The Science of Ventilation

When most people think about roofing ventilation, many different attic ventilation myths come to mind. We are going to discuss 5 of the biggest myths when it comes to roofing ventilation. Attic ventilation can be one of the most misunderstood aspects of your roofing system. The American Institute of Architects claims that over 90% of the homes within the United States have over the ideal level of moisture content in their homes.
Attic ventilation is an important component of any roof. Ventilation is required by the building code, as well as the shingle manufacturers to qualify for the manufacturer’s warranty. Appropriate attic ventilation will aid in getting the maximum life expectancy from your roof. Having the right ventilation will reduce the temperature gradient from outside air to the air inside your attic, the smaller the temperature difference from outside to inside the less likely moisture is to accumulate in your attic.
1. More Attic Ventilation is Good
The same as buying the right pair of pants, correct size shoes, making sure you have the right amount of attic ventilation for the size of home is equally as important in making sure things operate correctly. If your home has insufficient ventilation it can lead to a wide variety of problems, during the winter it can lead to excess moisture build up in the attic and during summer months can lead to decreased efficiency of your HVAC system. On the opposite side of the spectrum too much attic ventilation can be just as bad. In the most basic sense an additional roof vents creates an additional hole in your roof (roof penetration) which can act as another access point for water to enter into your home. You never want to increase the number of roof penetrations in your house for no reason, this can lead to leaks from blowing rain or as an access point for sparks or smoke during a fire.  Rodents and Birds often make their way into a homes attic through the delicate mesh filters in the attic vents.  The most common type of plastic attic vents can easily warp and crack in extreme temperatures, causing catastrophic leaks.

So, what amount of roof ventilation is the right amount? Without exception you should speak with a professional roofing contractor that has formal certifications in ventilation theory and have them conduct an inspection to determine the ventilation requirements of your home. As a general rule you can follow the 3-1 Ventilation Rule, so for every 300 sq/ft of ceiling space you want to have 1 sq/ft of attic ventilation. Ideally this would be 1 sq/ft of un obstructed intake ventilation (soffits) and 1st/ft of exhaust (roof vent). If your homes attic is sectioned off into various partitions determining the correct amount of ventilation can be more difficult.  Thickness of the insulation, height of the main ridge relative to the height of secondary ridges, vaulted ceilings…all of these things affect the basic ventilation rule.

2. Roof Vents and Attic Ventilation are for warmer climates
The majority of people believe that the only importance of roof attic ventilation is to increase the energy efficiency during the summer months. Although proper attic ventilation can help with this it is equally as important to take into consideration shingle color, sun exposure, and insulation. In fact, the colder the climate the more likely that your home will benefit from having the proper attic ventilation. When dealing with warmer climates you don’t need to deal with condensation – think about how often dew forms on your grass. In these climates, hot attic spaces are eliminated by installing a thermal barrier along the roof line, instead of the attic floor.

3. Roof Vents remove warm air during winter months.
Many people get this misconception that because hot air rises, the attic ventilation draws all the heat upward in your home and pulls it out of your home causing un-necessary drag on your homes furnace equipment. If you are noticing increased strain on your homes HVAC system during the winter months you have much larger problems than attic ventilation and you should really be focusing on the insulation. In the majority of homes, your furnace should not be heating your attic space, unless your home is designed with insulation directly on the roof deck and is designed without ventilation. The worst situation is when, due to poor insulation, warm humid air is allowed to enter the attic space from the interior of the home, when this warm moist air hits the roof it forms condensation which will deteriorate your roof decking, ruin the insulation and causing interior damage in the process. As an easy test for you wait until the sun goes down and enter into your attic with a digital thermometer and flashlight and take a reading, this reading should be almost the same as the exterior temperature.

4. Attic Ventilation Research
In some ways, roof attic ventilation is as much as an art as it is a science, and installing your own roof vents based on something you read online is like trying to diagnose a skin rash using WebMD. Finding a trusted and experienced roofer who has worked in your region for his or her entire career is a better judge for your particular roof than any research study or online “expert.”

5. I have Roof Vents so I must have Attic Ventilation
It is hard to find a group of people that will agree on what the best attic ventilation system is but everyone will agree on the importance of attic ventilation. For example systems such as a ridge vent which when installed correctly using the right product is arguably the best roofing system, while if you install a ridge vent without baffles that prevent outside air from crossing over the vent, a ridge vent may create almost no attic ventilation at all, as this outside air crosses over the vent and keeps air trapped inside your attic. Gable vents may circulate air through only a small percentage of your attic. Static, roof-line, vents are effective for ventilation, but generally aren’t recommended due to issues with leaks. Soffit vents may leave air trapped at the top of your attic. Most effective ventilation uses a ridge-and-soffit continuous ventilation system, but even these designs can vary from roof to roof.
If you don’t know how your roof vents work, or if you’re unsure about your attic ventilation in general, you should talk to a roof inspector about your current system and any inherent weaknesses that may be at work. The risk/reward for having no attic ventilation or poor attic ventilation, along with the negligible cost of installing a good-working attic ventilation system makes them one of the unforgivable sins of home maintenance negligence.