made for screens or other features of a vent which block the free flow of air.
The IRC contains an exception to the 1/150 ratio, permitting a ratio of 1/300 if a balance of exhaust and intake ventilation is achieved such that upper vents (exhaust vents) comprise between 40% – 50% of the total net free ventilation requirement. In climate zones 6, 7, and 8, a Class I or Class II vapor retarder may be installed on the warm-in-winter side of the ceiling to qualify for the exception, again permitting use of the less stringent 1/300 ratio. Refer to Section R806 Roof Ventilation of the IRC for full details.
In addition to the requirements and exceptions of the IRC, what is also prudent to consider is the variation of air volume between a low-slope roof and a steep-slope roof as the volume of air in an attic with a 10/12 roof pitch for example, is significantly more than that of an attic with a 4/12 pitch. For this reason it makes sense to voluntarily increase intake and exhaust ventilation for attics with greater air volumes than others. Some ventilation product manufacturers such as Air Vent, Inc. recommend a net free ventilation ratio of 1/150, coupled with a roof pitch adjustment factor to account for increased air volume to determine how much ventilation is actually needed. A free ventilation calculator is available on the Air Vent, Inc. website at www.airvent.com.
It is important to provide ventilation to each rafter bay and eliminate dead areas where moisture and heat may otherwise reside. This will sometimes require ventilation baffles as well as the prescribed amount of intake and exhaust ventilation.
Where should vents be installed?
Vent placement is critical to the elimination of dead zones and to achieve ventilation of all rafter bays and attic spaces. This begins by installing an equal ratio of upper ventilation (exhaust) to lower ventilation (intake), although some ventilation experts recommend installing a higher percentage of intake ventilation to exhaust ventilation in an effort to maximize the flow of air. In no case should exhaust ventilation exceed intake because air is likely to be drawn from one exhaust vent to another and bypass the intake vents in the eaves or lower attic. Installing both intake and exhaust vents in each roof section possible will reduce heat and moisture accumulation in climate zones where one or both of these are prevalent. There are ventilation products available for virtually any situation that a builder is likely to encounter.
What type of ventilation product is the best?
It depends! There are ventilation products available for many different ventilation scenarios including not only common ventilation products such as ridge vents, ventilated soffit, and static vents, but also hip vents, solar-powered vents, roof to wall vents, and edge vents which integrate into the first row of shingles.
When selecting intake vents, it is important to note that not all types of intake ventilation products are permitted where overhangs extend into the fire separation distance as defined in the 2009 IRC Table R301.1(1) since the underside of the overhang must be 1-hour fire-resistance rated.
Other considerations for intake vents include how foolproof they are to install and how resistant they are to damage or blockage. Factors involved in the selection of exhaust ventilation components include the length of the ridge and the volume of attic to be ventilated. Other important factors include the vent’s ability to resist penetration of wind and wind-driven moisture, the UV resistance of the vent and resistance to hail damage.
What else should I know about ventilation?
Never use exhaust vents as intake vents unless rated for use as both. If exhaust vents are used lower on the roof for intake, they are susceptible to penetration of wind-borne moisture as these are not always designed for intake.
Do not mix multiple types of exhaust vents in the same contiguous attic area as the stronger exhaust vent is prone to draw air through the weaker exhaust vent and not through the intake vents as intended.
As stated previously, intake air volume should equal or exceed exhaust air volume, but exhaust should never exceed intake or the intake vents may be bypassed and the attic not properly vented.