unfinished attic SHOULD I USE blown-in cellulose on the attic floor or closed-cell SPF? OR HYBRID? – GreenBuildingAdvisor

I have a Cape Cod home in Westchester County, New York (Climate Zone 4A), and I’m trying to determine whether the best approach for insulating my unfinished attic is blown-in cellulose on the attic floor or closed-cell spray foam applied directly to the underside of the roof plywood.

My primary concern is long-term roof durability, moisture management, proper ventilation, and preventing potential wood rot. I recently had an entirely new roof installed, including brand-new plywood roof decking and new asphalt shingles, so protecting this investment and maximizing the lifespan of the roof are especially important to me.

My main roof has exposed plywood sheathing and rafters, with gable vents but no soffit vents or ridge vent.

I recently added three dormers to the roofline, each with a different configuration:

• Office shed dormer: This new shed dormer connects to and vents into the main unfinished attic, which is ventilated through the existing gable vents.

• Bathroom gable dormer: This is a smaller, unvented dormer constructed with 2×8 roof framing and a finished cathedral ceiling. It does not vent into the main attic. Closed-cell spray foam was applied directly against the underside of its roof plywood sheathing.

• Primary bedroom (MBR) gable dormer: This is another smaller, unvented dormer with 2×8 roof framing and a finished cathedral ceiling. This new shed dormer connects to and vents into the main unfinished attic, which is ventilated through the existing gable vents.

Additionally, all of my new walls below the dormers have closed-cell spray foam insulation. The remaining existing walls throughout the Cape Cod home will be retroactively insulated with blown-in cellulose.

My Cape Cod knee walls are also insulated with closed-cell spray foam applied directly against the plywood sheathing.

I was recently advised by an insulation contractor that I should not mix insulation systems or thermal envelopes and that, because portions of the new construction already contain spray foam, I should insulate the entire main roof with spray foam rather than cellulose.

However, I am trying to follow the building-science guidelines and recommendations of Dr. Joseph Lstiburek, particularly his principles regarding moisture management, roof ventilation, air sealing, and the long-term durability of roof assemblies. Based on my understanding of his work, I am questioning whether converting my entire roof to an unvented spray-foam assembly is actually the best approach.

My preference would be to retain the existing closed-cell spray foam in the dormers and new walls, thoroughly air-seal the main attic floor, and install blown-in cellulose across the attic floor and above the applicable finished ceilings, while leaving the original main roof plywood exposed to the attic so it can dry inward.

I understand that the existing closed-cell foam limits inward drying of the two unvented dormer roof assemblies and that the ventilation and transitions between the different insulated assemblies need to be evaluated.

A) How should the transitions between the two unvented, spray-foamed cathedral-ceiling dormers, the office shed dormer, and the cellulose-insulated main attic floor be properly air-sealed and insulated to maintain a continuous thermal and air-control boundary?

B) Given this specific configuration and the conditions in Climate Zone 4A, would you recommend keeping the main attic vented and insulating the floor with cellulose, or using closed spray-foam on the underside of the entire roof line?

Is there any building-science reason these two insulation approaches cannot safely coexist?

C) Considering that I have brand-new plywood roof decking and asphalt shingles, which approach would provide the best long-term protection against condensation, moisture accumulation, wood rot, and premature roof deterioration?

I would also appreciate your opinion on whether my existing gable-only ventilation is adequate, particularly with the new office shed dormer connecting to the main attic, or whether additional ventilation or moisture-control measures are necessary.

My priority is not necessarily choosing the least expensive option, but rather following sound building-science principles and selecting the safest, most durable insulation system to protect my new roof for decades to come.