Have you ever gone up into your attic on a hot summer day? If so, you know how hot it can get inside an attic. This heat buildup is one of the main reasons why proper attic ventilation is critical to maintaining the health of your home. In this blog post, I will answer these questions:
- How does attic ventilation work?
- Why is attic ventilation necessary?
- How much attic ventilation is needed?
- Do all attics require ventilation?
In my almost twenty years as a home inspector, I have noticed that attics seem to be one of the most neglected systems in a home. I suppose “out of sight, out of mind” often applies to attics. Unless you store things up in your attic that you use on a fairly regular basis, it is possible that no one has been in your attic in years. Attics are hot, dark, and dirty. They are not pleasant places to be, so it is no wonder that they are so often neglected.
Attics provide the structure that supports your home’s roof. They provide a space to run utilities such as electrical wiring, ductwork, and plumbing. Attics also are one of the barriers between the outside and the living space of your home, providing a place for things like insulation which helps to keep your home comfortable during both winter and summer. In order for your attic to perform its job well during the life or your home, it is important that it be properly ventilated.
How Does Attic Ventilation Work?
Ventilation only works when air flows, and for air to be able to flow, there must be two types of openings: exhaust openings that allow air to leave the attic, and intake openings that allow air to enter the attic from outside.
Without both types of openings, an attic will never be properly ventilated. Without intake openings to bring air into the attic, air will not be able to leave the attic, and without exhaust openings to allow air to leave the attic, air cannot enter the attic.
With this in mind, it should be obvious that the amount/size of exhaust and intake openings should be equal in order to provide for balanced air flow. If ten cubic feet of air leave the attic, then ten cubic feet of air must enter the attic to replace it. Because the attic must remain at atmospheric pressure. You cannot have pressure or a vacuum in the attic.
As you know, hot air is lighter than cooler air, and therefore rises. This is the principle behind convection or natural circulation, or the stack effect – hot air rises and cooler air falls. The stack effect occurs when hot air rises and creates a higher pressure at high points in the attic. Hot air that escapes is called exhaust. As the hot air escapes, cooler, low-pressure air enters the attic is referred to as intake.
In order to be properly ventilated, an attic needs intake vents that are located in the lower portion of the attic, and exhaust vents that are located in the upper part of the attic. I have inspected many homes that only have one source of ventilation such as one soffit vents, gable vents, or turbine vents. I have also seen many attic with vents at approximately the same elevation, such as turbine vents and gable vents. Neither of these situations will ever provide adequate ventilation for an attic.
There are two primary methods to create airflow within an attic:
- Mechanical. Requires a power source.
- Natural. Whenever possible, natural roof ventilation is used. The stack effect and the wind effect work together to circulate air naturally.
The stack effect occurs when hot air rises and creates a higher pressure at high points in the attic. Hot air that escapes is known as exhaust. However, this hot air cannot escape without an inlet for cooler, low-pressure air. Cool air that enters is referred to as intake.
When the wind blows against the outside of a roof and increases the volume of intake and exhaust, it is known as the wind effect. Intake and exhaust generate the natural flow of air to create a well-ventilated attic.
Why Is Attic Ventilation Necessary?
Attic ventilation serves two main purposes: to remove heat and excess moisture from the attic. Excessive heat and moisture can both cause serious problems to homes as I will discuss below.
In all areas of the country, attics can get very hot in the summertime. As a home inspector, I have measured some attic air temperatures above 150 degrees during the summer. I have measured the temperature of some of the wood such as the roof decking much higher than that. As the sun beats down on the roof, the roof gets very hot. Much of this heat is transferred through the roof decking and into the attic.
Take a look at the photos below.
Based on these photos, you can see just how hot it can get in an attic. In the attic shown above, it is 142 degrees just about 6 inches above the living space in the home. This shows why attic ventilation (and insulation) is so important. Regardless, some of this heat will find its way into the attic, so the cooler we can keep it in the attic, the cooler our homes will be.
If the attic is properly ventilated, the stack effect will cause the hottest of the air to leave the attic through the upper ventilation openings (such as ridge vents, gable vents, or turbine vents). As the hot air leaves the attic, cooler air from the outside is pulled into the attic, moderating the temperature in the attic to a safe level.
In an improperly ventilated attic, the hot air is not able to escape as easily, so the heat essentially continues to build up in the attic until the sun goes down and stops heating up the roof. This results in superheated air in the attic. This leads to dangerously hot temperatures for anyone entering the attic, and this amount of heat can cause some serious problems to our homes.
Rapid Deterioration of Shingles
The first problem caused by excessive heat buildup in attics is that it can cause much more rapid deterioration of our home’s roof. The high heat essentially cooks the shingles and causes them to deteriorate much more rapidly than they otherwise would. The high heat can also damage the wood in the attic, making it almost brittle and/or causing it to warp.
Increased Cooling Costs
The second issue with high heat, is that if you have a large space above the living area of your home with temperatures around 140 – 160 degrees, it is certain that no matter how much insulation you have in your home, some of that heat is going to make it into the living area of your house. This can make your home hotter and less comfortable, and it will certainly increase your cooling costs.
Ice DamsThe third problem with heat buildup in the attic is ice dams. This is a problem that only occurs in colder climates. It is unavoidable that some heat will escape from the living area of your home and end up in the attic during the cold winter months. (The better your home is insulated, the less heat that will be able to escape, but some will always be able to escape.) If your attic is properly ventilated, this heat will also quickly escape the attic – keeping the temperature in the attic below freezing and preventing ice dams from forming.
An ice dam is a buildup of ice that forms near the edge of a roof and prevents water (from melting snow) from draining off the roof. Instead, the water accumulates behind the ice dam and can leak into a home, causing damage to the attic and to the home’s interior.
Shingles are designed to prevent water from leaking through them as it flows down a roof, but they are not designed to prevent water from leaking if it is flowing from a lower area to a higher area, or if it is simply sitting in a large puddle on the shingles. As a result, water is able to get between the shingles and start leaking through the roof decking and into the attic. As a result of these water leaks, ice dams can cause significant damage to the home.
In summary, proper ventilation is required in order to prevent excessive heat and moisture buildup in the attic as well as to prevent ice dams from forming on the roof. All three of these issues can be costly, cause discomfort for the home’s occupants, and cause significant damage to the home.
How Much Ventilation is Needed?
Current standards call for one square foot of ventilation for every 300 square feet of area in your attic. To clarify this, that is one square foot of intake, and one square foot of exhaust for every 300 square feet of space in your attic or home. (For a one-story home, the attic space is the same as the square feet of living area in your home plus any garage space or other space with an attic above it). For example, if you have an 1800 square foot home, then you would need six square feet (1800/300 = 6 ) of intake ventilation, and six square feet of exhaust ventilation for your attic.
There are some factors that can affect how much ventilation an attic needs. For example, if you have an electrically-powered fan installed to help ventilate your attic, this should lower the total area of ventilation required. You will have to look at the specification of your particular fan or other device to determine how it affects the required surface area.
Do All Attics Require Ventilation?
Here is an example of an attic insulated with spray foam. Notice how there is no insulation on the attic floor in the photo on the right.