N1102.2.1

Section N1102.2.1 is titled 'Ceilings with attic spaces' and covers the requirements for an R-value reduction allowance in attics, when using the component values from table N1102.1.2 as the compliance method. Where the table requires R-49, a reduction to R-38 is allowed; and where the table requires R-38, a reduction to R-30 is allowed. For the reductions to be allowed, the uncompressed insulation over the top plate (nearest to the eave) must meet the full R-value.

When applying this measurement, the air ventilation requirement of section R806.3 must also be taken into consideration. Presuming a vented attic is being constructed, a 1" space (measured perpendicular from the sheathing) must be provided between the sheathing and the insulation to allow for free air flow.

The model shows 4 rafters being installed - a 2x10 and 2x12 for an attic with R-30 insulation; and a 2x10 and 2x12 for an attic with R-38 insulation. The insulation being used has an R-value of 3.7/inch, hence thicknesses of 8.11" for R-30, and 10.27" for R-38 (the height of insulation is only shown at the top plate outer edge for clarity). In order to achieve compliance with both this section and R806.3, the rafters must be at a minimum pitch to attain the height needed. The rafters are highlighted at the top to show the 1" space that must be kept exposed to allow for air flow. The 2x10 with R-30 insulation actually has two angle ranges where it is compliant - flat to 2.4° and 44.27° or greater. The 2x12 with R-30 insulation is compliant at any angle. The 2x10 with R-38 insulation must be at a very steep angle of 59.32° or greater to be compliant; and the 2x12 with R-38 must be at 38° or greater to be compliant.