What is the Best Type of Attic Ventilation For My House?
September 17, 2008
According to most building codes, you need one square foot of vent area for each 150 square feet of attic floor space. The minimum is one square foot for every 300 square feet of attic floor space if there is a vapor retarder or the space is balanced between the ridge and intake vents. A balanced ventilation system means about 50 percent of the required ventilating area should be provided by exhaust vents in the upper portion of your attic with the remaining 50 percent provided by intake vents.
Ventilation: Exhaust vs. Intake
The above requirements are from Air Vent Inc and are the same as most every roofing ventilation manufacturer and most state building codes. I am in South Carolina, so yours may be a little different. But the main thing is to make sure you not only have a place for air to escape your attic, but also for air to enter. Just like a car engine has intake and exhaust, an attic has to have the same thing. If the air in your attic sits still it will condensate and cause serious damage to the shingles, sheathing, insulation and ceilings. Ventilation works best if there is at least as much room for air to enter the attic as there is for it to leave.
Air exhausts through power fans, dormers, turbines or ridge vent. Air comes in through soffit vents in the overhang of the house. Most homes built in the last 30 years or so, at least here in South Carolina, will have eaves that either have soffit vents or room to add them. On older homes, or bungalow style houses, you may have to put in a vented drip edge or plugs or something. If you aren’t sure what your house has, look out the window and look up. If you see vents then you should be fine; if not, then ask your contractor to evaluate the situation.
Which Type of Exhaust Ventilation is Best
It depends on the house, but I’ll explain what that means.
If you have a house like this brown brick house below, then ridge vent won’t do you a bit of good because there isn’t enough linear feet of ridge to vent a house this size. But, as you can see, the builder put a ridge vent on it and we had to change the type of ventilation when we re-roofed it. On this house, and any like it, the best option is most likely a power fan, either wired or solar-powered. They come in different strengths and material. They also can come thermostatically and/or humidistatically controlled. As for solar-powered vs. wired, that is for another day. Just understand that with a wired power fan, unless it is also regulated by humidity, it won’t run in the winter when moisture can be a problem. For that reason, some prefer a solar-powered fan –plus it doesn’t take electricity to run it.
But if you have a house like this one with the red roof, then ridge vent is most likely your best option because there appears to be enough ridge to meet the ventilation requirements from the roofing manufacturer.
Above I said it appears to be, because you really don’t know for sure until the attic is inspected and it is determined that the house has the ability for ridge vent to be installed. Your roofing contractor, hopefully, will go in the attic and look to make sure. That isn’t something you want them to guess on.
You could also put turbines on this house as well and be fine, but you would need about 8 of them. Air Vent Inc says 42 linear feet of ridge is equal to 5 turbines, so on a house that is 65 feet wide then you’d need about 8. (Remember that 15 roof louvers or 5 turbine vents would be needed to provide the same ventilation as 42 feet of ridge vent on the same house– Air Vent Inc) That would make your house look like a mushroom field.
Many roofers install ridge vent on homes that have no business having it on there, but they, roofers, just assume that ridge vent is a cure-all and don’t have any idea that it isn’t right for the house.
There are formulas that determine exactly how much ventilation is needed and which type of ventilation is best for the house. I’ll post a link to them at the bottom.
Mixing Different Types of Ventilation
(Click on the Pictures To Make Them Bigger)
But, one of the dumbest things I see roofers do is add ridge vent to a home and leave the turbines or power fans in place. I can only assume they believe you can never have too much ventilation. But, even as a casual observer, you can see from these pictures that it can’t be a good idea to have two different types of ventilation on the roof at the same time
Exhaust ventilation –ridge vents, turbines and power fans– pull air from the nearest air source. If there is no other type of exhaust vents on the roof then the ridge vent will pull air in through the soffit vents in the eaves, up the wood and back out the ridge. But if you add a turbine or power fan, it becomes the nearest air source and the ridge vent will merely circulate air at the top; and the bottom 2/3 of the house will be stagnate. It will essentially short circuit the air flow and negate any advantage it could have provided.
Also with a power fan, when it is running, it will pull air in through the ridge vent.
Calculating the Amount of Ventilation You Need, Both Exhaust and Intake
Air Vent recommends 1 square foot of attic ventilation for every 150 square feet of attic floor space divided evenly between intake vents at the roof’s edge or in the overhang or undereave and exhaust vents at or near the peak of the roof. Here’s what the formula looks like for a 1200 square foot attic space: 1200 divided by 150 = 8 square feet of attic ventilation. Then divide that number by 2 to provide half the ventilation for intake and half for exhaust. Thus, 8 divided by 2 = 4 square feet of attic ventilation for intake and 4 square feet of attic ventilation for exhaust.
The final step is to figure out how many vents would be needed to provide 4 square feet of attic ventilation. To do this let’s start by converting the number to square inches by multiplying by 144. Thus, 4 x 144 = 576 square inches of attic ventilation for intake and 576 for exhaust. Air Vent ridge vents provide 18 square inches of Net Free Area per linear foot. To determine how many feet of ridge vent would be needed the formula looks like this: 576 divided by 18 = 32 feet of ridge vent. A typical 8″ x 16″ undereave vent provides 56 square inches of Net Free Area per vent. To determine how many undereave vents would be needed, the formula looks like this: 576 divided by 56 = 10.2 (which can be rounded up to 11). (From Air Vent Inc)
As for power fans: unless you have a huge attic, one is enough. Power fans can vent attics upwards of 2100 square feet.
Ventilation is needed, recommended and required for a reason. But, like food, too much or the wrong type can be a bad thing. Just because you put something in your mouth does not mean it is good for you. Ventilation is the same way. When you re-roof your home, be sure the contractor does a Ventilation Assessment. That assessment will determine the right type and amount of ventilation for your home.
For Those In South Carolina
If you would like a representative to look at your home to do a ventilation assessment please click here. Also, if you are considering having a new roof installed please click here to have a representative contact you.