- International Residential Code for One- and Two-Family Dwellings
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 the following conditions are met:
- 1. The unvented attic space is completely contained within the building thermal 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 5.1, 5.2 or 5.3 shall be met, depending on the air permeability of the insulation directly under the structural roof sheathing.
- 5.1. Air-impermeable insulation only. Insulation shall be applied in direct contact with the underside of the structural roof sheathing.
- 5.2. Air-permeable 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 R806.4 for condensation control.
- 5.3. Air-impermeable and air-permeable insulation. The air-impermeable insulation shall be applied in direct contact with 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
CLIMATE ZONE MINIMUM RIGID BOARD ON AIR-IMPERMEABLE INSULATION R-VALUEa 2B and 3B tile roof only 0 (none required) 1, 2A, 2B, 3A, 3B, 3C R-5 4C R-10 4A, 4B R-15 5 R-20 6 R-25 7 R-30 8 R-35 a. Contributes to but does not supersede Chapter 11 energy requirements.