Why ventilate a roof? “The main purpose of roof ventilation is to keep the air space above the roof insulation more or less at the same temperature as the outside air.”
That’s Sarah Gray, an engineer with RDH Building Science, talking about best practices for venting a roof. “Roof ventilation matters in cold climates,” she says, “because … if warm air—particularly warm, moist air—gets into the attic, that can lead to ice damming and also to condensation on the underside of the sloped roof itself.” Moisture from leaks at ice dams or condensation is bad news for wood rafters and roof sheathing, not to mention insulation, drywall, and other finish materials. “If those elements get wet from condensation,” Gray says, “it can lead to deterioration over time.”
Roof ventilation also makes sense in hot climates. “In hot southern climates, attics can get really, really warm,” Gray says. “And it’s not just an issue of stopping condensation, it’s also an issue whereby—as the roof gets really, really hot—the materials on that roof can start to deteriorate.” That can cause the shingles to curl, crack, dry up, and wither away.
How Ventilation Works
One mechanism driving simple attic ventilation is convection. In cold climates, Gray explains, “As the house loses heat … into the attic, either by air leakage, conduction, or convection, the top part of the house and the roof will get warm.” In a ventilated roof assembly, the warm air escapes out at the top and replacement air is drawn in at the bottom. This constant air movement flushes heat buildup, which prevents ice dams in winter and reduces the load on the AC system in summer. And, Gray adds, “we also have some air movement to help clear any condensation or moisture out of the roof assembly.”
How to Ventilate
You can vent the attic, the roof, or the roof cladding—the choice partly depends on whether the attic is living space and on where the insulation is located. The simplest example is an unoccupied attic with an insulated floor. In this case, the area above the floor needs to be flushed with outdoor air .In a finished attic space or a room with a vaulted or cathedral ceiling, the insulation is often placed between the rafters. In this case, use baffles the entire length of each rafter bay to create a clear path for air from the soffits to the ridge vent. Where rigid roof insulation is placed on top of the roof sheathing, ventilation may still be a good idea because a thick blanket of snow can trap heat from sunlight and lead to melting at the roof surface, which can cause ice dams. To ventilate this kind of assembly, create a space between the insulation and the roof sheathing to allow air to move from soffits to ridge.
How Much Ventilation Is Enough?
Roof ventilation won’t work properly if there isn’t enough air movement or enough air. How much is enough? “The amount of venting required for roofs is typically based on the area of the floor space in the attic,” according to Gray. For intake and exhaust combined, the 2015 IRC sets the ratio at 1/150, or 1 square foot of ventilation “net free area” for every 150 square feet of attic floor. Ventilation products typically list square inches of net free area per linear foot, which doesn’t include anything that obstructs airflow, such as insect screen or louvers in the vent. You can cut the amount of ventilation area by half to 1/300 under certain conditions, but code varies by location so check your local requirements.