Please note: the following provides advice for homes with ventilated attics. There are some styles of vaulted ceiling homes or flat roof homes that have ventilation spaces within the roof itself, and no attic. These ventilation needs are calculated differently.
The key to proper ventilation of your roof and attic is balance: the amount of space you devote to intake must be equal to the amount you allow for exhaust, and these must be calculated according to the size of your attic and slope of your roof.
You can calculate attic ventilation requirements by determining the square footage of your attic floor (its width times its length), and compare that to the total required “net free area” (NFA). Vents are rated by their net free area, or the amount of space for air to flow in or out. This helps make it easy to calculate how many vents you need for your attic, once you’ve determined your requirements.
If your attic floor has a vapor barrier, you will need one square foot of NFA per every 300 square feet of attic floor area (half of that will be intake, half for exhaust). If there is no vapor barrier, double it to one square foot of NFA for every 150 square feet of attic floor space (half for intake, half for exhaust). These are broad guidelines, so be sure to check with your local building code.
(Source: NRCA, Stanley Complete Roofs and Siding, AirVent.com.)
Also, the NFA will be calculated differently if your roof slope is greater than average. If the slope of your roof is greater than 6:12 you will need more ventilation to reflect the extra volume of attic space: Calculate 20 per cent more for 7:12 to 10:12 pitches, and 30 per cent more for a pitch that is 11:12 or greater.
(Source: Asphalt Roofing and NRCA.)
Then, you can begin choosing the style and type of roof vents you want for your home.
It’s important to remember that it’s better to have too much intake ventilation than too little – and many homes often undercalculate the amount of intake needed — so worry less about having too much intake and err on the side of having more.
(Source: House Logic and ARCA.)
The most popular way to ventilate an attic is to take advantage of buoyancy, or warm, moist air’s natural tendency to rise, especially when pushed by cold air beneath it. Homes are naturally filled with warm moist air on a daily basis thanks to human physiology and activities such as cooking and showering.
To take full advantage of this effect, a roofer will generally install intake roof vents for houses lower on the roof (closer to the eaves) while placing exhaust vents higher (near the peak), to let the cold air push the hot air out more easily.
(Source: NRCA and Everybody Needs a Roof.)
How to Install Roof Vents
Before a roofer gets to work installing your roof vents, they will need to determine what type of vents your home needs (or which you prefer) and how many you require according to the calculations above.
Below we describe the different types of roof vents, which will vary according to the needs and architecture of your home, the geography and weather patterns of your region, and your roof structure.
These are general guidelines, as most manufacturers will offer instructions on how to best install roof vents and what measures are required to meet the limited warranty specifications.