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R-49 is the common target for the roof or ceiling in cold and mixed climates. That is about 20 inches of blown fibreglass, 15 inches of blown cellulose, 16 inches of fibreglass batt or about 8 inches of closed-cell spray foam. Air seal the ceiling plane before adding any of it.
The Roof Insulation Calculator works out how much insulation a roof needs to reach the recommended R-value for your climate zone: R-38 in mild zones and R-49 to R-60 in cold ones. Enter the area you are insulating (the sloped roof area for a roof deck or cathedral ceiling), the target R-value and what is already there, and pick fiberglass batts, mineral wool, blown cellulose, blown fiberglass or spray foam. It returns the R-value to add, the thickness, the bags or batts and the installed cost.
Insulating a vented attic floor rather than the roof deck? Use the attic insulation calculator on our sister site Smart Insulation Calculator, built for loose-fill depth, bag counts and topping up existing attic insulation.
Roof insulation is measured in R-value, which is resistance to heat flow. How thick that is depends entirely on the material — the same R-49 is about 20 inches of blown fibreglass or about 8 inches of closed-cell spray foam.
| Material | R per inch | Inches for R-49 | Coverage per unit |
|---|---|---|---|
| Blown fibreglass | 2.5 | 19.6 in | 40 sq ft per bag |
| Blown cellulose | 3.5 | 14 in | 14.3 sq ft per bag |
| Fibreglass batt | 3.1 | 15.8 in | 48 sq ft |
| Mineral wool batt | 3.3 | 14.8 in | 48 sq ft |
| Open-cell spray foam | 3.7 | 13.2 in | Applied by the board foot |
| Closed-cell spray foam | 6.5 | 7.5 in | Applied by the board foot |
R-49 is the common target for the roof or ceiling in cold and mixed climates; warmer zones use less and the coldest use more. Check the recommendation for your climate zone before ordering.
| Target R-value | Blown fibreglass (~28–30 lb bag) | Blown cellulose (25 lb bag) |
|---|---|---|
| R-30 | 16 bags (65.3 sq ft each) | 43 bags (23.3 sq ft each) |
| R-38 | 20 bags (51.6 sq ft each) | 55 bags (18.4 sq ft each) |
| R-49 | 25 bags (40 sq ft each) | 70 bags (14.3 sq ft each) |
| R-60 | 31 bags (32.7 sq ft each) | 86 bags (11.7 sq ft each) |
A bag holds a fixed amount of fibre, so it covers less floor the deeper you blow it. These counts follow the manufacturers' attic coverage charts (Owens Corning AttiCat for fibreglass; GreenFiber and Applegate cards scaled to a 25 lb bag for cellulose). Cellulose settles about 11%, so blow it that much deeper than the settled depth. The chart printed on your bag is the final word.
An under-insulated, badly ventilated attic is the direct cause of ice dams: heat escapes into the attic, melts the snow on the roof above, and the meltwater refreezes at the cold eave. Adding insulation and fixing the intake ventilation solves it. Heat cables treat the symptom.
It also shortens shingle life. An attic running far hotter than the outside air cooks the deck and the shingles from underneath, which is why ventilation and insulation belong in a roofing conversation rather than only an energy one.
R-49 is the common target in cold and mixed climates; warmer zones need less. The depth depends on the material — R-49 is about 20 inches of blown fibreglass or about 8 inches of closed-cell spray foam.
Yes, always. Sealing the gaps around wiring, pipes, the attic hatch and the top plates does more per pound spent than the insulation laid over them, and insulation laid over unsealed gaps does not stop the air moving.
Only when it blocks the soffit vents, which is extremely common. Insulation pushed into the eave closes off the intake air, the attic stays hot, and ice dams form at the eaves in winter. Baffles fix it.
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, and ventilation carries away what still gets through.
Closing up the walls and ceiling after insulating? The drywall calculator counts the sheets, mud, tape and screws for the room.
Hiring out the board? The drywall installation cost calculator prices hanging and finishing per square foot by finish level.
Choosing a product? The best attic insulation guide compares fiberglass, mineral wool, foam board and radiant barrier.
Working in the attic in October? Insulation and tools are in our Prime Big Deal Days tool deals hub.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.