8.5 Roof & Ceiling Insulation, Vapor Retarders, Air Sealing & Unvented Assemblies

Michigan Climate Zones (Table N1101.10)

Ceiling Insulation (Table N1102.1.1)

Key related rules:

• N1102.2.1: where the full height of uncompressed insulation extends over the wall top plate at the eaves, as with raised-heel trusses, R-30 satisfies an R-38 requirement and R-38 satisfies R-49.
• N1102.2.2: where a ceiling has no attic space and cannot fit the required insulation, R-30 is allowed, limited to 500 square feet or 20% of the insulated ceiling area, whichever is less.
• N1102.2.3 eave baffle: in vented attics with air-permeable insulation, a baffle goes next to the soffit and eave vents. It keeps an opening equal to or greater than the vent, extends over the top of the insulation, and may be any solid material.
• N1102.2.4: attic access doors from conditioned space are weatherstripped and insulated to match the surrounding surfaces. A retainer keeps loose-fill from spilling.
• R806.3: at eave vents, at least a 1-inch space stays between the insulation and the roof sheathing.

Air Sealing

Heat and moisture reach the attic mainly by air leakage through the ceiling: recessed lights, top plates, plumbing and wiring holes, chimney chases, bath fans, and attic hatches. The energy code requires a continuous air barrier with sealed breaks and joints, and it lists sealing points such as attic access openings and dropped soffits (Table N1102.4.1.1). Air-permeable insulation may not be used as a sealing material. Sealing the ceiling is the most effective step to prevent both winter condensation in the attic and ice dams.

Vapor Retarders

• Walls: Michigan enforces R601.3, which requires Class I or II vapor retarders on the interior side of frame walls in zones 5, 6, 7, and 8, with listed exceptions. LARA's errata note says R601.3, not R702.7, is the section to enforce.
• Ceilings: the code does not require a ceiling vapor retarder in a vented attic. In zones 6, 7, and 8, installing a Class I or II vapor retarder on the warm-in-winter side of the ceiling is one way to qualify for the 1/300 ventilation ratio (R806.2, Section 12.1).
• Put vapor retarders on the warm-in-winter side. Poly laid on top of attic insulation, or trapped against a cold deck, collects condensation.

Above-Deck Roof Insulation (R906)

Insulation installed above the roof deck, common on low-slope roofs, must be covered by an approved roof covering. It must comply with FM 4450 or UL 1256, which are fire and wind tests for roof deck constructions. Board standards include (Table R906.2):

Install boards with staggered joints, attached per the membrane system's requirements, and with any tapered layout for drainage (Section 10.3). Do not cover wet insulation. Remove and replace it.

Unvented Attics and Cathedral Ceilings (R806.5)

An attic or rafter assembly may be unvented only if all conditions are met. The main ones:

1. The unvented space is completely inside the building thermal envelope.
2. No interior Class I vapor retarder is on the ceiling side.
3. Wood shingles or shakes have at least a 1/4-inch vented airspace above the underlayment.
4. In zones 5 through 8, any air-impermeable insulation must be a Class II vapor retarder or have a Class II coating in direct contact with its underside.
5. Insulation is arranged one of these ways:
   • air-impermeable insulation (such as closed-cell spray foam) in direct contact with the underside of the deck;
   • air-permeable insulation under the deck plus rigid board or sheet insulation above the deck that meets Table R806.5;
   • a combination, with the air-impermeable layer against the deck meeting Table R806.5;
   • alternatively, enough continuous insulation above the deck to keep the deck's underside above 45°F on a monthly average basis.

Table R806.5 minimum rigid-board R-value for condensation control:

These values help prevent condensation. They do not replace the full energy-code insulation requirement.

Example

A Traverse City (zone 6A) cathedral ceiling uses fiberglass batts between the rafters with no vent space. To be unvented under R806.5, it needs at least R-25 rigid insulation above the deck, the ceiling must have no interior Class I vapor retarder, and the total assembly must still meet the energy code. Otherwise, provide a vented channel above the insulation from soffit to ridge.