Insulating exterior of 70's-built T&G cathedral ceiling, Zone 4a — Q1: Vented or Unvented? – GreenBuildingAdvisor

Long-time reader, first-time writer. Thanks in advance for your time, expertise, and consideration.

INSULATION DESIGN AND APPLICATION QUESTION

In Zone 4a, which insulation method is preferred for T&G cathedral ceiling?

4″ vented polyiso (~22-24 LTTR)

• 1st layer: 2″ Atlas ACFoam III

• 2nd layer: 3.5″ Atlas ACFoam CrossVent (2″ polyiso, 1″ vent, 7/16″ OSB)

• 2′ staggered seams per Atlas recommendation

 -OR-

5″ unvented polyiso (~28-30 LTTR)

• 1st layer: 2.5″ Atlas ACFoam III

• 2nd layer: 3″ Atlas ACFoam NailBase (2.5″ polyiso, 7/16″ OSB)

• 2′ staggered seams per Atlas recommendation

Replacing shingle roof in middle Tennessee (Zone 4a). Mixed roof design with 12:12 exposed-T&G cathedral and 3:12 over ceiling-insulated attic. Most of 2×6 T&G roof has no added insulation, inside or out. (Two portions covered by attic; see BUILDING DETAIL below). Insulation cannot be added to interior to preserve the appearance of the exposed wood. Desire to gain better thermal performance, mainly to increase comfort in the living space. Existing chimney height limits roof envelope increase to ~5.5″. Local residential roofers  seem uninformed and resistant to the idea of exterior insulation, though it seems common in local commercial/industrial buildings.

BUILDING DETAIL

I am replacing a roof in Zone 4a on a late 70’s-built home in middle Tennessee. The family room is in the middle of the house and has a 12:12 cathedral ceiling constructed of exposed 4×6 rafters (48″o.c.) and 2×6 T&G. There is a loft on one gable end and a brick chimney on the other. The ceiling is the roof: underlayment and shingles are applied directly to the T&G where exposed outside the envelope. There is no additional insulation except where enclosed by “attached” attics.

The rest of the house, on either side of the cathedral room, is ranch-style construction. Each side has 3:12 roof with attic typical for age and location (ridge vent, soffit vents, gable vent on open end, and blown insulation). The lower slope roofs are built onto/over the steep roof. In other words, there is a portion of the cathedral ceiling/roof that is part of each respective attic. There are two valleys where the rooflines intersect (4x valleys total).

During summer (cooling), the loft is unbearably hot in the making it unusable, though there are two supply ducts that “service” the space. The rest of the family room easily sits about 4° above the thermostat during the hottest part of the day. The rest of the house remains relatively comfortable or too cool being better insulated with the single stage HVAC running constantly during the day.

During winter (heating), the loft is tolerable whereas the rest of the family room is cold and drafty. The rest of the house remains mostly comfortable.

Along with insulation, air barrier improvement and mini-split are being added during this renovation to help improve the comfort and performance of this living space.