Roof box vents and continuous ridge vents are separated by a few differences, and deciding which one to put on your roof depends on which better fits the type and situational roof you have.
Box vents are not mechanical vents and work better when used with soffit ventilation. They're designed to work with open attics and do not need to be placed close to roof ridges to work optimally. They are also known as low profile vents because they are static and installed over a hole cut into the roof. They use natural winds and convection to move hot air and moisture out of the attic into the air outside. You will usually need more than one on your roof to remove all of the heat and moisture from your attic, with the exact number depending on the square footage of your attic.
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 150 square feet of attic space for traditional systems, but only one square foot of ventilation for every 300 square feet 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.
Roofing Ventilation Tips
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 150 square feet 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 300 square feet 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.
Other Types of Vents
- Soffit Vents: These vents are installed in the soffit (the enclosed portion under the roof overhang) and permit air to flow up under the roof and into the attic. They range in style from 6-inch round stainless steel vent covers that are placed in the soffit between each rafter to continuous vents that run the entire length of the soffit.
- Ridge Vents: These vents run the length of the ridge and replace the ridge shingles or tiles. They are designed with interior baffles that permit air to flow out but prevent rain from blowing in.
- Turbine Vents: Common on many roofs, the vent top spins on ball bearings. The slightest wind turns the vent, which in turn draws air from the attic.
- Eyebrow Vents: Also called turtle vents, they provide curved openings on roof slopes. They should be used in pairs with one on each side of the roof to facilitate air movement.