The stated vent areas for attics, roof/ceilings, and crawl spaces refer to net free areas through which air can pass unobstructed. When screening, louvers, or rain/snow shields cover vents, the area of vent openings should be increased to offset the area of the obstruction. Use manufacturers' stated net free areas whenever available. If no information is avail- able, use the conversion factors for determining gross area of vent openings listed in Table 1. Caution should always be exercised to assure that ventilators are not obstructed. Blocking or baffles should be used at the eaves to keep insulation material away from soffit vents. Recommendations for air space between the roof sheathing and the insulation may vary. As a general rule of thumb 1" is often used. (See Figure 2.)