Does your attic singe the brows from your face when you open the hatch? If so, you might want to learn a little about attic ventilation. Improperly ventilated attics can come with a mess of different problems such as:
Overworking your HVAC system,
Moisture build up in the attic, and
Whether it’s repair, removal, or replacement each problem will cost you money. The good news is all of this can be avoided by installing the proper ventilation and having a solid understanding of your home’s attic. Hopefully this article will serve you and many other homeowners as an informative crash course on the science behind attic ventilation.
Where To Begin!?
Because there is so much that goes into creating a proper ventilation system, it is difficult to narrow down a starting point. Before we even think about looking at the ventilation in your attic we need to evaluate your insulation situation. If your attic is not properly insulated and sealed then conditioned air from your living space will make its way into the attic. Any place the attic floor is penetrated due to a light fixture, an air duct, or chimney is a potential pathway for conditioned air to escape. Any kind of ventilation system will do more harm than good if these areas are not sealed properly. Without ventilation the warm air from your living space will be trapped and cause moisture buildup, ice dams, and harm to your roof. With ventilation the conditioned air is given a way to escape the house entirely. So, are you dammed if you do and dammed if you don’t? The answer is simply no. Most homes’ attics are insulated with blown-in cellulose or fiberglass. The problem with this insulation is you can not see the attic floor. The best thing you can do in this situation is hire a qualified insulation installer. They will go through and seal any pathways present in the attic floor. Air tightness is so important to the overall health of an attic it should really be the first step in the ventilation process.
For both your benefit and my own I am including a diagram of a Vented vs. Unvented home. A clear visual of the two should make the concepts discussed a little easier for you to follow and for me to explain.
In this diagram we can clearly see the movement of air as well as the transfer of heat in the way of arrows. We can also see the differences in an attic during the warm summer months and the cold winter months. The two extremes can affect your attic in different ways. Using the diagram as a reference, I will talk about how they affect your attic and how a proper ventilation system and insulation can spare you from sinister seasons.
On a hot summer day you can expect an unvented, poorly insulated attic to be at least 30 to 40 degrees F warmer than the outside temperature. If the outside temperature is 85°F then the attic could be anywhere between 125 and 135 degrees F. These are the brow singing temperatures we all know and love. There are two major issues you’re facing during the warm summer months: hot roofs and radiant heat.
A hot roof is created when the temperature beneath the roof decking is greater than the outside temperature. The excessive buildup of heat within the attic essentially bakes asphalt shingles. The shingles blister, buckle, and become brittle. At this point, a few more seasons of expansion and contraction will crack the shingles and give water a pathway into your home. In short, hot roofs will accelerate the aging process of your asphalt shingles and leave you needing a new roof sooner than expected.
Radiant heat is another issue facing unvented attics during the hot summer months. By definition radiant heat is “heat energy transmitted by electromagnetic waves in contrast to heat transmitted by conduction or convection”. I know, I know science was not everyone’s thing back in school. That is why I am here to explain how this pertains to you and, more importantly, your electric bill. When your attic is reaching temperatures of 120°F+ the trapped heat radiates down into your conditioned living space. Before long your HVAC system is in constant competition to keep up with the radiant heat and your electric bill skyrockets. If your HVAC system can not keep up with the constant demand for conditioned air then you are left with an uncomfortably warm living space.
The summer months can be hard on your attic, your roof, and your bank account but with proper ventilation it’s a breeze. An ideal ventilation system will keep an attic within 10 to 15 degrees F of the outside temperature. If the outside temperature is 85°F then the attic temperature will fall somewhere around 95 and 100 degrees F. With an ideal ventilation system a great toll on both your HVAC system and your bank account is lifted.
The cold winter months come with a mess of problems for an unvented attic. When we heat our homes the warmth from our living space can radiate into the attic and become trapped. Snow on the roof can magnify the problems as it helps hold the heat in the attic. Snow acts as a natural insulator for the heat within the attic. Some of the problems associated with an unvented attic during the winter months are moisture build up, ice damming, and mold.
Moisture Build Up
An unvented attic will experience moisture build up in the form of condensation. This is due to the transfer of radiant heat from the living space becoming trapped in the attic. The warm air in the attic comes in contact with the cooler roof decking and forms trace amounts of moisture. Given enough time that moisture will make its way down the rafters of your attic and into the living space. It can also lead to the rotting away of your roof decking. Whether it sits in your attic and rots away the roof decking or it finds its way into your living space, the end result is water damage.
To go along with moisture build up is everyone’s favorite, mold. Mold is a serious problem and given an enclosed, humid environment it will thrive. Unvented attics are especially prone to mold growth for that exact reason. Mold has the ability to cause structural damage as well as rot within the attic space. It may even spread its spores to the rest of the home given the HVAC system is located in the attic and is improperly sealed. Attic mold is a danger to your health and you must take the proper steps to prevent such a hazard.
Ice dams are by far the biggest concern for an unvented attic during the winter months. The roof of an unvented attic will be the warmest at its center and coolest at the edges. During the winter, snow melts at the warm center and then refreezes as ice at the cooler edges. Large formations of ice result in what is called an ice dam. Shingles are only meant to experience water one way, down. The water caught in the ice dam has nowhere to go but up and under the shingles. At this point, one of two things will happen: either the water follows the path of least resistance into the attic or the water reforms as ice under the shingles. If the water makes its way into the attic then the entire problem becomes compounded. The water from the ice dam will dampen the insulation, decreasing its R value, allowing more heat into the attic (as seen in the diagram). You will have water damage in your living space, attic, and possibly the interior of your walls. If the water reforms as ice under the shingles then the expansion will push the shingles upward and possibly loosen the nails securing them. Ice dams can do a fair amount of costly damage to your roof and in turn your bank account.
The great news is moisture build up, mold and ice dams can all be avoided through the installation of a proper ventilation system. A well vented home can withstand just about everything winter throws its way and, given your roof is in suitable condition, keep you leak free.
Now that we have discussed the dangers facing unvented attics, I think we should take a look at the kinds of ventilation available. While we are discussing the ventilation options, keep in mind that it could do more harm than good if your attic is not properly sealed and insulated.
Not all attics are alike, they all require a custom ventilation system which works with their design. This kind of service can be best completed by your local roofing specialist. Deciding between contractors can be a tricky business, that is why I have written a blog dedicated to guiding homeowners through that process. To read this blog posting see Choosing The Right Contractor on the Roofer Dude® Blog page.
The most efficient systems have both an intake and an output. The intake will be a pathway for fresh air to fill your attic space. The most common form of intake will be soffit vents. The output is a pathway for air within the attic space to escape. There are many forms of output such as ridge vents, power attic vents (PAVs), wind turbines (whirlybirds), gable vents, and box vents. Having a healthy combination of both intake and outake will produce the air circulation your attic needs.
Ridge vents are the most popular choice for ventilation and are installed on
nearly every new home. They are a low profile option and combined with soffit vents become a force to be reckoned with. While they have no moving parts, they do a great job at pulling air from all the nooks and crannies of any attic. Because they are located at the highest point in the attic, hot air finds its own way through convection. The soffit vents assist in this process by providing fresh, cool air from the lowest point in the attic. The combination of these two is seen in the diagram above. Together they create a healthy amount of circulation within the attic and promotes the prevention of ice dams.
Power Attic Vents (PAVs)
A PAV is another option available to you. They come in all shapes and sizes and are most efficient when installed near the ridge. There are electrically powered models which are hard wired into your home’s electrical and there are solar powered models which require no electrical connection. The solar powered vents are low profile and make very little sound. They really are a great choice if you decide to go with a PAV. To learn a little more about solar attic fans I recommend the Solar Attic Fans Just Make “Cents” blog post.
Wind Turbines (Whirlybirds)
Wind turbines, like solar powered PAVs, are another energy saving solution to attic ventilation. They use the power of the wind to pull out any air trapped within the attic. The quality of the vent varies between manufacturers so be careful when shopping around. They are prone to being a little on the noisy side and this is especially true for more economical models. They work best when they are located closest to the ridge and assisted by soffit vents.
Box vents have no moving parts and act only as a pathway for hot air to escape through convection. Like PAVs and whirlybirds, they work best near the ridge of the roof and with the assistance of soffit vents. If you are only going with box vents then the more the merrier. Individually they will have little effect on the circulation of the attic but in large quantities they will give sufficient output for a healthy amount of circulation.
Like box vents, gable vents have no moving parts and vent the roof by pulling air in one side of the attic and out the other. For this reason they do not pair well with a ridge and soffit vents. Ridge and soffit vents pull air vertically through convection while gable vents pull air horizontally through displacement. Adding more ventilation pathways doesn't always mean a better system. Gable vents are most commonly found on older homes with an outdated attic. With that said, outdated ventilation is better than no ventilation and if you're dead set on gable vents I say go for it.
There are a lot of great ventilation systems to choose from, it’s really a matter of finding what works best for your attic. And again that information is something a roofing professional can help with. Remember, the best ventilation systems have a healthy amount of both intake and output to create air flow within the attic space.
It might be worthwhile to take a walk around your home to see if you have a ventilation system for your roof. If not, hopefully this information will inspire you to do some more research into what will work best for your situation. If you do have a ventilation system, it may be a good idea to have it evaluated to be sure it’s working the way it should. In the end it will save you money whether it’s through saving energy, extending the life of your roof, or preventing water damage. Thanks for reading, please feel free to ask questions in the comments section below and if you found this blog posting informative there is a button for Facebook sharing above the comments.