This means that for every 300 square feet of ceiling area, one square foot of vent is required. The code allows for up to 75 percent of the vent area to be distributed at either the top or the bottom. However, it is generally accepted better practice to evenly distribute the vents between the soffit area and ridge. If it is not possible t o balance the venting arrangement, then it is best to install more of the vents at the soffit rather than the ridge, to reduce the risk of depressurization in the attic as a result of wind. While the 1:300 rule is a minimum code requirement, it should also not be exceeded by a large margin. Providing too much ventilation can cause moisture problems in coastal climates.

After air sealing measures have been performed, the amount of air leakage into the attic should be substantially reduced. In addition, the vapour barrier at the ceiling plane (painted ceiling finish, kraft faced batts, or polyethylene sheet) will limit the amount of vapour diffusion into the space.

Attic ventilation may still be beneficial even if the original attic was not ventilated. Vents could be added as part of the insulation retrofit. When attic vents are installed through the roofing material, a roofing contractor should be hired to properly install, shingle, and seal the vent in the existing roofing. It is common to see an attic vent cause a roof leak due to a poor installation. Holes are best cut through roofing at the time of re-roofing, but this does not always coincide with weatherization work.