A secondary cause of moisture in a poorly vented attic is by simple condensation. The air inside an improperly vented attic will be warmer than the air outside. When warm, moist air comes in contact with cold sheathing condensation occurs.
“The minimum required net free ventilating area shall be 1/300 of the area of space ventilated, provided a vapor barrier having a transmission rate not exceeding 1 perm in accordance with ASTM E 96 is installed on the warm side of the attic insulation and provided 50 percent of the required ventilating area are provided by ventilators located in the upper portion of the space to be ventilated at least tree feet (914mm) above the eave or cornice vents, with the balance of the required ventilation provided by eave or cornice vents.” If no vapor barrier is present the ratio is typically 1/150.
In the Canada the guideline comes from the National building code. In general terms it states that:
1) The obstructed vent area shall be not less then 1/300 of the insulated ceiling area.
2) Where the roof slope is less then 2 in 12 or in roofs that are constructed with roof joists, the unobstructed vent area shall be not less than 1/150 of the insulated ceiling area.
3) Required vents may be roof type, eave type, gable-end type or combination there of, and shall be distributed:
a) uniformly on opposite sides of the building
b) with not less than 25% of the required openings located at the top of the space, and
c) with not less than 25% of the required openings located at the bottom of the space.
4) Except where each joist space is separately vented, roof joist spaces shall be interconnected by installing purlins not less than 38 mm by 38mm on the top of the roof joists.
5) Vents shall comply with CAN3-A93-M “Natural Airflow Ventilators for Buildings”
In all cases it is recommended that the local building code and/or officials be consulted to ensure the correct ventilation guidelines are followed.
With some building designs the installation of soffit vents is not possible. In this case a roof side intake vent should be reviewed as a possible option.