Ventilation, Energy Code, and Moisture Control
Attic ventilation
IRC R806 requires net free vent area of 1/150 of the vented ceiling area, reducible to 1/300 where a Class I or II vapor retarder is on the warm side, 40 to 50 percent of the vent area sits in the upper portion at least 3 feet above the eave vents, and cross-ventilation is provided. Where eave vents are used, a clear 1-inch air channel must run from eave to ridge above the insulation - baffles, not compressed batts. Bath and kitchen exhaust must discharge outdoors, never into the attic.
Energy code
Base code is 780 CMR 13.00 (IECC-based). Massachusetts is Climate Zone 5, where base ceiling insulation is on the order of R-49 with air sealing, insulated and gasketed attic hatches, and airtight IC-rated recessed fixtures. Towns that adopted the Stretch Energy Code (225 CMR 22.00) impose performance requirements beyond the base code, so the same house needs a different roof-ceiling detail depending on the town.
Moisture and ice dams
Ice dams are a heat-loss problem: warm attic air melts snow, water refreezes at the eave and backs up under the shingles. Ice-and-water membrane at the eave is backup protection, not a cure - the cure is air sealing, insulation and ventilation. Vented assemblies need an unobstructed path; unvented cathedral assemblies must be insulated at the roof deck, airtight, and detailed to control condensation at the sheathing.
Worked example
A 1.5-story cape with ice dams every February. The defect is a leaky ceiling plane around the stair chase. Correct order: air seal, insulate to R-49, add baffles and a ridge vent, then install eave ice barrier during the re-roof.
Traps
Burying eave vents with insulation, assuming more vents cure an air leak, venting dryers into the attic, and applying Stretch Code assumptions to a town that never adopted it.