R806.3

Section R806.3 is titled 'Vent and insulation clearance' and covers the requirements for providing space for air flow through attics and rafter spaces. The requirement to not block the flow of air, and a minimum dimension that must be maintained between insulation and roof sheathing (measured perpendicular from the sheathing), as well as at vent locations is established. The purpose of this section is to prevent excessive moisture and condensation in attic and rafter spaces which could lead to premature degradation of wood framing, gypsum board, or other absorbent materials. (Note the importance of coordination of this section with the energy code requirements to assure a compliant roof framing design).

The model shows 4 rafters, 2 - 2x10s and 2- 2x12s, with the top 1" of each highlighted, and 2 different insulation heights (only shown at the wall top plate outer edge for clarity). To achieve the 1" clear space, the 2x10 with R-30 insulation actually has two angle ranges - flat to 2.4° and 44.27° or greater; the 2x12 with R-30 insulation achieves the 1" clear space at any angle; The 2x10 with R-38 insulation must be at a very steep angle of 59.32° or greater; and the 2x12 with R-38 insulation must be at 38° or greater.