Insulation in Michigan.

Snow itself is actually a decent insulator, a foot of fresh snow has roughly an R-1 per inch. The problem isn't the snow load; it's what melts unevenly. A poorly insulated attic lets warm indoor air rise to the roof deck, melts the snow above the heated portion, and refreezes at the cold eave. That's the recipe for ice dams. Properly insulating and air-sealing the attic floor keeps the roof deck cold, and snow stays as snow until it slides off in a thaw.

Yes, with a caveat. The biggest, fastest payback is sealing and insulating the rim joist (the band of wood at the top of the foundation where the floor framing sits). That single area is responsible for an outsized share of basement air leakage. Full basement wall insulation (typically rigid foam against the foundation, then framing inside that) is also valuable in MI, but it's a bigger project. Both pay back over time; the rim joist pays back almost immediately.

This is more nuanced than the old "always include a poly vapor barrier" rule. Modern building science recommends a vapor retarder on the warm side of the wall in zones 5 and 6, but the air barrier is far more important than the vapor barrier. Most moisture problems in MI walls come from air leakage, not vapor diffusion. Your installer will choose appropriate materials based on your specific assembly, typically smart vapor retarders or kraft-faced batts on the interior side, with attention paid to air sealing.

Michigan's residential code follows IECC 2015 with state amendments. For new construction in zone 5A: R-49 attic, R-20 walls (or R-13 + R-5 continuous), R-30 floors over unconditioned spaces. Zone 6A (northern MI and U.P.) bumps to R-49 attic, R-20 walls + R-5 continuous, R-30 floors. Most existing homes built before 1990 fall well short. Bringing them up to current values is where most of the savings come from.

For most MI attics, blown-in cellulose wins. Cellulose densifies into the cavity, conforms around obstructions (pipes, wires, framing irregularities), and has slightly better R-per-inch than loose-fill fiberglass at low temperatures, which matters in a state where attic temps regularly hit -10°F. Batts have their place when stud cavities need to be filled in walls, but for top-up attic work, blown cellulose is the standard answer.

Yes, when it's done right. Ice dams form when warm attic air melts snow on the upper roof, which then refreezes at the cold eaves. The fix has two parts: air sealing the attic floor (so warm indoor air can't rise into the attic), and then insulating to code-level R-values (so what little heat does escape doesn't warm the roof deck). Most ice-dam projects in MI are really air-sealing projects with insulation as the second-order fix. Done together, they typically eliminate ice dams entirely, saving you the gutter ice-removal calls every January.