Unvented attic assemblies (modern best practice): conditioned roof space with air-impermeable insulation at sheathing

Description

Unvented attic / unvented enclosed rafter assemblies are permitted with strict requirements: the space must be enclosed without vents, with air-impermeable insulation directly against the underside of the structural sheathing (or with a combination of air-impermeable + air-permeable insulation in specified ratios). Climate Zones 5, 6, 7, 8: any air-permeable insulation must be either Class II vapor retarder OR have a Class II retarder painted on. Climate Zones 1-3: air-permeable insulation under unvented attics has its own requirements.

Why this exists

Modern high-performance homes increasingly use unvented attic assemblies — bringing the attic into the conditioned envelope reduces duct losses + ice dams + the risk of insulation moisture damage. The detailing is fussy (the wrong insulation/vapor combination causes condensation in the sheathing) but the outcome is a quieter, more efficient home.

Categories

Applies to

• Jurisdiction: New York State

Source

Solver enforcement

Browsable only — the solver does not currently enforce this directive (no spec-level data to check against). This entry exists so the architect personas can cite it in conversation and the user can read what the rule says.

Related directives

• Natural light in habitable rooms · IRC R303.1
• Habitable space minimum ceiling height · IRC R305.1
• Carbon monoxide alarms in sleeping areas · IRC R315
• Wind design · IRC R301.2.1
• Seismic provisions · IRC R301.2.2