Ventilation Primer – How Much is Needed?

How Much Ventilation?

Probably less than 3 percent of the homes standing today (new and existing) have proper attic ventilation. Those that do can achieve this goal in several different ways.

The houses that have proper ventilation do so in the following way. Continuous or ample singular vents are placed at or near the top of the roof. A similar system of vents is then placed at the bottom or under side of the roof. This system works like a forced air heating or cooling system in your home. The air that leaves the upper vents is replaced by the same amount of air at the lower vents.

It is extremely important to have both upper and lower vents. This is what produces the flow through and continuous air flow in your attic. Furthermore, the vents should be installed in a specific fashion. Sixty percent of your venting area should be in the lower vents, while the remaining 40 percent should be at the top of your roof.

OK, so you understand that. Well, how much ventilation is required? Simple. The number (or ventilation area) is directly related to the size of your attic. In other words it is a simple ratio.

The amount of ventilation required depends upon whether or not you choose to use continuous or flow through ventilation. It is possible to achieve attic ventilation without flow through or continuous methods, but it is not as effective.

Let's assume you decide to go with continuous ventilation (Smart move!) Just about every model code, the Home Ventilating Institute and the FHA state that the minimum amount of ventilation required is a ratio of 1:300 (net free area). Hold on, I'll explain both.

The ratio 1:300 can be explained readily. It means simply this: for every 300 square foot of attic floor (in reality this equals the ceiling area of all insulated ceilings in your house) you need 1 square foot (free net area) of ventilation space.

Net free area is also easy to explain. Just about every ventilation product, whether it is a metal pot vent, a gable end vent or a continuous ventilation product has some type of screening or barrier which prevents insects from entering these 'holes' in your house. Well, you might not believe this, but the screening restricts or slows the movement of air through the ventilation product. So the manufacturers calculate the real air flow that goes through the hole. It will always be less than the size of the hole when using a non-powered ventilation product.

For example, most common metal pot vents that you see on roofs have a net free ventilating area of only 50 square inches. On the other hand, most continuous ridge ventilation products have a net free ventilation area of 18 square inches per lineal foot of vent.

For the formula of how to calculate ventilation area of a house, click here. This explanation illustrates how to determine the ventilation area needed for a typical ranch house.

Column B335