Context
User has a 24’x40’ garage with 10’ walls, an upstairs loft, and an A-frame roof in climate zone 6a. The structure has vented soffits and ridge vent, metal roofing, and will be finished later. They want to know the best approach to meet insulation code requirements (R-60 attic, R-49 cathedral slopes).
Key Points
- R-60 required for attic, R-49 for cathedral ceiling in Zone 6a.
- Flat ceilings: blown-in cellulose or fiberglass to ~16—18” depth.
- Angled roof sections: rafters too shallow for R-49 with fluffy insulation alone; need furring down or spray foam.
- Options:
- Fur-down rafters + batts/cellulose: cheapest, more labor.
- Closed-cell spray foam full-depth: most expensive, excellent air seal, meets R-49 in 2×10.
- Flash-and-batt: 2—4” spray foam for air seal + fluffy insulation; balances cost and performance.
- Exterior rigid foam (if reroofing): continuous insulation, strong option if metal roof not yet installed.
- Spray foam cost: ~$8—12/ft² for full-depth R-49; hybrids ~$4—8/ft²; batt/cellulose ~$3—6/ft².
- Strategic spray foam use (rim joists, knee wall backs, top plates, penetrations) recommended even if not full-depth.
Actions
- Decide conditioned vs unconditioned zones (garage shop vs loft apartment).
- Choose insulation method for angled rafters: fur-down + batts, flash-and-batt, or full foam.
- Plan air sealing details (rim joists, knee walls, penetrations).
- Confirm with inspector whether vented or unvented cathedral assembly is preferred.
- Price out insulation methods with local contractors.
Links
- Example diagram of furring down rafters: included in conversation.
- Flash-and-batt assembly diagrams: included in conversation.