The amount of ventilation one need in the attic has always been a topic of debate. When you think of it, is it alright to have a few extra vents in the attic to make sure that all the hot air gets exhausted properly?
It is alright to have too much ventilation in the attic as long as the intake vent area is equal to or more than the exhaust vent area. But one shouldn’t have too much exhaust ventilation even though having too much intake ventilation is okay.
I am pretty sure you will be wondering why is this the case. Let me explain further;
Why is too much exhaust ventilation a bad idea
This is because when you have a lot of exhaust ventilation they will disrupt the natural flow of air since some exhaust vents will then draw air through the other exhaust vents deranging the entire airflow.
Let us say, your attic has soffit vents for intake and roof vents for exhaust. The number of roof vents actually required is two but you installed four. So here you have an exhaust vent area twice the size of the intake.
Normally, when the wind blows over the roof vents it creates a negative pressure that draws out the hot air in the attic. Thus the inside of the attic will develop a negative pressure. This negative pressure will draw in cool air through the soffit vents. Here the path of the airflow is from the soffit vents to the roof vents.
But when you have too many exhaust vents, the air will flow from one exhaust vent to the other instead of from the soffit vents because they are closer and so they offer a path of least resistance. So the hot air inside the attic gets trapped there and as a result, the attic stays hot which nullifies the whole point of having attic vents.
How much ventilation is too much?
The minimum ventilation needed for an attic is specified by the building code of that area. Normally it is 1 sq.ft of vent area per 150 sq.ft of attic area. However, this value can be 1/300 of the attic area if both of the following two conditions are met;
- Installation of a Class 1 or 2 vapor retarder on the warm-in-winter side of the roof of those houses in climate zones 6, 7 and 8
- The exhaust area should be no less than 40% and no more than 50%, and they shouldn’t be more than 3 feet away from the top of the roof when measured vertically (source)
|Area of the attic||Min. vent area needed (1:150)||Min. vent area needed (1:300)|
|1000 sq.ft||6.67 sq.ft||3.34 sq.ft|
|1500 sq.ft||10 sq.ft||5 sq.ft|
|2000 sq.ft||13.34 sq.ft||6.67 sq.ft|
|2500 sq.ft||16.67 sq.ft||8.34 sq.ft|
|3000 sq.ft||20 sq.ft||10 sq.ft|
However, it is alright to have more ventilation than that is stipulated above if the total intake vent area is equal to or more than the total exhaust area. If the exhaust area is considerably larger than the intake area, that is when the attic ventilation is considered too much.
So if you have an attic of 1000 sq.ft area it should have 3.34 sq.ft of soffits and 3.34 sq.ft of ridge vents (if we are using 1:150).
Can you combine two or more types of exhaust vents on a roof?
While you can use more than one type of exhaust vents on a roof it is not recommended. This is because these exhaust vents may start drawing air from each other jeopardizing attic ventilation.
For example, if you use an attic fan along with ridge vents, the attic fan is most probably going to suck air in through the ridge vents as well in addition to the soffit vents. This not only disrupts the ventilation process but also may draw in water and other pollutants through the ridge vents causing damages to the attic.