In an older home with a poorly insulated attic and minimal ventilation, heat loss through the ceiling may warm incoming ventilation air enough so it absorbs and carries away surplus moisture. This process isn't very reliable. It also fails to stop moist air leaking up from the house, which is the real cause of most attic moisture problems.
To stop interior moisture from damaging attic surfaces, carefully seal up every ceiling air leakage path to keep humid interior air out of the attic. Make sure exhaust fans don't vent moisture to the attic. No moisture, no problem! Attic venting attacks the symptoms, not the root cause of the problem. Air sealing also improves comfort ... and saves energy!
Good roof ventilation keeps the entire underside of the roof near outdoor air temperature, so snow on the roof isn't melted by interior heat escaping past poorly installed or inadequate attic insulation. Without melted water running down the roof, ice dams can't form on the cold eaves. Roof ventilation is a very useful component of most effective ice dam prevention strategies.
Good ventilation will reduce attic air temperatures somewhat. But ventilation by itself usually won't stop your home from overheating on hot days unless the ceiling is well insulated and air tight.
Studies have found that ventilation has little effect on the surface temperature of asphalt shingles installed over wood decking. Roof colour and the type of roof surface have a much larger impact.
The performance of loose fill or batt insulation can be degraded by cold ventilation air blowing through it. Ceiling air leakage can also be increased by suction from roof-top ventilators.
Prevent "wind washing" at roof edges by using solid materials to direct ventilation air over the insulation and prevent it from blowing through the edges. Baffles can also stop blown insulation from clogging vents.
For maximum performance, use solid materials or pre-formed foam ventilation chutes to physically separate cold ventilation air from rafter or cathedral ceiling insulation. Seal up attic floor air leakage paths to keep heat, and moisture, inside the home.
Yes, IF the ceiling air barrier is virtually perfect and the roof is highly insulated. However, in real life, it is difficult to guarantee perfection. Attic ventilation provides low cost insurance against a small amount of moist air leakage, but it is still vital that the ceiling air barrier be as tight as possible.
The 1995 National Building Code requires that roofs have an unobstructed vent area equal to at least 1/300 of the insulated ceiling area. This ratio increases to 1/150 for low slope roofs. Vents are required to be distributed on opposite sides of the building and at least 25 per cent of the required openings must be at both the top and bottom of the attic space. At least 2.5 inches of clearance is required between insulation and the underside of the roof deck.
For locations with high winter storm exposure, ridge and gable vents can sometimes provide a path for snow to enter the attic. When the snow melts, it rains in your house. If nearby buildings experience this problem, vent type and location must be selected carefully. In extremely exposed sites, the only solution may be an unvented roof, or relying on soffit vents only.