Roof vents provide ventilation for the space underneath a roof. If an attic is present, roof vents, soffit vents and gable vents also provide attic ventilation. Roof ventilation and attic ventilation are necessary to help remove moisture in the air that may damage wood, insulation and other building materials underneath the roof, susceptible to damage or deterioration from moisture or condensation. The air conditioned living space underneath your roof or attic is usually at a much different temperature than the airspace underneath the roof or in the attic and can lead to condensation forming from humidity rising up from the living space below or especially in summer months from humid outside air. Winter attic ventilation must be sufficient to remove moisture vapor rising from the living space to the attic. In general, ventilation adequate for summer cooling is more than adequate for winter ventilation. Constant air movement or ventilation can help keep the building materials under a roof or in the attic dryer. In hot summer months, a cooler attic or space underneath your roof will also help keep trapped super heated air from pushing down into your living space and will promote more efficient cooling of your living space below. Attics or spaces under a roof can reach temperatures of 150 to 160 degrees F during a summer day, although outside air temperatures may only be 95 to 97 degrees F. The cooling load for a home air conditioning depends on the difference in temperature between the inside and outside air, and reduction of roof space or attic temperatures from 155 degrees to 105 degrees F will result in a significant reduction in cooling load. In a home with poor ceiling insulation, heat movement through ceilings may account for 30 percent or more of the total cooling cost. Heat movement through well insulated ceilings may still account for 12 to 15 percent of the total cooling cost. Roof shingles, roofing tile and other roof structure materials benefit from optimum roof ventilation, which can help prevent deterioration from this excess heat and lengthens the life of roofing. Natural cross ventilation for roofs is typically supplied with high mounted roof vents such as ridge vents or domed pot vents and low mounted roof edge vents or soffit vents, typically mounted in the overhangs (soffits or eaves) of a building. The cooler air enters through the low mounted soffit vents and exits through the high mounted roof vents, providing cross ventilation. Heated air becomes less dense and rises and wind movement around and over a home creates areas of high and low pressure. Natural cross ventilation utilizing this chimney effect and wind movement is the most common and energy efficient method of achieving lower attic temperatures and moisture control. Air powered turbine vents can provide increased ventilation over regular vents.
A rough rule of thumb to figure out much roof ventilation is needed is to take the square footage underneath your roof and divide that by 300 (for new homes with good vapor barriers). That gives you the square footage of total ventilation opening or net free area required. About 60% of that total ventilation opening should be for low mounted roof vents or soffit vents and the other 40% for high mounted roof vents. For example say a home has dimensions of 40 feet by 75 feet and 3000 square feet of space under the roof. Divide 3000 by 300 to get 10 square feet of ventilation space or opening needed. 60% of that 10 square feet or 6 square feet should be low mounted or soffit vents and 40% or 4 square feet should be high mounted roof events. If there are no soffits or overhangs on a building then roof vents alone may be used but the recommended square footage of roof ventilation doubles. So in this example of a 40 foot by 75 foot, 3000 square foot building, 20 square feet of roof ventilation without soffit vents is suggested rather than 10 square feet with soffit vents. Older homes or those without good vapor barriers should use 150 (instead of 300) to divide into the total home square footage to get ventilation area. Local building codes specify the minimum attic ventilation requirements and while you should always at least meet building code requirements, a larger total roof ventilation area usually will provide even better performance.