Moisture Control Begins at the Roof Deck
In a conditioned attic, unvented roof assemblies can deliver excellent comfort and airtightness—if they’re detailed to keep the roof sheathing safely above the dew point. The core principle is continuous insulation + continuous air barrier. Start by confirming roof structure and sheathing condition; repair soft spots, verify fastener pull-out, and document all penetrations you’ll need to seal later.
Insulation Ratios That Protect Sheathing
To prevent wintertime condensation, maintain the required ratio of exterior (or foam-in-bay) insulation to interior insulation so the sheathing stays warm. A common stack for re-roof projects is rigid insulation above the deck (polyiso or GPS) with taped seams as part of the air barrier, then dense batt or additional foam under the rafters to hit target R-values. For interiors-only work, closed-cell spray foam against the deck can provide both air seal and the warm-side R needed, followed by a service cavity below for wiring and fixtures.
Vapor Control and Drying Paths
Use vapor-smart membranes on the interior side so assemblies can dry to the room in shoulder seasons. Avoid poly sheeting that traps moisture; instead, pair your membrane with meticulous sealing at joints, top plates, and skylight shafts. Around chimneys and flues, respect clearances and use noncombustible air-seal methods per listings.
Penetrations, Skylights, and Air Tightness
Every hole is a risk. Box and gasket electrical fixtures (IC/AT where insulation contacts housings), use compatible tapes and sealants for ducts and vent stacks, and specify factory pan flashing kits at skylights. Before drywall, run a blower-door test to find and fix leaks while access is easy. Commission bath exhausts so humid air leaves the building—not the soffit or attic cavity.
For step-by-step assemblies and checklists, see our attic waterproofing and moisture control in NYC guide.