So your ready for a new roof and what you don't know about that contractors bid is the ventilation system is about to change! Why is your contactor changing your existing ventilation system when the old one works fine? Because it will look better? Sadly looks are not everything and the majority of the roof installers couldn't even tell you your homes square footage let alone the cubic feet of air intake currentry active with your building design. Ridge vent must have at least an equal amount of soffit ventilation to balance the airflow. There can be more soffit ventilation, but NEVER less soffit ventilation then ridge ventilation. PNWIG has never seen any roof in over 25yrs of inspecting make changes to the air intake when replacing roof cover. So what does this mean for you? ATTIC VENTILATION ISSUES and MOLD!! So your contractor that has bid to change your ventilation needs to know your attics total SF. the square footage of house under roof X .48 divide this number by 20 (for V-600® products), or by 13.5 (for V-300® or Fold-A-Vent products) this will give the lineal feet of product needed to meet a 1/150 vent ratio. If you are only going for a 1/300 (code minimum) vent ratio divide the above result by two. REMEMBER there MUST be an equal amount of soffit vent (in NFVA) to balance the ridge vent. Also a concern is replacing a shake roof that itself breaths differs than the new composition roof about to be installed. Now your contractor is not only about to change your ventilation but also in most cases add roof sheathing and new roof cover that doesn't let the attic breathe as easy.
Continuous ridge vents are more effective because they are installed at the peak of a roof's ridge, allowing for warm air to escape from the attic. It also works better because it creates a vacuum. It has the ability work with vaulted ceilings, and you only need one of them to get the job done for ventilation, as compared to multiple box vents. It is the more expensive option of the two, but it is still non-mechanical, so you will not need to spend money on electrical issues or failures in the system. These systems are best for shingled roofs.
Roofing manufacturers' warranties require a minimum of one square foot of ventilation for every 150sf of attic space for traditional systems, but only one square foot of ventilation for every 300sf when the ridge vent system is used. That is a good indication of the difference in efficiency. There are many good ridge vents on the market, and the continuous vent, as opposed to the individual roof vent, is the most effective. Continuous ridge vents allow hot air to readily escape from the roof peak. Most manufacturers now use a durable plastic, or aluminum, to avoid rust. If using plastic in cold climates, make sure it is rated to withstand subfreezing temperatures.
Improper ventilation results in warm, moist air generated in the house meeting with colder air in the attic, which causes condensation. Ventilation problems can be seen in a variety of ways, such as blistering and peeling paint on the gable end exteriors because of excessive heat and moisture. Buckling roof shingles are another sign of poor ventilation, as are ice dams in the Midwest and Northeast. Water stains on a ceiling may actually be a result of condensation moisture dripping from under the roof rather than a leak.
Ideally, air circulates under the roof by being drawn up through a continuous soffit vent, through the attic space, and then out the ridge through a continuous ridge vent. This is the smoothest and most efficient system. But few houses have continuous soffit and ridge vents. Most houses, however, do have some type of vent in the roof or an opening at gable ends. Houses with no rafter overhang, or a very short one, may not have soffits. Older houses may not have vents between the rafters leading into the attic space. Venting was not a major problem in older houses because they leaked air everywhere. But with the advent of better roofs and tighter house construction, moisture became a more significant issue.
Calculating the number and size of roof vents is not an exact science, but instead is influenced by the climate, the roof pitch, available locations for vents, and the house's orientation to prevailing winds. The Asphalt Roofing Manufacturers Association states:
"In most cases, a minimum free-flow ventilation area equal to one square foot per 150sf of attic floor area must be designed and properly installed to provide proper ventilation. Where a properly designed and installed eave and ridge ventilation system is employed, a free-flow ventilation area equal to at least one square foot per 300sf of attic floor area is often sufficient. Combination eave and ridge venting is generally recognized as a superior venting technique."
Vent screens should be cleaned regularly because accumulated dust and grime can significantly restrict air movement. Fans connected to thermostats can be placed at gable end openings to draw hot air out of the attic space when temperatures reach a preset level.