If you have properly sealed the attic you should not need more attic ventilation. Attic ventilation is overrated. In winter, the cold outside air cannot hold much humidity or carry moisture away from the attic. In summer, attic temperatures are more affected by the sun and shingle colour than by the amount of ventilation.

Recent research shows that identical attics, with one unvented and the other vented to code, have much the same humidity and temperature. Attic computer models show that attics in damp coastal climates may actually be drier with less ventilation.

Building codes require attic ventilation. Ventilation may make a difference in a borderline situation.

Attic ventilation is driven primarily by wind. To ensure thorough venting, have openings at the soffits and then higher on the roof at the ridge, gable end, or high on the roof surface. The requirement for attic vent sizing is nominally 1:300 (or one square foot of vent size for every 300 square feet of attic floor area). If you wish to improve your attic venting, ensure that it is as well distributed as possible. Do not worry about meeting the 1:300 requirement exactly. Vents should be screened to keep out animals and insects. If you are using soffit vents, make sure that there is a space between the roof sheathing and the insulation for the ventilation air to pass. Commercially available plastic or card board forms can be used, or the extruded polystyrene board option.