Attic Ventilation, The Basics...

It is estimated that 90% of the homes in the U.S.A. do not have proper attic ventilation. It is required in many cases to limit the temperature in the attic as well as to prevent the buildup of moisture.

To understand why it is important, we must look at the basics of heat transfer. Basically, a larger temperature difference between two areas results in a larger amount of heat being transferred from the warmer area to the colder area. The temperature difference between your attic and the living areas affects the rate of heat transfer between them.

In dominantly cold areas, attic ventilation is neither required nor desirable. In these areas, moisture removal is not required under normal conditions. (That is if bath fans are used and properly installed.) It typically does not get hot enough in these areas to require it in the summer time.  

The disadvantage of having a vented attic in these areas is that the attic temperature is kept colder in the winter time which allows more heat to be transferred from the living space to the attic.

This in turn requires more operation of the heating system to replace the heat that was transferred to the attic.

In moderate climates, ventilation is required but the proper design is critical. If there is too little air flow to cool the attic in the summer, the life of roofing materials can be shortened and your cooling bills can be higher. This is because the attic temperature will be higher and more heat will be transferred from the attic to the living areas. If there is too much ventilation, your winter heating bills will be higher than they should be because the attic temperature will be lower at this time of year. Proper ventilation is a balance between the summer and winter requirements.

In hot climates where there is little or no need for heating systems, maximum attic ventilation is recommended. This will minimize the air temperature in the attic and lower the load on the cooling system. (In this case, more is better.)

Building Code Requirements...

Many building codes have standards for attic ventilation but, in general, very few builders, hvac companies, or inspectors pay much attention to the quantity. (And even fewer understand the basic design principles and their importance.)

The HOUSING AND URBAN DEVELOPMENT (HUD) requirements are based on the square footage of the attic floor. For homes where there is no attic vapor barrier, the requirement is 1 square foot of net free area per 150 square feet of attic floor area. For homes where there is an attic vapor barrier, the standard is 1 square foot of net free area per 300 square feet of attic floor area.

What is properly designed attic ventilation?

It is where the required amount of air is introduced to the attic and, at the same time, the proper amount of air is exhausted to maintain desired the desired temperature and moisture level. It is a balanced system.

In most cases, the air is supplied to the attic through vents located in the soffit. The soffit is the area of the roof that extends out from the exterior walls. Typically this is from 10 to 16 inches beyond the exterior wall and is sometimes referred to as overhang. Soffit panels are pieces of metal or vinyl that cover this area of the home. They are normally available in three styles which are solid panels, center vented panels, and fully vented panels. Each type of panel has a ventilation rating that is expressed as net free area per square foot.

There are several ways of exhausting the air from the attic. Passive systems use louvers located in the ends of the home called gables or a vent at the top of the roof called a ridge vent. These louvers have a vent rating that is typically expressed as venting area (V/A) in square inches. Both of these methods use the simple fact that warm air rises and lets this rising air escape.

Active systems include fans that are powered by the home's electricity or, in some cases, they have a built in solar collector. These units are normally controlled by a thermostat to turn them on and off as required to maintain a set temperature. Other systems are called turbines and use the action of the wind instead of electric motors.

The turbine models are great and free to operate but the biggest drawback is the fact that the wind is not always blowing and so on many days you do not have the proper ventilation. Traditional electric powered units are good at providing ventilation when it is required but they cost more to operate. The perfect solution is the solar powered attic fan which provides attic ventilation whenever it is necessary but has no operating costs. They tend to cost more but the energy savings especially during the cooling season can offset the cost. For a comparison of the brands, you can check out our solar attic fan ratings page.

Possible Effects of insufficient ventilation...

So, what can happen if the attic ventilation system is not balanced? In cases where an active system is used, these fans can pull conditioned air out of the living space to make up for a lack of air entering from the outside of the home. The result of this can be backdrafting of appliances that burn gas or fuel oil such as water heaters or furnaces. This can be a potentially dangerous condition and a source of carbon monoxide which can have serious health effects and even cause death.