The International Residential Code requires that contractors ventilate attics to reduce moisture buildup. Equal amounts of both air inlets, located at the eaves of the roof, and air outlets, located at the peak of the roof (such as ridge vents) are preferred.

Attic ventilation is covered under Section R806.2, Attic Spaces, of the Michigan Residential Code. This section of the code requires that enclosed attics and enclosed rafter spaces have cross ventilation. These ventilating openings must be protected against entrance of rain and snow. Blocking and bridging must not interfere with the air flow.

Section R806.3 states the requirement that a minimum 1 inch of air space be present between the insulation and the roof sheathing.

The net free ventilating area must not be less than 1/150 of the area of the space ventilated. This area can be reduced to 1/300 of the area of the space ventilated when a vapor retarder with a transmission rate of not exceeding 1 perm is installed on the warm side of the attic insulation. In addition, at least 50 percent (50%) but not more than 80 percent (80%) of the required ventilating area must be provided by ventilators located in the upper portion of the space, located at least 3 feet above the eave or cornice vent.

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