The most important part of your roof is located beneath it.
“A vented attic, where insulation is placed on an air-sealed attic floor, is one of the most underappreciated building assemblies that we have in the history of building science…A vented attic works in hot climates, mixed climates, and cold climates. It works in the Arctic and in the Amazon. It works absolutely everywhere – when executed properly”
—Joe Lstiburek, Ph.D., P.Eng.
Each year, Dreamworx Roofing inspects hundreds of roofs in the midstate area of Pennsylvania. What is the number one thing we notice with all of these roofs? Inadequate ventilation. Ventilation is a topic that is as important to the performance of a roofing system as the shingles themselves. This is the reason we refer to it as a “roofing system.” A system is defined “as a set of things working together as parts of a mechanism or an interconnecting network.” In reference to a home, a professionally installed roof is just one part of the system that protects and insulates your home. The other half of that system is the movement of air within the attic, known as ventilation.
Why Roof Ventilation is Required
As mentioned above, ventilation in its simplest form is the movement of air in and out of an attic space. Without this movement of air, a whole host of problems can begin to form in the attic space. Warm, moist air from the occupied space below can migrate into the attic space through ceiling penetrations such as light fixtures, fans, pipe chases, etc. This warm air, especially in colder northern climates, can begin to condense on the underside of the roof sheathing. Much like the way water droplets form on a soda can that has been removed from a refrigerator, that same condensation can form and soak into your roof deck. In addition to water condensation inside, improper ventilation can lead to ice dams on the roof in the winter months. With an ice dam, melting snow on a roof runs down until it is refrozen at the eave edge. With the attic temperature being higher than the temperature outside, the melting snow can become trapped behind the ice formed at the gutter/eave and begin to back up under the roof shingles. This can lead to damage to the roof decking, walls, ceilings and insulation. A properly vented attic will help keep the ambient temperature in the attic close to that of the outside air, thus slowing or stopping the freeze-thaw cycle that causes ice damming. Self-adhering ice and water membrane can help protect the roof deck in the event there is still some naturally occurring freeze-thaw cycle.
The ABC’s of Attic Ventilation
A proper roof ventilation system is important to the longevity of the roof structure as well as the shingles themselves. We like to think of a well performing roof system in terms of the ABC’s, which stands for:
“A” – Ample amount of ventilation (at least code required minimum ventilation)
“B” – Balanced ventilation between intake and exhaust
“C” – Control (control of moisture and air flow between the attic and occupied space)
A. We want to make sure there is an ample amount of ventilation so that air can flow freely between the soffits and the ridge vent. This creates a continuous flow of air that can move the warm, moist air trapped inside the attic and exhaust it through the ventilation at the ridge.
B. Now that we know there is ample room for the air to move within the attic, we need to make sure we have a balanced amount of intake and exhaust. A balanced system means there is an equal amount of intake and exhaust. This is usually achieved by calculating 25% of your intake needs on one side of your soffits, 25% on the other side of your soffits, and 50% for your ridge vent. A balanced system is achieved by ensuring there is unobstructed intake and clear continuous exhaust and that they are working in equal capacity. (Example: if only one side of your homes soffits are ventilated and you have a continuous ridge vent installed, the system is only 75% complete and will not effectively move all of the air in the attic.) In cases where there are no soffits to use for intake, we will install what is known as “intake vent.” This will create air intake on the roof deck itself as opposed to the soffits.
C. Now that we have an ample and balanced amount of ventilation, we need to control the amount of warm, moist air being introduced into the attic space. Controlling moisture is done by ensuring things like bathroom vents are being vented through the roofs surface and not just into the attic space. Air-sealing penetrations in ceilings such as lights and ductwork can also help in controlling the amount of indoor air entering the attic.
Creating a properly ventilated system
If the attic space under the roof is not adequately ventilated, we can create a properly ventilated system in many ways.
If no exhaust ventilation is present at the ridge or peak of the roof, contractors should take care to cut out enough sheathing (1-1/2″ – 2″) on both sides of the peak to allow air to escape.
What this all means to you
When qualifying contractors for a roof replacement project, it is imperative they be knowledgeable in these basics of attic ventilation. Every shingle available on the market has requirements for ventilation for their warranties to be honored. In fact, some manufacturers will completely void a shingle warranty if the home they are installed on, is found to not be within the proper standards for ventilation. Identifying and fixing any deficiencies with attic ventilation is the first step in ensuring your roof will perform as intended and provide you and your family the peace of mind of a quality, leak free roof over your head for years to come.