One of the most critical factors in roof system durability is proper ventilation. Without it, heat and moisture build up in an attic area and combine to cause rafters and sheathing to rot, shingles to buckle, and insulation to lose its effectiveness. Therefore, it is important never to block off sources of ridge vents or soffit vents, even in winter. Proper attic ventilation will help prevent structural damage caused by moisture, increase roofing material life, reduce energy consumption and enhance the comfort level of the rooms below the attic. In addition to the free flow of air, insulation plays a key role in proper attic ventilation.

An ideal attic has:

1. A gap-free layer of insulation on the attic floor to protect the house below from heat gain or loss.
2. A vapor retarder under the insulation and next to the ceiling to stop moisture from rising into the attic.
3. Enough open, vented spaces to allow air to pass in and out freely.
4. A minimum of 1 inch between the insulation and roof sheathing.

The requirements for proper attic ventilation may vary greatly, depending on the part of the United States in which a home or building is located, as well as the structure's conditions, such as exposure to the sun, shade and atmospheric humidity. Nevertheless, the general ventilation formula is based on the length and width of the attic. We recommend a minimum of 1 square foot of free vent area for each 300 square feet of attic floor—with vents placed proportionately at the eaves (e.g., soffits) and at or near the ridge.
(Note: Minnesota requires an R-Value of 38 in attics and ceilings.)

Venting - Explained Venting for most house attics is provided by the combined use of both soffit and roof vents. The code calls for this level of venting;

1. When roof vents are used alone: the code requires 1 square foot of venting for each 150 square feet of building imprint. Be sure to know the level venting provided by the vents you are using.
2. When roof vents are used with soffit venting: the code requires 1 square foot of venting for each 300 square feet of building imprint with the roof venting providing at least 50%, but no more than 80%, of the venting. Remember, if more soffit venting is required, it is the responsibility of the contractor to provide it at the soffit area or provide more venting on the roof.
3. Determining level of venting: To determine the structure imprint, simply measure the width and length of the structure and multiply the 2 measurements.

Example: A structure having a width of 24’ and a length of 40’ would have an imprint of: 24’ X 40’ total square feet 960.
Homes With 50% Soffit Venting would need:
5 roof louvers - turtle vents or 2 wind turbines - whirlybirds or continuous Ridge Vent (with 2” opening both sides of the ridge).
Homes With No Soffit Venting would need:
10 roof louvers - turtle vents or 4 turbines - whirlybirds or continuous ridge vent (with 2” opening both sides of the ridge). 
Remember: All roofs located over enclosed spaces (i.e. any roof with a ceiling beneath it) shall have cross ventilation for each separate space. Unheated spaces do apply. Commonly missed areas are garages and sheds.