The corresponding code commentary reproduced below provides vital clarification of this statement.

Commentary: Item 2, which applies to all climate zones, prohibits the installation of a vapor retarder where it is typically installed at the ceiling level (attic floor) of a traditional ventilated attic. This assures that no barrier is installed which would separate the conditioned attic area from the remaining portion of the home. This requirement gives the attic space a limited potential to dry into the space beneath the attic so that small amounts of excess moisture can be removed from the attic.

Authors’ interpretation: Vapor retarders limit vapor diffusion and, when properly placed, can prevent the formation of condensation on interior surfaces. Examples of Class I vapor retarders include polyethylene film or foil-faced insulation. When improperly placed within a roofing assembly or combined with another strong vapor retarder, such as a roof covering, they may lead to condensation or trapped moisture. Barring use of a Class I vapor retarder on the attic floor of the roof assembly seeks to limit the potential for trapped moisture from accumulating in the attic space (Figure 3).

3. Where wood shingles or shakes are employed, a minimum ¹/₄-in. (6.4-mm) vented airspace separates the shingles or shakes and the roofing underlayment above the structural sheathing.

Authors’ interpretation: Providing a ventilation layer, specifically below wood shingles or shakes, allows back-drying of the wood shingles or shakes between rain events to limit moisture accumulation in the roof assembly and wood decay.

4. In Climate Zones 5, 6, 7, and 8, any air-impermeable insulation shall be a Class II vapor retarder or shall have a Class II vapor retarder coating or covering in direct contact with the underside of the insulation.

Authors’ interpretation: Inclusion of a Class II vapor retarder on the winter-warm side of the assembly produces favorable hygrothermal performance and lowers risk of condensation in cold climates.

Examples of air-impermeable insulation include expanding sprayfoam (open- and closed-cell) and insulated sheathing panels. Open- and closed-cell sprayfoams may be air impermeable when installed in thicknesses greater than manufacturers’ minimum requirements. Closed-cell sprayfoam may form a Class II vapor retarder at certain minimum thicknesses, while open-cell sprayfoams are generally vapor permeable.

Air-permeable insulations include fiberglass batts, semi-rigid mineral wool boards, and most cellulose insulation products.

5. Insulation shall be located in accordance with the following:

5.1 Item 5.1.1, 5.1.2, 5.1.3 or 5.1.4 shall be met, depending on the air permeability of the insulation directly under the structural roof sheathing.

Authors’ interpretation: If insulating below the roof sheathing deck, designers must select an approved insulation method described in the subsections.

5.1.1. Where only air-impermeable insulation is provided, it shall be applied in direct contact with the underside of the structural roof sheathing.

Authors’ interpretation: Insulation that does not allow air movement through it (e.g. closed-cell sprayfoam) must be located on the underside of the roof deck (Figure 4).

5.1.2. Where air-permeable insulation is provided inside the building thermal envelope, it shall be installed in accordance with Item 5.1. In addition to the air-permeable insulation installed directly below the structural sheathing, rigid board or sheet insulation shall be installed directly above the structural roof sheathing in accordance with the R-values in Table 1203.3 for condensation control.