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This guide explains how to choose the right roof and attic insulation by climate zone, material type, and budget. Cold zones (5-8) require R-49 to R-60 in the attic; mixed zones (3-4) need R-38 to R-49; warm zones (1-2) need R-30 to R-38. Material options include fiberglass batt (cheapest, DIY-friendly), blown cellulose (best for retrofits), blown fiberglass (mold-resistant), and closed-cell spray foam (highest R per inch but most expensive). Pair with our Insulation Calculator.
Attic insulation and roof ventilation are the same conversation. Insulation keeps heat in the house; ventilation carries away whatever gets past it. Get one right and the other wrong and you end up with condensation, ice dams, or both.
| Material | R per inch | Inches for R-49 |
|---|---|---|
| Blown fibreglass | 2.5 | 19.6 in |
| Blown cellulose | 3.5 | 14 in |
| Fibreglass batt | 3.1 | 15.8 in |
| Mineral wool batt | 3.3 | 14.8 in |
| Open-cell spray foam | 3.7 | 13.2 in |
| Closed-cell spray foam | 6.5 | 7.5 in |
R-49 is a common target for attics in cold and mixed climates. Warmer zones need less and the coldest need more — check the recommendation for your climate zone before ordering.
Insulation slows heat conduction; it does not stop air moving. Warm moist air escaping through gaps around wiring, plumbing, the attic hatch and the top plates carries far more heat — and far more moisture — than conduction through the ceiling does. Sealing those gaps before laying insulation does more per pound spent than the insulation itself.
Ice dams are an insulation and ventilation failure, not a roofing one. Heat escaping into the attic melts the snow above it; the meltwater runs to the cold eave and refreezes; the dam pushes water back under the shingles. Adding insulation and air sealing stops the heat, and ventilation carries away what still gets through. Heat cables treat the symptom and run up a bill doing it.
R-49 is a common target in cold and mixed climates, less in warmer zones. The depth depends on the material — R-49 is about 20 inches of blown fibreglass or about 8 inches of closed-cell spray foam.
Always. Insulation slows conduction; it does not stop air moving. Sealing gaps around wiring, plumbing, the attic hatch and the top plates does more per pound spent than the insulation laid over them.
On the attic floor in a vented attic — that is the thermal boundary. Insulating the roof slope instead is a different assembly with different ventilation rules and is only correct in a deliberately conditioned attic.
It is half the answer. Ice dams form when heat escaping into the attic melts snow that then refreezes at the cold eave. Insulation and air sealing stop the heat; ventilation carries away what still gets through.
Also cutting heat gain from above? See energy-efficient roofing for cool roofs and reflective shingles.