9.19.1.3. Clearances
This Article requires that sufficient clearance be provided to ensure adequate #ventilation flow. Unless air can flow freely between the #insulation and the roof deck to reach a vent to the exterior, venting will not be effective. As this space is reduced, the resistance to #airflow increases.
Where thick insulation is used in #attics, particularly attics with low roof slopes, the insulation could completely close off the air space at the lower part of the roof so that the air from the #soffit vents cannot reach the attic space. The insulation thickness must be reduced at these locations either by cutting the insulation on the slope to provide clearance between the insulation and the roof deck, or by depressing the insulation by means of specially designed void forms installed between the roof framing against the deck (Figure 9.19.-5).
Insulation must be installed in a manner that ensures that a 63 mm (2 1/2 in.) air space is provided between the insulation and roof #sheathing. Where #baffles (Figure 9.19.-5) are used to contain the insulation at the #junction of a sloped roof and an exterior wall, the clearance can be reduced to 50 mm (2 in.), as long as the overall ventilation area meets the requirements of NBC Article 9.19.1.2. Alternatively, the roof framing members can be increased in depth to create additional insulation space.
Quoted from Illustrated Users' Guide - NBC2015
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