The International Residential Code for One- and Two-family Dwellings, 2006 Edition, contains the following text regarding unvented, conditioned attic assemblies.

"R806.4 Conditioned attic assemblies. Unvented conditioned attic assemblies (spaces between the ceiling joists of the top story and roof rafters) are permitted under the following conditions:
1. No interior vapor retarders are installed on the ceiling side (attic floor) of the unvented attic assembly.
2. An air-permeable insulation is applied in direct contact to the underside/interior of the structural roof deck. "Air-impermeable" shall be defined by ASTM E283.
   Exception: In Zones 2B and 3B, insulation is not required to be air impermeable.
3. In the warm humid locations as defined in Section N1101.2.1:
   
   3.1  For asphalt roofing shingles: A 1-perm (5.7 x 10^-11 kg/s•m²•Pa) or less vapor retarder (determined using procedure B of ASTM E96) is placed to the exterior of the structural roof deck; that is, just above the roof structural sheathing.
   3.2  For wood shingles and shakes: a minimum continuous 1/4-inch (6 mm) vented air space separating the shingles/shakes and the roofing felt placed over the structural sheathing.
4. In Zones 3 through 8 as defined in Section N1101.2, sufficient insulation is installed to maintain the monthly average temperature of the condensing surface above 45?F (7?C). The condensing surface is defined as either the structural roof deck or the interior surface of an air-impermeable insulation applied in direct contact with the underside/interior of the structural roof deck. "Air-impermeable" is quantitatively defined by ASTM E283. For calculation purposes, the interior temperature of 68?F (20?C) is assumed. The exterior temperature is assumed to be the monthly average outside temperature.”

"R806.4 Unvented attic assemblies: Unvented attic assemblies (spaces between the ceiling joists of the top story and the roof rafters) shall be permitted if all of the following conditions are met:
1. The unvented attic space is completely contained within the building envelope.
2. No interior vapor retarders are installed on the ceiling side (attic floor) of the unvented attic assembly.
3. Where wood shingles or shakes are used, a minimum 1/4 inch (6 mm) vented air space separates the shingles or shakes and the roofing underlayment above the structural sheathing.
4. In climate zones 5, 6, 7 and 8, any air-impermeable insulation shall be a vapor retarder, or shall have a vapor retarder coating or covering in direct contact with the underside of the insulation.
5. Either Items a, b or c shall be met, depending on the air permeability of the insulation directly under the structural sheathing.
   a. Air-impermeable insulation only. Insulation shall be applied in direct contact to the underside of the structural roof sheathing.
   b. Air-impermeable insulation only. In addition to the air-permeable installed directly below the structural sheathing, rigid board or sheet insulation shall be installed directly above the structural roof sheathing as specified in Table 806.4 for condensation control.
   c. Air-impermeable and air-permeable insulation. The air-impermeable insulation shall be applied in direct contact to the underside of the structural roof sheathing as specified in Table R806.4 for condensation control. The air-permeable insulation shall be installed directly under the air-impermeable insulation.

TABLE R806.4
INSULATION FOR CONDENSATION CONTROL

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